Fracture prediction in the tight-oil reservoirs of the Triassic Yanchang Formation in the Ordos Basin, northern China

Wen-Tao Zhao; Gui-Ting Hou

doi:10.1007/s12182-016-0141-2

Fracture prediction in the tight-oil reservoirs of the Triassic Yanchang Formation in the Ordos Basin, northern China

Zhao, Wen-Tao; Hou, Gui-Ting 2017-01-12 00:00:00 Pet. Sci. (2017) 14:1–23 DOI 10.1007/s12182-016-0141-2 ORIGINAL PAPER Fracture prediction in the tight-oil reservoirs of the Triassic Yanchang Formation in the Ordos Basin, northern China 1,2,3 1 Wen-Tao Zhao Gui-Ting Hou Received: 3 September 2015 / Published online: 12 January 2017 The Author(s) 2017. This article is published with open access at Springerlink.com Abstract It is important to predict the fracture distribution distribution in the study area, which further determine the in the tight reservoirs of the Ordos Basin because fracturing fracture distribution in the stable Ordos Basin. The pre- is very crucial for the reconstruction of the low-perme- dicted fracture density and the two-factor method can be ability reservoirs. Three-dimensional ﬁnite element models utilized to guide future exploration in the tight-sand are used to predict the fracture orientation and distribution reservoirs. of the Triassic Yanchang Formation in the Longdong area, southern Ordos Basin. The numerical modeling is based on Keywords Ordos Basin Yanchang Formation Fracture the distribution of sand bodies in the Chang 7 and 7 prediction Finite element modeling Two-factor method 1 2 members, and the different forces that have been exerted Tight-sand reservoirs along each boundary of the basin in the Late Mesozoic and the Cenozoic. The calculated results demonstrate that the fracture orientations in the Late Mesozoic and the Ceno- 1 Introduction zoic are NW–EW and NNE–ENE, respectively. In this paper, the two-factor method is applied to analyze the Unconventional oil and gas resources, such as tight gas, distribution of fracture density. The distribution maps of tight oil and shale oil, have been successfully developed predicted fracture density in the Chang 7 and 7 members commercially in the USA, Canada, Australia and some 1 2 are obtained, indicating that the tectonic movement in the other countries. The production of tight oil soared from 30 Late Mesozoic has a greater inﬂuence on the fracture million tons in 2011 to 96.9 million tons in 2012 in the development than that in the Cenozoic. The average frac- USA by using new unconventional technologies (Du et al. ture densities in the Chang 7 and 7 members are similar, 2014). In China, the Ordos Basin, the Junggar Basin, the 1 2 but there are differences in their distributions. Compared Songliao Basin, the Sichuan Basin, the Qaidam Basin, etc., with other geological elements, the lithology and the layer have abundant tight-oil resources with an output of 97 thickness are the primary factors that control the stress million tons, accounting for 22% of the nationwide total oil output (Jia et al. 2014). In the Ordos Basin, the tight-oil reservoirs in the Triassic Yanchang Formation have & Gui-Ting Hou become a major target of petroleum exploration and gthou@pku.edu.cn development in recent years (e.g., Guo et al. 2012; Yao The Key Laboratory of Orogenic Belts and Crustal Evolution et al. 2013). of Ministry of Education, School of Earth and Space Since tight-oil reservoirs in the Ordos Basin are of low- Sciences, Peking University, Beijing 100871, China -3 2 permeability (\2 9 10 lm ) and low-porosity (\10%) China Huaneng Clean Energy Research Institute, overall, fracturing is crucial for the reservoir reconstruc- Beijing 102209, China tion, even though the reservoirs are formed with compli- PetroChina Research Institute of Petroleum Exploration and cated mechanisms (e.g., Yao et al. 2013; Ezulike and Development, Beijing 100083, China Dehghanpour 2014). Therefore, it is important to predict the natural fracture distribution in reservoirs, including Edited by Jie Hao 123 2 Pet. Sci. (2017) 14:1–23 their orientation and density, for future exploration and Li 2009). Some small-scale paleo-faults exist, but no large development (e.g., Smart et al. 2009). Previous studies faults have been found within the basin (Wan and Zeng have focused on the geometrical or kinematic models, such 2002; Yang et al. 2013). as analyses of seismic techniques or of the layer curvature Contrary to the evolution of the eastern North China (e.g., Zahm et al. 2010; Pearce et al. 2011; Tong and Yin Craton including thickening, thinning and destruction, the 2011), and fracture prediction in the Ordos Basin has also Ordos Basin has evolved from three Mesoproterozoic been involved in some papers (e.g., Ju et al. 2014a). aulacogens to a Paleozoic–Mesozoic cratonic basin since However, since earlier fracture prediction was mainly the Middle Proterozoic (Menzies et al. 2007; Yang et al. carried out through layer curvature or two-dimensional 2013; Wang et al. 2014b). The interior part of the basin is (2D) models, which cannot meet the demands for the tight- characterized by horizontal or gently dipping strata (\3), oil study and exploration, it is necessary to build three- especially for the Mesozoic and Cenozoic strata, whereas dimensional (3D) mechanical models in order to achieve the strata along the margins have been subjected to sig- the accuracy needed for further research on the uncon- niﬁcant folding and faulting since the Late Triassic. ventional petroleum. Two distinct tectonic events took place from the Late Various factors, such as the proximity of faults, the Mesozoic to the Cenozoic, resulting in two different stress curvature of folds, the layer thickness and the lithology, are ﬁelds in these periods. In the Late Mesozoic, namely from deemed to control the fracture development in tight the Early Jurassic to the Late Cretaceous, the long-distance reservoirs (e.g., Ju et al. 2013), and the anisotropy or effect of subduction of the Izanagi Plate turned from north- heterogeneity should also be considered in the modeling northwestward to northwestward when the force arrived at (e.g., Glukhmanchuk and Vasilevskiy 2013). However, it is the Ordos Basin, resulting in the WNW-trending stress difﬁcult for 2D geomechanical models to fully consider all ﬁelds and the structural fractures in NW–EW trends (e.g., these factors, and the modeling results cannot be used Wan 1994; Hou et al. 2010; Sun et al. 2014; Zhao et al. successfully for exploration and production. Therefore, 3D 2016); while in the Cenozoic, the predominant tectonic models will be utilized in this paper, which take the event became the northeastward collision between the lithology, the thickness and the stress ﬁelds into Indian and the Eurasian Plate, which led to the NE-trending consideration. stress ﬁelds and the structural fractures in NNE–ENE The study area in this paper, namely the Longdong area, trends (e.g., Yuan et al. 2007; Wang et al. 2014b). Two is located in the southern Ordos Basin, where research on episodes of fractures are developed under distinct tectonic structural fractures in the tight reservoirs is still deﬁcient events, so the stress ﬁelds of different periods should be (e.g., Ren et al. 2014; Li et al. 2015). The structural frac- taken into consideration during the fracture prediction. tures in the Longdong area were mainly formed after the Late Triassic, as a result of multiple-stage tectonic events in the Late Mesozoic and the Cenozoic. These extensively 3 Fracture measurement developed fractures are mostly unﬁlled and effective, which noticeably improve the permeability of tight reser- The parameters of fracture characteristics are important in voirs in the Ordos Basin. the exploration and development of fractured tight reser- voirs. The fracture density is one of the signiﬁcant indicators to reﬂect the failure degree of rocks, which can be divided 2 Geological background into three types, including the linear density, the surface density and the bulk density of fractures. In this paper, the 5 2 The Ordos Basin, covering an area of 2.6 9 10 km ,is a surface density is utilized to describe the fracture distribution large N–S trending basin in the western North China in the Ordos Basin. The surface density is deﬁned as the ratio Craton, which is located between the Siberian Craton and between the cumulative fracture length and the cross-sec- the South China Craton (Hou et al. 2010) (Fig. 1). Three tional area of the matrix, which can better reﬂect the degrees orogenic belts have been developed along different of fracture development and be measured more effectively boundaries of the stable basin, including the Yinshan than others (Golf-Racht 1982). The fracture density from Mountain in the north, the Qinling Orogen in the south and core observations can be calculated as: P P the Liupanshan Mountain in the southwest (e.g., Nutman l l i i f ¼ ¼ ð1Þ et al. 2011) (Fig. 2). The basement of the basin is com- S 2pr þ 2pr L posed of Archean rocks with Proterozoic sedimentary where f is the fracture surface density, l is the length of cover. Although the margin underwent multiple tectonic each structural fracture, S is the surface area of the activities, the central part is still stable and is covered by observed core, r is the radius and L is the length of the core. shallow Paleozoic marine carbonate sediment (Kusky and 123 Qinling-Dabie Belt Qilianshan Orogen Himalayan Orogen Central Asian Pyeonrang YB CB Pet. Sci. (2017) 14:1–23 3 110°E 115°E 120°E 125°E 0 100 200 400 km City (b) 2.5-2.7 Ga Neo-Archean Greenstone Belt 2.5-2.7 Ga Neo-Archean Rocks Central 2.8-3.8 Ga Meso- and Eo-Archean Rocks Beijing WB Taiyuan EB Ordos Basin Qingdao Trans-North China Orogen 80°E 100°E 120°E 140°E (a) Yinan 40°N Linfen Xi’an 30°N Shanghai 500 1000 km 20°N Yangtze Craton 110°E 115°E 120°E 125°E Fig. 1 A geological summary of the Ordos Basin. a Tectonic framework of the major cratonic blocks in China (after Kusky 2011 and Santosh et al. 2012); b generalized geological and tectonic map of the North China Craton (after Zhao et al. 2005). TC Tarim Craton, NCC North China Craton, SCC South China Craton, YB Yangtze Block, CB Cathaysia Block, WB Western Block, EB Eastern Block In this paper, the Longdong area was selected as the 4 Modeling approach study area to carry out fracture measurements (Fig. 2). Sixty-six wells were chosen to study the distribution of Methods such as geological analysis, physical modeling structural fractures in the Chang 7 and 7 members and numerical simulation including the ﬁnite element 1 2 (Fig. 3). As shown in Fig. 4, the fracture density in the method (FEM) can be applied in the study of stress ﬁelds, tight-sandstone cores is relatively low (smaller than which is the foundation of fracture prediction. In this study, -1 0.5 m ), representing the general condition in the the ﬁnite element software ANSYS is used to calculate the Longdong area. The fracture density of the Chang 7 stress ﬁeld and predict the fracture distribution (Vela´zquez member is more concentrative than that of Chang 7 et al. 2009; Jarosinski et al. 2011). The basic concept of member, even though their average densities are similar FEM is that a geological body can be discretized into ﬁnite -1 in general (0.071 m for the Chang 7 member and continuous elements connected by nodes. The geometrical -1 0.081 m for the Chang 7 member). The difference of and mechanical parameters allocated on each element are fracture distribution between the Chang 7 and 7 consistent with the properties of real rocks. The continuous 1 2 members is obvious: The highest fracture density in the ﬁeld function of the geological area is ﬁrst transformed into Chang 7 member lies in the Laocheng and Qingyang linear functions at every node that contain displacement, areas, while that in the Chang 7 member lies in the stress and strain variables resulting from the applied forces Laocheng, the Qingyang and the Zhengning areas (Jiu et al. 2013), and then all these elements are used to (Fig. 4). obtain the stress distribution over the entire area. TC SCC NCC Asian Orogenic Inner Mongolia Suture Zone Orogenic Belt Belt Yinshan Block Imjngang Gyeonggi Yeongnam Su-Lu Orogen Su-Lu UHP Belt 35°N 40°N 35°N 40°N Liupan Mountain 4 Pet. Sci. (2017) 14:1–23 105°E 110°E 02 50 100 00 km Hetao Graben 40°N 40°N Alashan Block Yinchuan Wuqin Yulin Ordos Basin Dingbian Yanchang Qingyang Lanzhou Study area Linfen 35°N 35°N Xi'an Qinling Orogen Belt 105°E 110°E 1 2 3 4 5 6 7 8 Fig. 2 Tectonic framework of the Ordos Basin. The orange area denotes the Ordos Basin, and the light yellow ones denote the adjacent blocks. The light gray areas represent the graben system along the margins, and the dark gray ones represent the orogenic belts around the basin. The black frame denotes the study area (the Longdong area) (after Darby and Ritts 2002)(1 Boundary of orogenic belt, 2 Fault, 3 Normal fault, 4 Reversed fault, 5 Strike-slip fault, 6 Fold, 7 River, 8 City) 4.1 Geometrical model part. Therefore, the outline of the basin remained unchan- ged in these periods (Sun et al. 2014) (Fig. 5a). Since no The Ordos Basin is a near-rectangular basin in the western large faults or folds have been recorded inside the Ordos part of the North China Craton (Li and Li 2008; Tang et al. Basin, the sedimentary facies, the lithology and the distri- 2012) (Fig. 2). Although the Ordos Basin underwent multi- bution of sand bodies are the key factors to determine the stage tectonic movements in the Late Mesozoic–Cenozoic fracture development. eras, the deformation was conﬁned to the western margin In this paper, the Chang 7 and 7 members in the Tri- 1 2 and no signiﬁcant tectonic events occurred in the central assic Yanchang Formation, the major tight-oil members in Yinshan Orogen Belt Yinchuan Graben Weihe Graben Fenhe Graben Shanxi Block Yellow River Helan Mountain Lüliang Mountain Pet. Sci. (2017) 14:1–23 5 Lithology Sedimentary Epoch Formation Subsection Thickness, m column facies Anding Swamp 80-150 Middle Jurassic Zhiluo Fluvial 20-40 Lower Yan 1- Fluvial- Yan’an 250-300 Yan 10 Jurassic lacustrine- swamp Chang 1 0-245 120-160 Chang 2 Chang 3 100-170 Fluvial- Chang 4+5 lacustrine 90-130 Chang 6 180-200 Upper Yanchang Triassic Deep Chang 7 lacustrine Chang 8 Lacustrine 100-190 Chang 9 Chang 10 Fluvial 200-320 Conglomerate Sandstone Sandy mudstone Mudstone Oil shale Coal Fig. 3 Synthetical stratigraphic column and depositional environment of the Upper Triassic—Middle Jurassic in the Ordos Basin (after Duan et al. 2008) the Ordos Basin, are selected as the study strata, and the on the sandstone-mudstone ratio, it is assumed that the Longdong area is chosen to discuss the stress ﬁelds and the ratios between the sandstone and mudstone layers are fracture distribution (Figs. 2, 3, 5). Since the Yanchang 0.43–4.26 (average 1.27) in the Chang 7 member and Formation is characterized by strong heterogeneity with 0.54–9.00 (average 1.70) in the Chang 7 member (e.g., facies change, the simpliﬁed model with only one rock Guo et al. 2012; Li et al. 2015), and multiple-layer con- mechanical property is no longer suitable for the complex structions (four sandstone layers in Chang 7 member and interior of the Ordos Basin (Yang and Deng 2013). Based three sandstone layers in Chang 7 member) are applied in 123 6 Pet. Sci. (2017) 14:1–23 0 50 100 200 km 0 0 (a) (b) N N 0.085 0 B401 Hua56 Hua312 W98 0 W98 0.164 0 Huanxian 0 B36 Y433 L96 L96 B146 Huanxian B146 0.059 S142 0.068 M28 0.052 B170 B478 0.033 0.047 L189 Huachi L189 Huachi S160 L79 L79 0.053 0.036 B456 B456 0.018 0.129 0.070 0 0.062 0.020 0.043 0 W47 M40 L47 C87 L47 W67 C87 B117 0 Ze77 B117 0.464 0.201 0.151 0.316 0.199 X259 0.344 0.071 0.055 0.068 0.072 0 Z57 Z87 0 Ze97 0 Ze97 Z78 Z47 Z78 0.061 X270 X233 X233 Z79 T2 Laocheng 0 Z79 Z15 Ze298 0.026 0.122 Z186 0.113 0.050 Laocheng X261 Z24 X73 Qingcheng Z200 Qingcheng 0.052 Ze220 T15 X263 X263 0.046 0.064 Ze362 0.918 0.028 0.038 0 Ze95 0.025 0.271 Z148 Z230 Z230 0.320 0.080 X195 X140 Z172 0 Z172 Z233 Z233 0.155 Heshui Heshui Z124 Ze284 0.096 Ban12 Ban12 N76 0.210 N76 Qingyang 0.062 Qingyang 0 0.024 Ze118 0.015 0.037 0.026 0.036 0.157 N43 N78 N43 N78 X67 X67 N75 N75 0.180 0.385 0.057 0.055 0.137 0.050 0.137 Ningxian X69 0.053 N81N57 0 N57 Zeg70 Zeg70 N51 N51 Ningxian X65 N55 Zhengning Zhengning Chang 7 1 Chang 7 2 Sandstone thickness 0 5 10 15 20 m Fig. 4 Distribution of the measured fracture density and the sandstone thickness in a the Chang 7 and b Chang 7 members in the Longdong 1 2 area. The black solid dots denote the observed wells, while the hollow dots denote the cities. Measured fracture densities in the Chang 7 and 7 1 2 members are marked with red cylinders, and the contours represent the sandstone thickness in the study area the geometrical model to simulate the sandstone-mudstone consists of sedimentary cover, greenschist and granite, is interlayers (Fig. 5). To avoid the boundary effect, forces in 2750 kg/m (Hou et al. 2010; Wang et al. 2014b). Based on the models are set on the boundaries of the Ordos Basin, the velocities of P and S waves, the calculated average and the study area is nested inside the basin (Fig. 5). The Poisson’s ratio is 0.20 and the average Young’s modulus is sandstone layers, which are also the main layers in fracture 80 GPa for the whole basin (Liu et al. 2006). development, in the middle of each member, are selected to To subtly depict the distribution of structural fractures in display the modeling results in the following text, repre- the Longdong area, four more kinds of material elements senting the general situation of fracture development in the are involved in the 3D geometrical model, including the Chang 7 and 7 members. sandstones/mudstones of the Chang 7 member and the 1 2 1 sandstones/mudstones of the Chang 7 member. Tri-axial 4.2 Boundary conditions and modeling rock mechanical experiments were carried out by the Institute of Acoustics, Chinese Academy of Sciences, on 62 In order to predict the fracture distribution of the Ordos core samples collected from observed wells in the Long- Basin, it is assumed that the upper crustal thickness of the dong area (Fig. 4). In order to simulate the real conditions basin in the Late Mesozoic–Cenozoic era is 25 km (e.g., underground, in these experiments, conﬁning pressures Liu et al. 2006). The top of the model is set as a free corresponding to the original depth of the Yanchang For- surface, and the entire model is subjected to gravity load. mation are applied in the radial directions, and vertical The average density of the upper crust, which mainly pressures are applied in the axial directions of all samples. 123 Pet. Sci. (2017) 14:1–23 7 (a) (b) 20 km Longdong area Overlying layer Chang 7 1 Chang 7 2 Basement Sandstone layers Mudstone layers Nested model Fig. 5 Simpliﬁed geometrical model of the Longdong area (b) and its nested model (a) in the Ordos Basin Through statistical analysis and geological classiﬁcation, push from the northwestward subduction of the Izanagi ﬁve sets of rock mechanical properties, the average density, Plate in the Late Mesozoic (Zhang et al. 2007; Hou and Young’s modulus, Poisson’s ratio, internal friction angle Hari 2014). Hence, it is a compressive boundary with a and cohesion, are listed in Table 1 by layer and lithology. sinistral shearing component along the eastern edge of the Because the stress ﬁelds in the Late Mesozoic and basin. A deviatoric stress of an approximately 150 MPa the Cenozoic are strikingly different and both of them normal component with a 45 MPa shearing component is had a signiﬁcant effect on the fracture development in set along the eastern boundary (L2) (Fig. 6a). Based on the the Ordos Basin, the boundary conditions during these paleo-magnetic constraints, geological evidence and 40 39 two episodes along the basin need to be deﬁned (Zhao Ar/ Ar and U–Pb dating, it can be assumed that in the et al. 2016). southern part of the Ordos Basin, the Qinling Ocean ﬁnally As a result of intense compression from the Early closed during the Late Jurassic-Early Cretaceous period. Jurassic to the Middle Cretaceous, the Yinshan Orogen This indicates that the collision between the North China Belt was developed as thrust faults with dextral shearing Craton and the South China Craton continued up to the features in the Late Mesozoic (Darby and Ritts 2002; Cretaceous period (Huang et al. 2005; Liu et al. 2015). And Zhang et al. 2007; Faure et al. 2012). A uniform direction due to this collision, thrust faults with sinistral strike-slip and a constant magnitude of a 40 MPa normal component features were developed along the northern margin of the with a 10 MPa dextral shearing component are applied Qinling Orogen Belt (Malaspina et al. 2006; Yuan et al. along the northern side of the Ordos Basin (L1) (Fig. 6a). 2007). Therefore, a constant magnitude of normal stress The east-dipping thrusts, the NWW-dipping back-thrusts (60 MPa) with sinistral shearing stress (30 MPa) is applied and the associated folds developed along the Lu¨liang along the southern margin of the basin (L3) (Fig. 6a). In Mountain in the Jurassic show that the stress regime in the the western and southern margins, the long-distance effect eastern margin was related to the long-distance effect of the of collision from the Qiangtang Massif affected the Ordos Table 1 Rock mechanical parameters for the numerical modeling of the Chang 7 and 7 members in the Longdong area 1 2 Model position Lithology Density, g/cm Young’s modulus, GPa Poisson’s ratio Internal friction angle, Cohesion, MPa Chang 7 Sandstone 2.562 26.971 0.229 36.00 37.62 Chang 7 Mudstone 2.457 24.296 0.269 24.02 40.34 Chang 7 Sandstone 2.639 25.629 0.229 37.80 31.13 Chang 7 Mudstone 2.485 24.943 0.269 24.08 40.12 80 km Hangjinqi Ordos Basin Dingbian Wuqi Yanchang Qingyang Longdong area 8 Pet. Sci. (2017) 14:1–23 105°E 109°E 113°E 105°E 109°E 113°E (a) (b) Late Mesozoic Cenozoic 150 MPa 150 MPa L1 L7 Hangjinqi Hangjinqi Dongsheng Dongsheng L6 L12 Wushenqi Wushenqi L8 L2 Jingbian Jingbian Dingbian Dingbian Wuqi Wuqi Yanchang Yanchang L5 L11 Qingyang Qingyang Zhenyuan Zhenyuan Tongchuan Tongchuan L4 L9 L3 L10 Xi’an Xi’an 040 80 km 040 80 km 105°E 109°E 113°E 105°E 109°E 113°E Fig. 6 Boundary conditions of the Ordos Basin in a the Late Mesozoic and b the Cenozoic eras. Arrows indicate the tectonic forces and the length represents the stress magnitude. The crosses represent the ﬁxed boundaries in different models Basin (Zhang et al. 2007; Li and Li 2008), so a compres- was locked by the adjacent blocks in the Late Mesozoic sive traction with a uniform direction and a constant (Fig. 6a). magnitude of deviatoric stress of 30 MPa on the south- The stress ﬁeld in the Cenozoic era, which is regarded as western boundary (L4) and 75 MPa on the western a consequence of the Indo-Asian collision, is strikingly boundary (L5) are applied along the basin (Fig. 6a). On the different from that in the Late Mesozoic era (e.g., Darby basis of SHRIMP zircon U–Pb ages and other geochrono- and Ritts 2002; Bao et al. 2013) (Fig. 6b). During the logical data, it can be presumed that the closure of the Cenozoic, the extension along the margins of the Ordos Paleo-Asian Ocean ﬁnally took place after the Early Per- Basin triggered the formation of the Hetao, the Weibei and mian. Due to this episode of closure, the northward the Yinchuan Grabens, which in turn transposed reverse movement of the Alashan Block (Fig. 2) was arrested by faults to normal faults in the Helan Mountain and the the Siberian Craton in the Late Mesozoic (e.g., Zheng et al. Qinling Mountain (Rao et al. 2014). Therefore, a tensile 2014). The ﬁnal closure of the Qilian Ocean took place at traction with a uniform direction and a constant magnitude the end of the Ordovician, and after that, the Qaidam of 5 MPa is applied on the northern, the southern and the Block, which was adjacent to the Alashan Block, restricted northwestern margins, respectively (L7, L9 and L12) the southward movement of the Alashan Block (Song et al. (Fig. 6b). The subduction of northwestern Paciﬁc Plate 2013). The nonidentical apparent polar wandering paths of restricted the further eastward movement of the Ordos the Tarim Block and the Alashan Block up to the Jurassic Basin (Fournier et al. 2004; Schellart and Lister 2005). The period clearly indicates that the amalgamation of these two current GPS horizontal velocity ﬁeld map shows that the blocks might have occurred during the Jurassic (Gilder eastward velocity of the Shanxi Block (Fig. 2) is relatively et al. 2008). As a result of amalgamation in the Jurassic, the smaller than that of the Ordos Basin (e.g., Zhu and Shi wedge-shaped Alashan Block was trapped between the 2011; Wang et al. 2014c). The velocity differences Siberian Plate, the Qaidam Block, the Tarim Block and the between the Shanxi Block and the Ordos Basin suggest that Ordos Block (Zhang et al. 2007). Therefore, the north- the northeastward motion of the Ordos Basin, which was western boundary (L6) is kept ﬁxed as the Alashan Block pushed by the Tibet Plateau, was restricted by the Shanxi 34°N 38°N 42°N 34°N 38°N 42°N 34°N 38°N 42°N 34°N 38°N 42°N Pet. Sci. (2017) 14:1–23 9 Table 2 Shortening rates of Proﬁles Shortening rate, % Average shortening rate, % different proﬁles in the mid- south section of the western Tianshuibao (A–A ) 30.4–50.6 42.4 margin (L11) in the Ordos Basin Shibangou (B–B ) 32.8 32.8 (Source: Feng et al. 2013) Shajingzi (C–C ) 16.5–38.6 29.3 Pengyang (D–D ) 12.9–17.9 15.4 Block due to the westward subduction of the Paciﬁc Plate Cenozoic era (Yuan et al. 2007; Li and Li 2008). When the in the Cenozoic (Hou et al. 2010). Accordingly, the eastern western boundary of the basin is taken into consideration, edge of the basin is kept ﬁxed for the Cenozoic era (L8) as the shortening rate of the northern section (Tianshuibao (Fig. 6b). On the basis of massive fault-striation data, it can Proﬁle: 30.4%–50.6%) is greater than that of the southern be interpreted that the southern margin, namely the Weihe one (Pengyang Proﬁle: 12.9%–17.9%) (Feng et al. 2013) Graben, turned into a sinistral shearing tensile boundary (Table 2; Fig. 7), a compressive traction with a uniform (e.g., Mercier et al. 2013; Rao et al. 2014), and hence, a direction and a gradient magnitude from 80 to 55 MPa is constant left-lateral shearing stress of 30 MPa is set on the applied on the western boundary (L11), whereas a com- southeastern border of the basin (L9) (Fig. 6b). pressive traction with a constant magnitude of 80 MPa is Due to the impact of collision between the Indian Plate applied on the southwestern margin (L10) (Fig. 6b). and the Eurasian Plate, the Liupanshan Thrust-Fold Belt (namely the Liupan Mountain in Fig. 2) was developed 4.3 Theory of fracture prediction along the southwestern margin of the Ordos Basin, which resulted in the transformation of the west-southwestern Lagrangian formulations are used in ANSYS to simulate the margin into a strongly compressive boundary during the three-dimensional, plane strain deformation, applying 5 10 20 km IV Tianshuibao Profile Lingwu A A’ K T T P Qingtongxia P C T C O C O Zhongwei II A A’ J J P T T C O III A A’ Shortening rate: 30.41 %-50.62 % Pengyang Profile B’ Haiyuan D D’ 6 Huanxian K K T T T C C C O O O Є Є D D’ Guyuan C C Boundary C C O Zhenyuan Fault City Shortening rate: 12.87 %-17.91 % Fig. 7 Maps of tectonic units, two relevant proﬁles and their corresponding balanced sections in the mid-south section along the western margin (L11) of the Ordos Basin (1 Western Liupanshan Fault, 2 Eastern Liupanshan Fault, 3 Haiyuan Fault, 4 Qingshuihe Fault, 5 Yantongshan- Yaoshan Fault, 6 Qingtongxia-Guyuan Fault, 7 Hui’anbao-Shajingzi Fault) (modiﬁed after Feng et al. 2013). Area: I Tianhuan Depression, II Thrust Belt of Western Margin, III Qilianshan Orogen, IV Alashan Block. Age: O Ordovician, C Carboniferous, P Permian, T Triassic, J Jurassic, K Cretaceous C’ D’ D 10 Pet. Sci. (2017) 14:1–23 8-node isotropic elements to represent each lithological mechanism, is an effective criterion to predict the develop- layer. The mechanical behavior in the elastic domain is ment and the distribution of tensile fractures; however, this dominated by the generalized Hook’s law. As the Yanchang criterion, which in nature is equivalent with the theory of Formation is generally less than 3000 m in depth where the maximum tensional stress, is only suitable for the tensile plastic deformation is not obvious and the structural fractures fractures (Grifﬁth 1920). Although tensile fractures are found in the Chang 7 and 7 members are chieﬂy shearing frac- in some areas of the Ordos Basin, they are limited to the 1 2 tures based on ﬁeld measurements and core observations contact surfaces of sandstone and mudstone layers, and more (Fig. 8), the mechanical behavior follows the elastic model, than 95% of structural fractures in the Longdong area are which is described by the generalized Hook’s law. shearing fractures, whose rupture is controlled by the Mohr– Various methods for fracture prediction have been pro- Coulomb failure criterion (Xie et al. 2008). Therefore, only posed in previous literature, such as the conventional logging Mohr–Coulomb failure criterion is taken into consideration in method, the stress ﬁeld method, the principle curvature this study, which follows the equation (Coulomb 1776): method, the geostatistical method, etc. (e.g., Savage et al. ½ s ¼ C þ r tan u ð2Þ 2010; Zahm et al. 2010; Jiu et al. 2013). The two-factor where [s] represents the critical shearing stress, C repre- method, involving the rupture value and the strain energy sents the cohesion, r represents the stress normal to the density, is used in this paper to predict the distribution of n shearing fractures and u represents the internal friction structural fractures in the Ordos Basin (Ding et al. 1998). angle (Table 1). Shearing fracture is triggered once the shearing stress exceeds the critical shearing stress ([s]) in 4.3.1 Rupture value Eq. (2). r can be obtained via the maximum principal stress (r ) and the minimum principal stress (r ) according Tensile fractures and shearing fractures conform to different 1 3 to Wang et al. (2004): criteria. Grifﬁth’s criterion, which is derived from the micro- Fig. 8 Photographs of structural fractures in outcrops and cores of the Ordos Basin. a Conjugate fractures indicate the maximum principal compressive stress of WNW orientation in the Late Mesozoic; b conjugate fractures indicate the maximum principal compressive stress of NE orientation in the Cenozoic; c near-vertical fracture plane in a core from the Longdong area; and d moderate-dipping fracture plane in a core from the Longdong area 123 Pet. Sci. (2017) 14:1–23 11 105°E 109°E 113°E 105°E 109°E 113°E (a) (b) Late Mesozoic Cenozoic 4000 m 4000 m Hangjinqi Dongsheng Hangjinqi Dongsheng Wushenqi Wushenqi Jingbian Jingbian Dingbian Dingbian Yanchang Wuqi Wuqi Yanchang Qingyang Qingyang Zhenyuan Zhenyuan To n g ch u an Tongchuan Linfen Linfen Xi’an Xi’an 040 80 km 040 80 km 105°E 109°E 113°E 105°E 109°E 113°E Fig. 9 Displacement ﬁelds of the two models for the Ordos Basin in the Late Mesozoic and the Cenozoic eras. Thin black arrows indicate the calculated displacement directions and their lengths indicate the magnitude of displacement. Thick black arrows outside the basin denote the rotation of the basin during these periods. Black crosses represent the ﬁxed boundaries in different models 4.3.2 Strain energy density r ¼ðÞ r þ r =2 ðÞ r r sin u=2 ð3Þ n 1 3 1 3 The shearing stress (s ) can also be obtained via the two It is generally accepted that the rocks with relatively high principal stresses according to Wang et al. (2004): strain energy density are more likely to develop structural s ¼ðÞ r r cos u=2 ð4Þ fractures than those with a lower one. The strain energy n 1 3 density, namely the strain energy per unit volume, is Following the Mohr–Coulomb failure criterion, the rock described as follows (Prince and Rhodes 1966): will break when the shearing stress is equal or greater than 2 2 2 U ¼ r þ r þ r 2vðÞ r r þ r r þ r r the critical shearing stress in Eq. (2), so the rupture value X Y Y Z Z X X Y Z ð6Þ 2 2 2 (I) is introduced in order to measure the probability of þ 21ðÞ þ v s þ s þ s =2E XY YZ ZX rock’s rupture according to Ding et al. (1998): where U is the strain energy density, v is Poisson’s ratio, I ¼ s =½ s ð5Þ r , r and r are the normal stress components in x, y and X Y Z The possibility of rock’s failure is very small when the z directions, respectively, and s , s and s are the XY YZ ZX rupture value (I) is far smaller than 1, whereas the possi- shearing stress components in the corresponding directions. bility is relatively larger when the rupture value (I) exceeds Strain energy density (U) could be utilized to indicate the 1. The fracture density (f) and the rupture value (I) may fracture distribution. have a positive correlation, so the rupture value (I)isan Rupture value (I) stands for the possibility of rock effective index for fracture prediction through empirical failure, whereas the strain energy density (U) stands for the formulas established between them. developing ability of structural fractures. In this study, 34°N 38°N 42°N 34°N 38°N 42°N 34°N 38°N 42°N 34°N 38°N 42°N 12 Pet. Sci. (2017) 14:1–23 105°E 109°E 113°E 105°E 109°E 113°E (a) (b) Late Mesozoic Cenozoic Hangjinqi Dongsheng Dongsheng Hangjinqi 31.5-50.9 MPa (Zhang et al., 2014) 84.2-85.6 MPa Wushenqi (Zhou et al., 2009b) Wushenqi 84.4-97.9 MPa 89.4-103.9 MPa (Zhou et al., 2009b) Jingbian (Zhou et al., 2009b) Jingbian 22.3-55.1 MPa Dingbian Dingbian (Zhou et al., 2009a) 79.1-90.0 MPa 30.0-50.0 MPa 40.0-70.0 MPa (Zhou et al., 2009b) (Wang et al., 2014c) (Wang et al., 2014c) Wuqi Wuqi Yanchang Yanchang 57.4-58.5 MPa (Zhou et al., 2009a) Qingyang Qingyang Zhenyuan Zhenyuan Tongchuan Tongchuan Xi’an Xi’an 040 80 km 040 80 km 105°E 109°E 113°E 105°E 109°E 113°E 0 13.3 26.7 40.0 53.3 66.7 80.0 93.3 107.0 120.0 MPa Fig. 10 Maximum principal stress distribution of the two models in a the Late Mesozoic and b the Cenozoic eras. Red frames denote the areas where the calculated stress magnitudes match well with the Acoustic Emission paleo-stress magnitudes in earlier literature (Zhou et al. 2009a,b; Wang et al. 2014a; Zhang et al. 2014) syntheses of the rupture value and the strain energy density, necessary to verify the correctness of the two models namely the two-factor method, are applied, in order to proposed in this paper, including the Late Mesozoic and build ﬁnite element models for fracture prediction in the the Cenozoic ones, by comparing the results of ﬁnite ele- Ordos Basin (Ding et al. 1998). ment modeling with earlier published data. The calculated displacement directions reveal that the relative rotation directions in these periods are (1) anti- 5 Results and analyses clockwise from the Early Jurassic to the Cretaceous and (2) clockwise in the Cenozoic era (Fig. 9). These results are in Because the orientation and the distribution of structural good agreement with earlier ﬁndings (e.g., Pei et al. 2011; fractures are the key elements in fracture prediction, the Li et al. 2014; Yang et al. 2014). fracture orientation and the estimated density have been Acoustic Emission (AE) is an important technique in calculated with the ﬁnite element modeling and will be rock mechanics and experimental seismology, which can compared with the observed data in outcrops and cores. offer rock mechanical parameters, such as the maximum With the two-factor method, modeling results, including principal stress magnitudes generated in the geological the principal compressive stress orientations, the rupture history. The maximum principal stress magnitudes of the values, the strain energy density and the fracture density, Late Mesozoic era after pore-pressure correction range are presented as maps, which can imply the relative from 40.0 to 103.9 MPa in the Yanhewan, the Dingbian, degrees of fracture development in the Longdong area. the Dongsheng areas, etc. (Fig. 10a). The Cenozoic stress magnitudes remain in a limited range of 22.3–58.5 MPa 5.1 Validity of models within the Wuqi-Yanhewan, the Zhenyuan, the Wushengqi areas, etc. (Zhou et al. 2009a, b; Wang et al. 2014a; Zhang Since reliable numerical models are the basis of further et al. 2014) (Fig. 10b). The calculated maximum principal study on the fracture prediction in the Longdong area, it is stress magnitudes in the Late Mesozoic and the Cenozoic 34°N 38°N 42°N 34°N 38°N 42°N 34°N 38°N 42°N 34°N 38°N 42°N Pet. Sci. (2017) 14:1–23 13 105°E 109°E 113°E 105°E 109°E 113°E (a) (b) Late Mesozoic Cenozoic Hangjinqi Dongsheng Hangjinqi Dongsheng Wushenqi Wushenqi Dingbian Jingbian Jingbian Dingbian Yanchang Wuqi Yanchang Wuqi Zhenyuan Qingyang Qingyang Zhenyuan Tongchuan Ruishuihe Tongchuan Ruishuihe Xi’an Xi’an 040 80 km 040 80 km 105°E 109°E 113°E 105°E 109°E 113°E 1 2 3 4 5 6 7 8 9 10 11 12 Fig. 11 Maximum principal compressive stress trajectory maps of the two models for the Ordos Basin in the a Late Mesozoic and b Cenozoic eras. Green and red arrows represent the two major orientations of horizontal maximum principal stress (S ) through conjugate joint Hmax measurements. Short black bars indicate the calculated S , and bars in other colors represent the observed S in previous literature. 1 Late Hmax Hmax Mesozoic S from Wan (1994), 2 Late Mesozoic S from Hou et al. (2010), 3 Late Mesozoic S from Sun et al. (2014), 4 Late Hmax Hmax Hmax Mesozoic S from Zhou et al. (2009b), 5 Late Mesozoic S deduced from conjugate joints in our ﬁeld measurements, 6 Cenozoic S Hmax Hmax Hmax from Wang et al. (2008), 7 Cenozoic S from Xie et al. (2011), 8 Cenozoic S from Sun et al. (2014), 9 Cenozoic S from Yang et al. Hmax Hmax Hmax (2014), 10 Cenozoic S from Zhou et al. (2009a), 11 Cenozoic S deduced from conjugate joints in our ﬁeld measurements, 12 City Hmax Hmax are in agreement with the range of stress magnitudes compressive stress and the measured ones, including the measured by AE technology (Fig. 10). The above-men- stress orientations in previous literature (e.g., Wan 1994; tioned evidence strengthens the validity of our calculated Hou et al. 2010; Sun et al. 2014) and the measured data in results in the models. the present study, are in general less than 5, proving the In addition, earlier published stress orientation data reliability of the Late Mesozoic and Cenozoic models (Wan 1994; Hou et al. 2010; Sun et al. 2014) are also used (Fig. 11). as evidence to substantiate our models (Fig. 11a). These Evidence including the rotation directions, the measured stress orientation data suggest that the dominant orientation maximum principal stress magnitudes and the previous of maximum principal compressive stress in the Late stress data is gathered to prove the authenticity of the two Mesozoic is WNW. Current stress ﬁeld data can also be stress ﬁelds in the Late Mesozoic–Cenozoic models, and it utilized to interpret the Cenozoic stress ﬁelds because the is found that the calculated results are reliable. Despite basin has been stable during this period (Wang et al. 2008; slight differences between the calculated and observed Xie et al. 2011; Sun et al. 2014; Yang et al. 2014). Based maximum principal compressive stress, the modeling on the borehole collapse and multiple strain analyses in the results of the Late Mesozoic stress ﬁelds indicate that the Yanhewan area, it can be inferred that the dominant ori- orientation of the maximum principal compressive stress in entation of maximum principle compressive stress in the the Ordos Basin is WNW, whereas in the Cenozoic model, Cenozoic is NE (Zhou et al. 2009a). All these orientations the orientation is NE. Based on the above-mentioned are presented in the stereonets (Fig. 11). The differences proofs, the validity of the two models in the Late Mesozoic between the calculated orientations of maximum and the Cenozoic can be corroborated. 34°N 38°N 42°N 34°N 38°N 42°N 34°N 38°N 42°N 34°N 38°N 42°N 14 Pet. Sci. (2017) 14:1–23 0 50 100 200 km Cenozoic Late Mesozoic (a) (b) B504 B504 Huanxian B286 L124 Huanxian L124 B286 B478 L89 Huachi Huachi B270 B270 L179 C77 X251 Z17 Z17 Ze130 X251 Laocheng Laocheng Qingcheng Qingcheng X266 Z166 X266 Ze265 Ze265 Z197 X46 Z197 X46 N68 N68 Heshui Qingyang Qingyang Heshui X93 Ningxian Ningxian Zhengning X66 Zhengning Fig. 12 Calculated maximum principal compressive stress orientations in a the Late Mesozoic and b the Cenozoic within the Longdong area are compared with the observed fracture orientations in 19 wells. The observed orientations are obtained from imaging logging (FMI technology) data, which are shown as rose diagrams in the ﬁgures. The green rose diagrams denote the structural fractures in NW–EW trends, whereas the red ones denote those in NNE–ENE trends 5.2 Maximum principal stress orientations utilized to indicate the stress orientations in the Late Mesozoic–Cenozoic. From numerical modeling, the ori- Tectonic events of different episodes have distinct effects entations of calculated maximum compressive stress in the on the principal stress orientations in the Ordos Basin. Late Mesozoic are mainly WNW, while those in the Since there is little difference between the Chang 7 and 7 Cenozoic are mainly NE (Zhao et al. 2013, 2016) (Fig. 12). 1 2 members except for lithology and layer thickness, the In outcrops and cores, the observed fractures developed in pattern of principal stress orientation during the same the Late Mesozoic are chieﬂy in NW–EW trends and those period is similar in each layer of the Longdong area. Thus, in the Cenozoic are chieﬂy in NNE–ENE trends (Fig. 8). the Chang 7 member is taken as an example to demon- Our ﬁeld measurements also corroborate that the ENE- strate the distribution of maximum principal compressive trending structural fractures developed later than the NW- stress in the study area (Fig. 12). trending ones. Therefore, it can be concluded that the NW On the basis of paleo-magnetic evidence in earlier to EW fractures were developed in a Late Mesozoic stress studies, although the Ordos Basin experienced rotation in ﬁeld, whereas the NNE to ENE ones were developed in a different directions from the Late Mesozoic to the Ceno- Cenozoic stress ﬁeld. Despite tiny differences between the zoic, the rotation angle of the basin is less than 5 in the calculated and the observed data, in general, modeling Late Mesozoic–Cenozoic eras (e.g., Huang et al. 2005). results ﬁt well with the dominant orientations of observed Therefore, the present stress data, including fracture trends fractures which are obtained from the FMI technology and Formation Microscanner Image (FMI) data, can also be (Fig. 12). 123 Pet. Sci. (2017) 14:1–23 15 Structural fractures in the Ordos Basin were developed d). The distribution of sand bodies and the thickness of in multiple orientations under different stress ﬁelds, pri- sandstone layers have a distinct impact on the distribution marily in the Late Mesozoic and the Cenozoic episodes, of rupture values within the Longdong area. Both in the and this intersection pattern will contribute to wider Chang 7 and 7 members, the rupture values are rela- 1 2 opening and better connectivity of the fractures. The tively higher where sand bodies are developed and the formed fracture networks provide a path for ﬂuid trans- thickness of sandstone layers is relatively larger, due to mission and enhance the permeability, which will have the brittleness of sandstones (Fig. 4). The regional stress notably improved the fractured tight-oil reservoirs in the ﬁelds during different periods also inﬂuence the rupture Ordos Basin (e.g., Izadi and Elsworth 2014). values, resulting in the Cenozoic rupture values being smaller than the Late Mesozoic ones. However, the 5.3 Rupture values inﬂuence of regional stress ﬁelds is not as remarkable as that of lithology, because regional stress ﬁelds determine Since the rupture value is an important parameter to indi- only the magnitudes, not the distribution of rupture values cate the fracture development in the study area, comparison in the Chang 7 and 7 members within the study area 1 2 between the calculated rupture values and the observed (Fig. 13). core fracture density is informative to help analyze the reliability of the models (Figs. 13, 14). 5.4 Strain energy density In the maps of rupture values in the Chang 7 member during the Late Mesozoic–Cenozoic era, the highest Because rocks with higher strain energy density are more rupture values are situated in the east and center of the likely to form structural fractures than those with a lower study area, mainly concentrated in the Qingyang, the one, the strain energy density can be used as another Laocheng and the Zhengning areas (Fig. 13a, b), while parameter to predict the fracture density. the highest rupture values in the Chang 7 member are Similar to the rupture value, there is obvious positive chieﬂy situated in the mid-southern area, particularly in correlation between the strain energy density and the the Qingyang-Heshui and the Ningxian areas (Fig. 13c, thickness of sandstone layers. The strain energy density is 0 50 100 200 km Chang 7 1 in Late Mesozoic Chang 7 1 in Cenozoic N N (a) (b) Huanxian Huanxian Huachi Huachi Laocheng Qingcheng Laocheng Qingcheng Heshui Qingyang Heshui Qingyang Ningxian Ningxian Zhengning Zhengning 0.95 0.98 1.01 1.04 1.07 1.10 1.13 1.16 1.19 1.22 0.95 0.98 1.01 1.04 1.07 1.10 1.13 1.16 1.19 1.22 Chang 7 2 in Late Mesozoic Chang 7 2 in Cenozoic N N (c) (d) Huanxian Huachi Huanxian Huachi Laocheng Laocheng Qingcheng Qingcheng Heshui Heshui Qingyang Qingyang Ningxian Ningxian Zhengning Zhengning 0.95 0.98 1.01 1.04 1.07 1.10 1.13 1.16 1.19 1.22 0.95 0.98 1.01 1.04 1.07 1.10 1.13 1.16 1.19 1.22 Fig. 13 Distribution of rupture value of the Late Mesozoic and the Cenozoic in the Chang 7 and 7 members within the Longdong area. 1 2 a Rupture values of the Late Mesozoic in the Chang 7 member, b rupture values of the Cenozoic in the Chang 7 member, c rupture values of 1 1 the Late Mesozoic in the Chang 7 member and d rupture values of the Cenozoic in the Chang 7 member 2 2 123 16 Pet. Sci. (2017) 14:1–23 02 50 100 00 km Chang 7 1 in Late Mesozoic Chang 7 1 in Cenozoic (a) (b) N N Huanxian Huanxian Huachi Huachi Laocheng Qingcheng Qingcheng Laocheng Heshui Heshui Qingyang Qingyang Ningxian Ningxian Zhengning Zhengning 7.0 7.3 7.6 7.9 8.2 8.5 8.8 9.1 9.4 9.7 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5 5.7 5.9 Chang 7 2 in Late Mesozoic Chang 7 2 in Cenozoic (c) (d) N N Huanxian Huanxian Huachi Huachi Laocheng Laocheng Qingcheng Qingcheng Heshui Heshui Qingyang Qingyang Ningxian Ningxian Zhengning Zhengning 7.0 7.3 7.6 7.9 8.2 8.5 8.8 9.1 9.4 9.7 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5 5.7 5.9 4 3 Fig. 14 Distribution of strain energy density (10 J/m ) of the Late Mesozoic and the Cenozoic in Chang 7 and 7 members within the 1 2 Longdong area. a Strain energy density of the Late Mesozoic in the Chang 7 member, b strain energy density of the Cenozoic in the Chang 7 1 1 member, c strain energy density of the Late Mesozoic in the Chang 7 member and d strain energy density of the Cenozoic in the Chang 7 2 2 member Table 3 Curve-ﬁtting Layers Curve-ﬁtting relations Correlation coefﬁcient relationships of the measured 2 2 fracture densities, the calculated Chang 7 D = 3.493 I - 0.049 U - 6.241 I ? 0.695 U ? 0.270 0.947 1 M rupture values and the strain 2 2 D = 3.581 I - 0.123 U - 7.105 I ? 3.240 U ? 1.054 0.904 energy densities of Chang 7 2 2 Chang 7 D = 48.429 I - 0.039 U - 100.308 I ? 0.734 U ? 48.585 0.871 2 M and 7 members in the 2 2 Longdong area D = 22.944 I ? 0.450 U - 46.876 I - 4.094 U ? 33.241 0.941 -1 -1 D (m ) and D (m ) represent the measured fracture densities in cores of the Late Mesozoic and the M C Cenozoic periods, respectively. I and U denote the calculated rupture values and the strain energy densities 4 3 (10 J/m ), respectively higher where the sand bodies are developed as a whole 5.5 Predicted fracture distribution (Figs. 4, 14). Although the Cenozoic stress ﬁeld of the Ordos Basin is strikingly different from the Late Mesozoic In order to predict the fracture distribution in the Yanchang Formation within the Longdong area, connection between one, the impact of regional stress is mainly limited to the magnitudes, not the distribution of strain energy density in the calculated and the measured fracture density in cores the Longdong area. The distribution of strain energy den- must be established to study their relationship. In this sity in the Late Mesozoic and the Cenozoic periods is paper, the two-factor method is utilized to compare the similar, but the Late Mesozoic strain energy density is calculated data (including the rupture value and the strain larger than the Cenozoic one both in the Chang 7 and 7 energy density) and the measured fracture density (Ding 1 2 members, implying that the strain energy density is more et al. 1998). Since structural fractures in the Ordos Basin were chieﬂy developed during two stages of stress ﬁelds, inﬂuenced by the movement in the Late Mesozoic than that in the Cenozoic (Fig. 14). namely the Late Mesozoic and the Cenozoic ones, two 123 Pet. Sci. (2017) 14:1–23 17 Table 4 Overview of predicted and measured fracture densities in the Chang 7 member in the Longdong area Well Measured Predicted Absolute Relative Well Measured Predicted Absolute Relative -1 -1 -1 -1 -1 -1 name density, m density, m error, m error, % name density, m density, m error, m error, % B117 0.020 0.045 0.025 122 W98 0.000 0.000 0.000 – B146 0.000 0.005 0.005 – X140 0.000 0.000 0.000 – B170 0.000 0.008 0.008 – X195 0.028 0.015 0.013 46 B456 0.000 0.012 0.012 – X233 0.000 0.000 0.000 – B478 0.059 0.085 0.026 44 X259 0.000 0.027 0.027 – Ban12 0.320 0.305 0.015 5 X261 0.000 0.005 0.005 – C87 0.070 0.058 0.012 18 X263 0.000 0.000 0.000 – Hua56 0.000 0.007 0.007 – X67 0.015 0.008 0.007 47 L189 0.068 0.069 0.001 2 X73 0.026 0.057 0.031 118 L47 0.018 0.042 0.023 128 Y433 0.085 0.052 0.033 39 L79 0.033 0.023 0.010 29 Z124 0.000 0.013 0.013 – L96 0.000 0.008 0.008 – Z148 0.000 0.020 0.020 – M28 0.000 0.005 0.005 – Z15 0.068 0.099 0.031 46 M40 0.036 0.022 0.014 39 Z172 0.038 0.048 0.010 26 N43 0.062 0.090 0.029 46 Z186 0.344 0.372 0.028 8 N51 0.137 0.110 0.027 20 Z200 0.122 0.112 0.009 8 N57 0.055 0.080 0.025 45 Z230 0.046 0.054 0.009 19 N75 0.037 0.047 0.010 27 Z233 0.080 0.084 0.004 5 N76 0.155 0.155 0.000 0 Z24 0.061 0.086 0.025 41 N78 0.210 0.176 0.033 16 Z47 0.199 0.132 0.067 34 N81 0.057 0.084 0.027 47 Z57 0.151 0.121 0.030 20 S142 0.164 0.136 0.028 17 Z78 0.316 0.231 0.085 27 S160 0.000 0.000 0.000 – Z79 0.071 0.036 0.035 49 T15 0.113 0.097 0.016 14 Z87 0.201 0.234 0.033 16 T2 0.055 0.063 0.008 14 Ze220 0.050 0.026 0.024 48 W47 0.053 0.079 0.026 49 Ze97 0.000 0.000 0.000 – W67 0.000 0.024 0.024 – Zeg70 0.180 0.132 0.048 27 -1 ‘‘–’’ means that the relative errors do not exist in these wells because the corresponding measured fracture densities are 0 m , and large errors, including absolute and relative errors, are denoted in bold type in this table episodes of fractures should be ﬁtted separately and then be D D p m e ¼ 100% ð8Þ added up by weight. By multiple regression analyses, bi-quadratic rela- D denotes the absolute error and e denotes the relative tionships between the rupture values, the strain energy error. D and D represent the predicted and the measured P M density and the measured fracture density in the Chang fracture densities, respectively. Generally, when e is less 7 and 7 members of different episodes have been built 1 2 than 50%, we can consider that the predicted data match and the empirical formulas are shown in Table 3.Cor- the measured ones and the modeling results are reliable to a relation coefﬁcients in all curve-ﬁtting relationships are certain extent. larger than 0.87, which means that there is a signiﬁcant The differences between the measured and the predicted correlation between the calculated and the measured fracture densities are shown in Tables 4 and 5. For most of data. the wells in the Chang 7 member, the predicted and the To further illustrate the reliability of our models, error measured data match quite well. In the 54 measured wells, analyses are carried out as follows. Both the absolute error -1 only 2 wells exceed 0.05 m in absolute errors and 3 and the relative error were applied to reﬂect the accuracy of wells exceed 50% in relative errors (Table 4). The differ- fracture prediction. Absolute error is calculated by: ences between them may be caused by the stress concen- D ¼ D D ð7Þ p m tration in some areas, such as Well Z47 and Z78, where numerous fractures are found. As for the Chang 7 And relative error can be described as follows: 123 18 Pet. Sci. (2017) 14:1–23 Table 5 Overview of predicted and measured fracture densities in the Chang 7 member in the Longdong area Well Measured Predicted Absolute Relative Well Measured Predicted Absolute Relative -1 -1 -1 -1 -1 -1 name density, m density, m error, m error, % name density, m density, m error, m error, % B117 0.062 0.083 0.021 34 X233 0.000 0.049 0.049 – B146 0.000 0.033 0.033 – X263 0.000 0.004 0.004 – B36 0.000 0.000 0.000 – X270 0.000 0.041 0.041 – B401 0.000 0.026 0.026 – X65 0.053 0.040 0.013 25 B456 0.000 0.000 0.000 – X67 0.026 0.031 0.005 20 Ban12 0.918 0.903 0.016 2 X69 0.157 0.080 0.077 49 C87 0.043 0.036 0.007 15. Z172 0.025 0.022 0.003 12 Hua312 0.000 0.000 0.000 – Z230 0.064 0.037 0.027 42 L189 0.052 0.037 0.015 29 Z233 0.271 0.109 0.162 60 L47 0.000 0.009 0.009 – Z78 0.464 0.464 0.000 0 L79 0.047 0.039 0.008 16 Z79 0.072 0.000 0.072 100 L96 0.000 0.000 0.000 – Ze118 0.000 0.045 0.045 – N43 0.000 0.000 0.000 – Ze284 0.000 0.023 0.023 – N51 0.385 0.337 0.048 12 Ze298 0.000 0.097 0.097 – N55 0.000 0.009 0.009 – Ze362 0.000 0.131 0.131 – N57 0.137 0.100 0.036 27 Ze77 0.129 0.072 0.057 44 N75 0.036 0.149 0.113 317 Ze95 0.052 0.036 0.016 30 N76 0.093 0.038 0.055 59 Ze97 0.000 0.029 0.029 – N78 0.024 0.056 0.032 132 Zeg70 0.050 0.036 0.014 27 W98 0.000 0.044 0.044 – -1 ‘‘–’’ means that the relative errors do not exist in these wells because the corresponding measured fracture densities are 0 m , and large errors, including absolute and relative errors, are denoted in bold type in this table member, predicted data of only 8 wells in the 39 measured the Cenozoic ones are NNE–ENE (Fig. 12b) which are -1 wells are more than 0.05 m in absolute errors, and data consistent with the regional stress ﬁelds of the Ordos Basin of only 5 wells are more than 50% in relative errors in the corresponding periods (e.g., Zhang et al. 2003). In (Table 5). Most of these wells are with extraordinarily high the maps of predicted fracture density in different periods, fracture density, which results in large errors between the the average density of the Cenozoic fractures is larger than predicted and the measured fracture densities. Some large that of the Late Mesozoic ones (Fig. 15). By comparison errors may be caused by non-structural factors, such as between the distribution maps of predicted total fracture various sedimentary phenomena. Cross bedding and len- densities in the Chang 7 and 7 members within the study 1 2 ticular bedding appeared widely in Well Ze77, etc., which area (Fig. 16), the predicted fracture density in each may lead to the difference between the predicted and member is alike as a whole; however, their fracture dis- measured data. Despite these differences, the tendency of tributions are signiﬁcantly distinct. In the Chang 7 mem- predicted fracture distribution is still in accordance with the ber, the maximum fracture density is located in the center measured one. In short, the errors between the predicted and the east of the Longdong area (Fig. 16a), while in the and the measured fracture densities are within accept- Chang 7 member, the maximum density is situated in the able limits, implying that the modeling results are suit- southern-central section of the study area (Fig. 16b). able for the fracture prediction in the Yanchang Formation In addition, by comparing the predicted fracture density of the Ordos Basin. with the distribution of sand bodies, their similarity reveals that the lithology is a key factor in controlling the fracture distribution in the Ordos Basin. Structural fractures are 6 Discussion more likely to be developed in the sandstones rather than in the mudstones. Where thicker sandstone layers are devel- As is shown in the maps of maximum principal compres- oped, the fracture density is relatively higher than other sive stress orientations in the Chang 7 and 7 members in areas (Figs. 4, 16). However, there is still a difference 1 2 the Longdong area, the dominant orientations of the Late between the predicted fracture distribution and the outline Mesozoic fractures are NW–EW (Fig. 12a) and those of of sand bodies, indicating that the regional stress ﬁeld also 123 Pet. Sci. (2017) 14:1–23 19 0 50 100 200 km Chang 7 1 in Late Mesozoic Chang 7 1 in Cenozoic N N (a) (b) Huanxian Huanxian Huachi Huachi Laocheng Laocheng Qingcheng Qingcheng Heshui Heshui Qingyang Qingyang Ningxian Ningxian Zhengning Zhengning 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 Chang 7 2 in Late Mesozoic Chang 7 2 in Cenozoic (c) (d) N N Huanxian Huanxian Huachi Huachi Laocheng Qingcheng Laocheng Qingcheng Heshui Heshui Qingyang Qingyang Ningxian Ningxian Zhengning Zhengning 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 -1 Fig. 15 Distribution of predicted fracture density (m ) of the Late Mesozoic and the Cenozoic in the Chang 7 and 7 members within the 1 2 Longdong area. a Predicted fracture density of the Late Mesozoic in the Chang 7 member, b predicted fracture density of the Cenozoic in the Chang 7 member, c predicted fracture density of the Late Mesozoic in the Chang 7 member and d predicted fracture density of the Cenozoic in 1 2 the Chang 7 member plays a role in the fracture development, even though its act as a reference for future regional-scale petroleum inﬂuence is limited compared with the lithology and the exploration, while the method of fracture prediction, layer thickness. including the two-factor method and the empirical formulas In brief, the stress ﬁelds determine the overall fracture can be used at well scale. Structural fractures play an orientations, and the lithology distribution and the thick- important role in reconstructing the tight clastic reservoirs, ness of sandstone layers in the study area play a predom- especially in their permeability (Reda 2013). inant role in the distribution of predicted fracture density. The controlling factors of fracture development are Some potential factors which are not covered in these complex owing to the complicated geological background. numerical models may restrict the accuracy of predicted Fault systems can be a vital factor in developing fractures results, including: where tectonic movements are strong such as the Kuqa Depression of the northern Tarim Basin in the northwestern 1. The complicated heterogeneity of each layer; China (Ju et al. 2014b) and the Upper Rhine Graben in 2. The extreme stress in some areas; France and Germany (Johanna et al. 2015); ﬂow may 3. The interaction between the two episodes of structural notably promote fracture development where ﬂuid ﬂow or fractures; and lava ﬂow appears (e.g., Agosta et al. 2010). However, in 4. The inﬂuence of deep paleo-faults. the Ordos Basin, where the tectonic events are rather weak Since the modeling is on a relatively large-scale while the and the dips of the Mesozoic–Cenozoic strata are less than outline of sand bodies is depicted in considerable detail, the 3, the lithology and the layer thickness are the dominant modeling results, including the rupture values and the strain factors in governing the distribution of fracture density. energy density, can still be used to guide further exploration The relationship between the lithology and the fracture in spite of the four above-mentioned restrictions. Mean- density is still obscure, but it may be related to the dif- while, the qualitative fracture prediction obtained from the ference of rock physical parameters (Table 1) according to numerical modeling may also be applicable. These results previous study (e.g., Zeng et al. 2008). The different grain 123 20 Pet. Sci. (2017) 14:1–23 0 50 100 200 km Chang 7 1 Hua56 W98 (a) Y433 S142 L96 M28 B170 B146 Huanxian B478 L189 S160 L79 Huachi B456 W47 M40 L47 C87 B117 W67 X259 Z47 Z57 Z87 Ze97 Z78 X233 Z79 T2 Z15 Z186 Laocheng Z24 X261 X73 Z200 Ze220 T15 X263 Qingcheng Z148 Z230 X195 X140 Z172 Z233 Heshui Z124 Ban12 N76 Qingyang N43 N78 X67 N75 Ningxian N81 N57 Zeg70 N51 Zhengning 0.00 0.04 0.08 0.12 0.16 0.20 0.24 0.28 0.32 0.36 Chang 7 2 Hua312 B401 (b) W98 B36 L96 B146 Huanxian L189 Huachi L79 B456 C87 L47 Ze77 B117 Ze97 Z78 Laocheng X270 X233 Z79 Ze298 Qingcheng X263 Ze362 Ze95 Heshui Z230 Z172 Z233 Ze284 N76 Ban12 Qingyang Ze118 N43 N78 X67 N75 X69 Ningxian N57 X65 Zeg70 N51 N55 Zhengning 0.00 0.04 0.08 0.12 0.16 0.20 0.24 0.28 0.32 0.36 -1 Fig. 16 Distribution of predicted total fracture density (m )in a the Chang 7 and b Chang 7 members within the Longdong area. Black solid 1 2 dots represent the measured wells in the study area 2. Two episodes of structural fractures have been devel- sizes in various clastic rocks may be the micro-mechanism that causes the distribution of fracture density in the Ordos oped since the Late Triassic: The dominant orienta- tions of the Late Mesozoic fractures in the Yanchang Basin (Zhao et al. 2013; Ju et al. 2015). Formation are NW–EW, whereas those of the Ceno- zoic fractures are NNE–ENE, both of which are in agreement with the modeling results. 7 Conclusions 3. Structural fractures in the Ordos Basin are controlled by the regional stress ﬁelds, and the lithology and the The predicted fracture distribution provides a clear view of layer thickness have a signiﬁcant impact on the the fracture concentration and fracture development. Sev- distribution of structural fractures, because the stress eral primary conclusions can be drawn from the modeling distribution will be affected by the inhomogeneity of results: lithology and layer thickness. This conclusion is shown 1. A ﬁnite element modeling technique, applying the two- in the similarity between the maps of predicted fracture factor method, is suitable for the fracture prediction of density and observed sand bodies in the Yanchang the Ordos Basin, based on comparison between the Formation within the study area. calculated and the measured fracture densities of the 4. The average fracture density is close in the Chang 7 Chang 7 and 7 members in the Longdong area. and 7 members, but there are obvious differences in 1 2 2 123 Pet. Sci. (2017) 14:1–23 21 Faure M, Lin W, Chen Y. Is the Jurassic (Mesozoic) intraplate their fracture distributions. In the Chang 7 member, tectonics of North China due to westward indentation of North the maximum fracture density is concentrated in the China block? Terra Nova. 2012;24(6):456–66. doi:10.1111/ter. center and the east of the Longdong area, particularly in the Qingyang, the Laocheng and the Zhengning Feng JP, Ouyang ZY, Huang ZL. The application of balance -1 geological section technology in the mid-south section of the areas (up to 1.5 m ), while in the Chang 7 member, western margin of Ordos Basin. Geotecton Metallog. the maximum value is located in the central and 2013;37(3):393–7 (in Chinese). southern part of the area. Fournier M, Jolivet L, Davy P, et al. Back-arc extension and collision: 5. The modeling results and the predicted fracture density an experimental approach to the tectonics of Asia. Geophys J Int. 2004;157:871–89. doi:10.1111/j.1365-246X.2004.02223.x. can be utilized to guide future regional exploration, Gilder SA, Gomez J, Chen Y, et al. A new paleogeographic and the method of fracture prediction, namely the two- conﬁguration of the Eurasian landmass resolves a paleomagnetic factor method, can be referred for further study of the paradox of the Tarim Basin (China). Tectonics. tight-sand reservoirs. 2008;27(1):1256. doi:10.1029/2007TC002155. Glukhmanchuk ED, Vasilevskiy AN. 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Publisher: Springer Journals
Copyright: Copyright © 2017 by The Author(s)
Subject: Earth Sciences; Mineral Resources; Industrial Chemistry/Chemical Engineering; Industrial and Production Engineering; Energy Economics
ISSN: 1672-5107
eISSN: 1995-8226
DOI: 10.1007/s12182-016-0141-2
Publisher site: See Article on Publisher Site

Abstract

Pet. Sci. (2017) 14:1–23 DOI 10.1007/s12182-016-0141-2 ORIGINAL PAPER Fracture prediction in the tight-oil reservoirs of the Triassic Yanchang Formation in the Ordos Basin, northern China 1,2,3 1 Wen-Tao Zhao Gui-Ting Hou Received: 3 September 2015 / Published online: 12 January 2017 The Author(s) 2017. This article is published with open access at Springerlink.com Abstract It is important to predict the fracture distribution distribution in the study area, which further determine the in the tight reservoirs of the Ordos Basin because fracturing fracture distribution in the stable Ordos Basin. The pre- is very crucial for the reconstruction of the low-perme- dicted fracture density and the two-factor method can be ability reservoirs. Three-dimensional ﬁnite element models utilized to guide future exploration in the tight-sand are used to predict the fracture orientation and distribution reservoirs. of the Triassic Yanchang Formation in the Longdong area, southern Ordos Basin. The numerical modeling is based on Keywords Ordos Basin Yanchang Formation Fracture the distribution of sand bodies in the Chang 7 and 7 prediction Finite element modeling Two-factor method 1 2 members, and the different forces that have been exerted Tight-sand reservoirs along each boundary of the basin in the Late Mesozoic and the Cenozoic. The calculated results demonstrate that the fracture orientations in the Late Mesozoic and the Ceno- 1 Introduction zoic are NW–EW and NNE–ENE, respectively. In this paper, the two-factor method is applied to analyze the Unconventional oil and gas resources, such as tight gas, distribution of fracture density. The distribution maps of tight oil and shale oil, have been successfully developed predicted fracture density in the Chang 7 and 7 members commercially in the USA, Canada, Australia and some 1 2 are obtained, indicating that the tectonic movement in the other countries. The production of tight oil soared from 30 Late Mesozoic has a greater inﬂuence on the fracture million tons in 2011 to 96.9 million tons in 2012 in the development than that in the Cenozoic. The average frac- USA by using new unconventional technologies (Du et al. ture densities in the Chang 7 and 7 members are similar, 2014). In China, the Ordos Basin, the Junggar Basin, the 1 2 but there are differences in their distributions. Compared Songliao Basin, the Sichuan Basin, the Qaidam Basin, etc., with other geological elements, the lithology and the layer have abundant tight-oil resources with an output of 97 thickness are the primary factors that control the stress million tons, accounting for 22% of the nationwide total oil output (Jia et al. 2014). In the Ordos Basin, the tight-oil reservoirs in the Triassic Yanchang Formation have & Gui-Ting Hou become a major target of petroleum exploration and gthou@pku.edu.cn development in recent years (e.g., Guo et al. 2012; Yao The Key Laboratory of Orogenic Belts and Crustal Evolution et al. 2013). of Ministry of Education, School of Earth and Space Since tight-oil reservoirs in the Ordos Basin are of low- Sciences, Peking University, Beijing 100871, China -3 2 permeability (\2 9 10 lm ) and low-porosity (\10%) China Huaneng Clean Energy Research Institute, overall, fracturing is crucial for the reservoir reconstruc- Beijing 102209, China tion, even though the reservoirs are formed with compli- PetroChina Research Institute of Petroleum Exploration and cated mechanisms (e.g., Yao et al. 2013; Ezulike and Development, Beijing 100083, China Dehghanpour 2014). Therefore, it is important to predict the natural fracture distribution in reservoirs, including Edited by Jie Hao 123 2 Pet. Sci. (2017) 14:1–23 their orientation and density, for future exploration and Li 2009). Some small-scale paleo-faults exist, but no large development (e.g., Smart et al. 2009). Previous studies faults have been found within the basin (Wan and Zeng have focused on the geometrical or kinematic models, such 2002; Yang et al. 2013). as analyses of seismic techniques or of the layer curvature Contrary to the evolution of the eastern North China (e.g., Zahm et al. 2010; Pearce et al. 2011; Tong and Yin Craton including thickening, thinning and destruction, the 2011), and fracture prediction in the Ordos Basin has also Ordos Basin has evolved from three Mesoproterozoic been involved in some papers (e.g., Ju et al. 2014a). aulacogens to a Paleozoic–Mesozoic cratonic basin since However, since earlier fracture prediction was mainly the Middle Proterozoic (Menzies et al. 2007; Yang et al. carried out through layer curvature or two-dimensional 2013; Wang et al. 2014b). The interior part of the basin is (2D) models, which cannot meet the demands for the tight- characterized by horizontal or gently dipping strata (\3), oil study and exploration, it is necessary to build three- especially for the Mesozoic and Cenozoic strata, whereas dimensional (3D) mechanical models in order to achieve the strata along the margins have been subjected to sig- the accuracy needed for further research on the uncon- niﬁcant folding and faulting since the Late Triassic. ventional petroleum. Two distinct tectonic events took place from the Late Various factors, such as the proximity of faults, the Mesozoic to the Cenozoic, resulting in two different stress curvature of folds, the layer thickness and the lithology, are ﬁelds in these periods. In the Late Mesozoic, namely from deemed to control the fracture development in tight the Early Jurassic to the Late Cretaceous, the long-distance reservoirs (e.g., Ju et al. 2013), and the anisotropy or effect of subduction of the Izanagi Plate turned from north- heterogeneity should also be considered in the modeling northwestward to northwestward when the force arrived at (e.g., Glukhmanchuk and Vasilevskiy 2013). However, it is the Ordos Basin, resulting in the WNW-trending stress difﬁcult for 2D geomechanical models to fully consider all ﬁelds and the structural fractures in NW–EW trends (e.g., these factors, and the modeling results cannot be used Wan 1994; Hou et al. 2010; Sun et al. 2014; Zhao et al. successfully for exploration and production. Therefore, 3D 2016); while in the Cenozoic, the predominant tectonic models will be utilized in this paper, which take the event became the northeastward collision between the lithology, the thickness and the stress ﬁelds into Indian and the Eurasian Plate, which led to the NE-trending consideration. stress ﬁelds and the structural fractures in NNE–ENE The study area in this paper, namely the Longdong area, trends (e.g., Yuan et al. 2007; Wang et al. 2014b). Two is located in the southern Ordos Basin, where research on episodes of fractures are developed under distinct tectonic structural fractures in the tight reservoirs is still deﬁcient events, so the stress ﬁelds of different periods should be (e.g., Ren et al. 2014; Li et al. 2015). The structural frac- taken into consideration during the fracture prediction. tures in the Longdong area were mainly formed after the Late Triassic, as a result of multiple-stage tectonic events in the Late Mesozoic and the Cenozoic. These extensively 3 Fracture measurement developed fractures are mostly unﬁlled and effective, which noticeably improve the permeability of tight reser- The parameters of fracture characteristics are important in voirs in the Ordos Basin. the exploration and development of fractured tight reser- voirs. The fracture density is one of the signiﬁcant indicators to reﬂect the failure degree of rocks, which can be divided 2 Geological background into three types, including the linear density, the surface density and the bulk density of fractures. In this paper, the 5 2 The Ordos Basin, covering an area of 2.6 9 10 km ,is a surface density is utilized to describe the fracture distribution large N–S trending basin in the western North China in the Ordos Basin. The surface density is deﬁned as the ratio Craton, which is located between the Siberian Craton and between the cumulative fracture length and the cross-sec- the South China Craton (Hou et al. 2010) (Fig. 1). Three tional area of the matrix, which can better reﬂect the degrees orogenic belts have been developed along different of fracture development and be measured more effectively boundaries of the stable basin, including the Yinshan than others (Golf-Racht 1982). The fracture density from Mountain in the north, the Qinling Orogen in the south and core observations can be calculated as: P P the Liupanshan Mountain in the southwest (e.g., Nutman l l i i f ¼ ¼ ð1Þ et al. 2011) (Fig. 2). The basement of the basin is com- S 2pr þ 2pr L posed of Archean rocks with Proterozoic sedimentary where f is the fracture surface density, l is the length of cover. Although the margin underwent multiple tectonic each structural fracture, S is the surface area of the activities, the central part is still stable and is covered by observed core, r is the radius and L is the length of the core. shallow Paleozoic marine carbonate sediment (Kusky and 123 Qinling-Dabie Belt Qilianshan Orogen Himalayan Orogen Central Asian Pyeonrang YB CB Pet. Sci. (2017) 14:1–23 3 110°E 115°E 120°E 125°E 0 100 200 400 km City (b) 2.5-2.7 Ga Neo-Archean Greenstone Belt 2.5-2.7 Ga Neo-Archean Rocks Central 2.8-3.8 Ga Meso- and Eo-Archean Rocks Beijing WB Taiyuan EB Ordos Basin Qingdao Trans-North China Orogen 80°E 100°E 120°E 140°E (a) Yinan 40°N Linfen Xi’an 30°N Shanghai 500 1000 km 20°N Yangtze Craton 110°E 115°E 120°E 125°E Fig. 1 A geological summary of the Ordos Basin. a Tectonic framework of the major cratonic blocks in China (after Kusky 2011 and Santosh et al. 2012); b generalized geological and tectonic map of the North China Craton (after Zhao et al. 2005). TC Tarim Craton, NCC North China Craton, SCC South China Craton, YB Yangtze Block, CB Cathaysia Block, WB Western Block, EB Eastern Block In this paper, the Longdong area was selected as the 4 Modeling approach study area to carry out fracture measurements (Fig. 2). Sixty-six wells were chosen to study the distribution of Methods such as geological analysis, physical modeling structural fractures in the Chang 7 and 7 members and numerical simulation including the ﬁnite element 1 2 (Fig. 3). As shown in Fig. 4, the fracture density in the method (FEM) can be applied in the study of stress ﬁelds, tight-sandstone cores is relatively low (smaller than which is the foundation of fracture prediction. In this study, -1 0.5 m ), representing the general condition in the the ﬁnite element software ANSYS is used to calculate the Longdong area. The fracture density of the Chang 7 stress ﬁeld and predict the fracture distribution (Vela´zquez member is more concentrative than that of Chang 7 et al. 2009; Jarosinski et al. 2011). The basic concept of member, even though their average densities are similar FEM is that a geological body can be discretized into ﬁnite -1 in general (0.071 m for the Chang 7 member and continuous elements connected by nodes. The geometrical -1 0.081 m for the Chang 7 member). The difference of and mechanical parameters allocated on each element are fracture distribution between the Chang 7 and 7 consistent with the properties of real rocks. The continuous 1 2 members is obvious: The highest fracture density in the ﬁeld function of the geological area is ﬁrst transformed into Chang 7 member lies in the Laocheng and Qingyang linear functions at every node that contain displacement, areas, while that in the Chang 7 member lies in the stress and strain variables resulting from the applied forces Laocheng, the Qingyang and the Zhengning areas (Jiu et al. 2013), and then all these elements are used to (Fig. 4). obtain the stress distribution over the entire area. TC SCC NCC Asian Orogenic Inner Mongolia Suture Zone Orogenic Belt Belt Yinshan Block Imjngang Gyeonggi Yeongnam Su-Lu Orogen Su-Lu UHP Belt 35°N 40°N 35°N 40°N Liupan Mountain 4 Pet. Sci. (2017) 14:1–23 105°E 110°E 02 50 100 00 km Hetao Graben 40°N 40°N Alashan Block Yinchuan Wuqin Yulin Ordos Basin Dingbian Yanchang Qingyang Lanzhou Study area Linfen 35°N 35°N Xi'an Qinling Orogen Belt 105°E 110°E 1 2 3 4 5 6 7 8 Fig. 2 Tectonic framework of the Ordos Basin. The orange area denotes the Ordos Basin, and the light yellow ones denote the adjacent blocks. The light gray areas represent the graben system along the margins, and the dark gray ones represent the orogenic belts around the basin. The black frame denotes the study area (the Longdong area) (after Darby and Ritts 2002)(1 Boundary of orogenic belt, 2 Fault, 3 Normal fault, 4 Reversed fault, 5 Strike-slip fault, 6 Fold, 7 River, 8 City) 4.1 Geometrical model part. Therefore, the outline of the basin remained unchan- ged in these periods (Sun et al. 2014) (Fig. 5a). Since no The Ordos Basin is a near-rectangular basin in the western large faults or folds have been recorded inside the Ordos part of the North China Craton (Li and Li 2008; Tang et al. Basin, the sedimentary facies, the lithology and the distri- 2012) (Fig. 2). Although the Ordos Basin underwent multi- bution of sand bodies are the key factors to determine the stage tectonic movements in the Late Mesozoic–Cenozoic fracture development. eras, the deformation was conﬁned to the western margin In this paper, the Chang 7 and 7 members in the Tri- 1 2 and no signiﬁcant tectonic events occurred in the central assic Yanchang Formation, the major tight-oil members in Yinshan Orogen Belt Yinchuan Graben Weihe Graben Fenhe Graben Shanxi Block Yellow River Helan Mountain Lüliang Mountain Pet. Sci. (2017) 14:1–23 5 Lithology Sedimentary Epoch Formation Subsection Thickness, m column facies Anding Swamp 80-150 Middle Jurassic Zhiluo Fluvial 20-40 Lower Yan 1- Fluvial- Yan’an 250-300 Yan 10 Jurassic lacustrine- swamp Chang 1 0-245 120-160 Chang 2 Chang 3 100-170 Fluvial- Chang 4+5 lacustrine 90-130 Chang 6 180-200 Upper Yanchang Triassic Deep Chang 7 lacustrine Chang 8 Lacustrine 100-190 Chang 9 Chang 10 Fluvial 200-320 Conglomerate Sandstone Sandy mudstone Mudstone Oil shale Coal Fig. 3 Synthetical stratigraphic column and depositional environment of the Upper Triassic—Middle Jurassic in the Ordos Basin (after Duan et al. 2008) the Ordos Basin, are selected as the study strata, and the on the sandstone-mudstone ratio, it is assumed that the Longdong area is chosen to discuss the stress ﬁelds and the ratios between the sandstone and mudstone layers are fracture distribution (Figs. 2, 3, 5). Since the Yanchang 0.43–4.26 (average 1.27) in the Chang 7 member and Formation is characterized by strong heterogeneity with 0.54–9.00 (average 1.70) in the Chang 7 member (e.g., facies change, the simpliﬁed model with only one rock Guo et al. 2012; Li et al. 2015), and multiple-layer con- mechanical property is no longer suitable for the complex structions (four sandstone layers in Chang 7 member and interior of the Ordos Basin (Yang and Deng 2013). Based three sandstone layers in Chang 7 member) are applied in 123 6 Pet. Sci. (2017) 14:1–23 0 50 100 200 km 0 0 (a) (b) N N 0.085 0 B401 Hua56 Hua312 W98 0 W98 0.164 0 Huanxian 0 B36 Y433 L96 L96 B146 Huanxian B146 0.059 S142 0.068 M28 0.052 B170 B478 0.033 0.047 L189 Huachi L189 Huachi S160 L79 L79 0.053 0.036 B456 B456 0.018 0.129 0.070 0 0.062 0.020 0.043 0 W47 M40 L47 C87 L47 W67 C87 B117 0 Ze77 B117 0.464 0.201 0.151 0.316 0.199 X259 0.344 0.071 0.055 0.068 0.072 0 Z57 Z87 0 Ze97 0 Ze97 Z78 Z47 Z78 0.061 X270 X233 X233 Z79 T2 Laocheng 0 Z79 Z15 Ze298 0.026 0.122 Z186 0.113 0.050 Laocheng X261 Z24 X73 Qingcheng Z200 Qingcheng 0.052 Ze220 T15 X263 X263 0.046 0.064 Ze362 0.918 0.028 0.038 0 Ze95 0.025 0.271 Z148 Z230 Z230 0.320 0.080 X195 X140 Z172 0 Z172 Z233 Z233 0.155 Heshui Heshui Z124 Ze284 0.096 Ban12 Ban12 N76 0.210 N76 Qingyang 0.062 Qingyang 0 0.024 Ze118 0.015 0.037 0.026 0.036 0.157 N43 N78 N43 N78 X67 X67 N75 N75 0.180 0.385 0.057 0.055 0.137 0.050 0.137 Ningxian X69 0.053 N81N57 0 N57 Zeg70 Zeg70 N51 N51 Ningxian X65 N55 Zhengning Zhengning Chang 7 1 Chang 7 2 Sandstone thickness 0 5 10 15 20 m Fig. 4 Distribution of the measured fracture density and the sandstone thickness in a the Chang 7 and b Chang 7 members in the Longdong 1 2 area. The black solid dots denote the observed wells, while the hollow dots denote the cities. Measured fracture densities in the Chang 7 and 7 1 2 members are marked with red cylinders, and the contours represent the sandstone thickness in the study area the geometrical model to simulate the sandstone-mudstone consists of sedimentary cover, greenschist and granite, is interlayers (Fig. 5). To avoid the boundary effect, forces in 2750 kg/m (Hou et al. 2010; Wang et al. 2014b). Based on the models are set on the boundaries of the Ordos Basin, the velocities of P and S waves, the calculated average and the study area is nested inside the basin (Fig. 5). The Poisson’s ratio is 0.20 and the average Young’s modulus is sandstone layers, which are also the main layers in fracture 80 GPa for the whole basin (Liu et al. 2006). development, in the middle of each member, are selected to To subtly depict the distribution of structural fractures in display the modeling results in the following text, repre- the Longdong area, four more kinds of material elements senting the general situation of fracture development in the are involved in the 3D geometrical model, including the Chang 7 and 7 members. sandstones/mudstones of the Chang 7 member and the 1 2 1 sandstones/mudstones of the Chang 7 member. Tri-axial 4.2 Boundary conditions and modeling rock mechanical experiments were carried out by the Institute of Acoustics, Chinese Academy of Sciences, on 62 In order to predict the fracture distribution of the Ordos core samples collected from observed wells in the Long- Basin, it is assumed that the upper crustal thickness of the dong area (Fig. 4). In order to simulate the real conditions basin in the Late Mesozoic–Cenozoic era is 25 km (e.g., underground, in these experiments, conﬁning pressures Liu et al. 2006). The top of the model is set as a free corresponding to the original depth of the Yanchang For- surface, and the entire model is subjected to gravity load. mation are applied in the radial directions, and vertical The average density of the upper crust, which mainly pressures are applied in the axial directions of all samples. 123 Pet. Sci. (2017) 14:1–23 7 (a) (b) 20 km Longdong area Overlying layer Chang 7 1 Chang 7 2 Basement Sandstone layers Mudstone layers Nested model Fig. 5 Simpliﬁed geometrical model of the Longdong area (b) and its nested model (a) in the Ordos Basin Through statistical analysis and geological classiﬁcation, push from the northwestward subduction of the Izanagi ﬁve sets of rock mechanical properties, the average density, Plate in the Late Mesozoic (Zhang et al. 2007; Hou and Young’s modulus, Poisson’s ratio, internal friction angle Hari 2014). Hence, it is a compressive boundary with a and cohesion, are listed in Table 1 by layer and lithology. sinistral shearing component along the eastern edge of the Because the stress ﬁelds in the Late Mesozoic and basin. A deviatoric stress of an approximately 150 MPa the Cenozoic are strikingly different and both of them normal component with a 45 MPa shearing component is had a signiﬁcant effect on the fracture development in set along the eastern boundary (L2) (Fig. 6a). Based on the the Ordos Basin, the boundary conditions during these paleo-magnetic constraints, geological evidence and 40 39 two episodes along the basin need to be deﬁned (Zhao Ar/ Ar and U–Pb dating, it can be assumed that in the et al. 2016). southern part of the Ordos Basin, the Qinling Ocean ﬁnally As a result of intense compression from the Early closed during the Late Jurassic-Early Cretaceous period. Jurassic to the Middle Cretaceous, the Yinshan Orogen This indicates that the collision between the North China Belt was developed as thrust faults with dextral shearing Craton and the South China Craton continued up to the features in the Late Mesozoic (Darby and Ritts 2002; Cretaceous period (Huang et al. 2005; Liu et al. 2015). And Zhang et al. 2007; Faure et al. 2012). A uniform direction due to this collision, thrust faults with sinistral strike-slip and a constant magnitude of a 40 MPa normal component features were developed along the northern margin of the with a 10 MPa dextral shearing component are applied Qinling Orogen Belt (Malaspina et al. 2006; Yuan et al. along the northern side of the Ordos Basin (L1) (Fig. 6a). 2007). Therefore, a constant magnitude of normal stress The east-dipping thrusts, the NWW-dipping back-thrusts (60 MPa) with sinistral shearing stress (30 MPa) is applied and the associated folds developed along the Lu¨liang along the southern margin of the basin (L3) (Fig. 6a). In Mountain in the Jurassic show that the stress regime in the the western and southern margins, the long-distance effect eastern margin was related to the long-distance effect of the of collision from the Qiangtang Massif affected the Ordos Table 1 Rock mechanical parameters for the numerical modeling of the Chang 7 and 7 members in the Longdong area 1 2 Model position Lithology Density, g/cm Young’s modulus, GPa Poisson’s ratio Internal friction angle, Cohesion, MPa Chang 7 Sandstone 2.562 26.971 0.229 36.00 37.62 Chang 7 Mudstone 2.457 24.296 0.269 24.02 40.34 Chang 7 Sandstone 2.639 25.629 0.229 37.80 31.13 Chang 7 Mudstone 2.485 24.943 0.269 24.08 40.12 80 km Hangjinqi Ordos Basin Dingbian Wuqi Yanchang Qingyang Longdong area 8 Pet. Sci. (2017) 14:1–23 105°E 109°E 113°E 105°E 109°E 113°E (a) (b) Late Mesozoic Cenozoic 150 MPa 150 MPa L1 L7 Hangjinqi Hangjinqi Dongsheng Dongsheng L6 L12 Wushenqi Wushenqi L8 L2 Jingbian Jingbian Dingbian Dingbian Wuqi Wuqi Yanchang Yanchang L5 L11 Qingyang Qingyang Zhenyuan Zhenyuan Tongchuan Tongchuan L4 L9 L3 L10 Xi’an Xi’an 040 80 km 040 80 km 105°E 109°E 113°E 105°E 109°E 113°E Fig. 6 Boundary conditions of the Ordos Basin in a the Late Mesozoic and b the Cenozoic eras. Arrows indicate the tectonic forces and the length represents the stress magnitude. The crosses represent the ﬁxed boundaries in different models Basin (Zhang et al. 2007; Li and Li 2008), so a compres- was locked by the adjacent blocks in the Late Mesozoic sive traction with a uniform direction and a constant (Fig. 6a). magnitude of deviatoric stress of 30 MPa on the south- The stress ﬁeld in the Cenozoic era, which is regarded as western boundary (L4) and 75 MPa on the western a consequence of the Indo-Asian collision, is strikingly boundary (L5) are applied along the basin (Fig. 6a). On the different from that in the Late Mesozoic era (e.g., Darby basis of SHRIMP zircon U–Pb ages and other geochrono- and Ritts 2002; Bao et al. 2013) (Fig. 6b). During the logical data, it can be presumed that the closure of the Cenozoic, the extension along the margins of the Ordos Paleo-Asian Ocean ﬁnally took place after the Early Per- Basin triggered the formation of the Hetao, the Weibei and mian. Due to this episode of closure, the northward the Yinchuan Grabens, which in turn transposed reverse movement of the Alashan Block (Fig. 2) was arrested by faults to normal faults in the Helan Mountain and the the Siberian Craton in the Late Mesozoic (e.g., Zheng et al. Qinling Mountain (Rao et al. 2014). Therefore, a tensile 2014). The ﬁnal closure of the Qilian Ocean took place at traction with a uniform direction and a constant magnitude the end of the Ordovician, and after that, the Qaidam of 5 MPa is applied on the northern, the southern and the Block, which was adjacent to the Alashan Block, restricted northwestern margins, respectively (L7, L9 and L12) the southward movement of the Alashan Block (Song et al. (Fig. 6b). The subduction of northwestern Paciﬁc Plate 2013). The nonidentical apparent polar wandering paths of restricted the further eastward movement of the Ordos the Tarim Block and the Alashan Block up to the Jurassic Basin (Fournier et al. 2004; Schellart and Lister 2005). The period clearly indicates that the amalgamation of these two current GPS horizontal velocity ﬁeld map shows that the blocks might have occurred during the Jurassic (Gilder eastward velocity of the Shanxi Block (Fig. 2) is relatively et al. 2008). As a result of amalgamation in the Jurassic, the smaller than that of the Ordos Basin (e.g., Zhu and Shi wedge-shaped Alashan Block was trapped between the 2011; Wang et al. 2014c). The velocity differences Siberian Plate, the Qaidam Block, the Tarim Block and the between the Shanxi Block and the Ordos Basin suggest that Ordos Block (Zhang et al. 2007). Therefore, the north- the northeastward motion of the Ordos Basin, which was western boundary (L6) is kept ﬁxed as the Alashan Block pushed by the Tibet Plateau, was restricted by the Shanxi 34°N 38°N 42°N 34°N 38°N 42°N 34°N 38°N 42°N 34°N 38°N 42°N Pet. Sci. (2017) 14:1–23 9 Table 2 Shortening rates of Proﬁles Shortening rate, % Average shortening rate, % different proﬁles in the mid- south section of the western Tianshuibao (A–A ) 30.4–50.6 42.4 margin (L11) in the Ordos Basin Shibangou (B–B ) 32.8 32.8 (Source: Feng et al. 2013) Shajingzi (C–C ) 16.5–38.6 29.3 Pengyang (D–D ) 12.9–17.9 15.4 Block due to the westward subduction of the Paciﬁc Plate Cenozoic era (Yuan et al. 2007; Li and Li 2008). When the in the Cenozoic (Hou et al. 2010). Accordingly, the eastern western boundary of the basin is taken into consideration, edge of the basin is kept ﬁxed for the Cenozoic era (L8) as the shortening rate of the northern section (Tianshuibao (Fig. 6b). On the basis of massive fault-striation data, it can Proﬁle: 30.4%–50.6%) is greater than that of the southern be interpreted that the southern margin, namely the Weihe one (Pengyang Proﬁle: 12.9%–17.9%) (Feng et al. 2013) Graben, turned into a sinistral shearing tensile boundary (Table 2; Fig. 7), a compressive traction with a uniform (e.g., Mercier et al. 2013; Rao et al. 2014), and hence, a direction and a gradient magnitude from 80 to 55 MPa is constant left-lateral shearing stress of 30 MPa is set on the applied on the western boundary (L11), whereas a com- southeastern border of the basin (L9) (Fig. 6b). pressive traction with a constant magnitude of 80 MPa is Due to the impact of collision between the Indian Plate applied on the southwestern margin (L10) (Fig. 6b). and the Eurasian Plate, the Liupanshan Thrust-Fold Belt (namely the Liupan Mountain in Fig. 2) was developed 4.3 Theory of fracture prediction along the southwestern margin of the Ordos Basin, which resulted in the transformation of the west-southwestern Lagrangian formulations are used in ANSYS to simulate the margin into a strongly compressive boundary during the three-dimensional, plane strain deformation, applying 5 10 20 km IV Tianshuibao Profile Lingwu A A’ K T T P Qingtongxia P C T C O C O Zhongwei II A A’ J J P T T C O III A A’ Shortening rate: 30.41 %-50.62 % Pengyang Profile B’ Haiyuan D D’ 6 Huanxian K K T T T C C C O O O Є Є D D’ Guyuan C C Boundary C C O Zhenyuan Fault City Shortening rate: 12.87 %-17.91 % Fig. 7 Maps of tectonic units, two relevant proﬁles and their corresponding balanced sections in the mid-south section along the western margin (L11) of the Ordos Basin (1 Western Liupanshan Fault, 2 Eastern Liupanshan Fault, 3 Haiyuan Fault, 4 Qingshuihe Fault, 5 Yantongshan- Yaoshan Fault, 6 Qingtongxia-Guyuan Fault, 7 Hui’anbao-Shajingzi Fault) (modiﬁed after Feng et al. 2013). Area: I Tianhuan Depression, II Thrust Belt of Western Margin, III Qilianshan Orogen, IV Alashan Block. Age: O Ordovician, C Carboniferous, P Permian, T Triassic, J Jurassic, K Cretaceous C’ D’ D 10 Pet. Sci. (2017) 14:1–23 8-node isotropic elements to represent each lithological mechanism, is an effective criterion to predict the develop- layer. The mechanical behavior in the elastic domain is ment and the distribution of tensile fractures; however, this dominated by the generalized Hook’s law. As the Yanchang criterion, which in nature is equivalent with the theory of Formation is generally less than 3000 m in depth where the maximum tensional stress, is only suitable for the tensile plastic deformation is not obvious and the structural fractures fractures (Grifﬁth 1920). Although tensile fractures are found in the Chang 7 and 7 members are chieﬂy shearing frac- in some areas of the Ordos Basin, they are limited to the 1 2 tures based on ﬁeld measurements and core observations contact surfaces of sandstone and mudstone layers, and more (Fig. 8), the mechanical behavior follows the elastic model, than 95% of structural fractures in the Longdong area are which is described by the generalized Hook’s law. shearing fractures, whose rupture is controlled by the Mohr– Various methods for fracture prediction have been pro- Coulomb failure criterion (Xie et al. 2008). Therefore, only posed in previous literature, such as the conventional logging Mohr–Coulomb failure criterion is taken into consideration in method, the stress ﬁeld method, the principle curvature this study, which follows the equation (Coulomb 1776): method, the geostatistical method, etc. (e.g., Savage et al. ½ s ¼ C þ r tan u ð2Þ 2010; Zahm et al. 2010; Jiu et al. 2013). The two-factor where [s] represents the critical shearing stress, C repre- method, involving the rupture value and the strain energy sents the cohesion, r represents the stress normal to the density, is used in this paper to predict the distribution of n shearing fractures and u represents the internal friction structural fractures in the Ordos Basin (Ding et al. 1998). angle (Table 1). Shearing fracture is triggered once the shearing stress exceeds the critical shearing stress ([s]) in 4.3.1 Rupture value Eq. (2). r can be obtained via the maximum principal stress (r ) and the minimum principal stress (r ) according Tensile fractures and shearing fractures conform to different 1 3 to Wang et al. (2004): criteria. Grifﬁth’s criterion, which is derived from the micro- Fig. 8 Photographs of structural fractures in outcrops and cores of the Ordos Basin. a Conjugate fractures indicate the maximum principal compressive stress of WNW orientation in the Late Mesozoic; b conjugate fractures indicate the maximum principal compressive stress of NE orientation in the Cenozoic; c near-vertical fracture plane in a core from the Longdong area; and d moderate-dipping fracture plane in a core from the Longdong area 123 Pet. Sci. (2017) 14:1–23 11 105°E 109°E 113°E 105°E 109°E 113°E (a) (b) Late Mesozoic Cenozoic 4000 m 4000 m Hangjinqi Dongsheng Hangjinqi Dongsheng Wushenqi Wushenqi Jingbian Jingbian Dingbian Dingbian Yanchang Wuqi Wuqi Yanchang Qingyang Qingyang Zhenyuan Zhenyuan To n g ch u an Tongchuan Linfen Linfen Xi’an Xi’an 040 80 km 040 80 km 105°E 109°E 113°E 105°E 109°E 113°E Fig. 9 Displacement ﬁelds of the two models for the Ordos Basin in the Late Mesozoic and the Cenozoic eras. Thin black arrows indicate the calculated displacement directions and their lengths indicate the magnitude of displacement. Thick black arrows outside the basin denote the rotation of the basin during these periods. Black crosses represent the ﬁxed boundaries in different models 4.3.2 Strain energy density r ¼ðÞ r þ r =2 ðÞ r r sin u=2 ð3Þ n 1 3 1 3 The shearing stress (s ) can also be obtained via the two It is generally accepted that the rocks with relatively high principal stresses according to Wang et al. (2004): strain energy density are more likely to develop structural s ¼ðÞ r r cos u=2 ð4Þ fractures than those with a lower one. The strain energy n 1 3 density, namely the strain energy per unit volume, is Following the Mohr–Coulomb failure criterion, the rock described as follows (Prince and Rhodes 1966): will break when the shearing stress is equal or greater than 2 2 2 U ¼ r þ r þ r 2vðÞ r r þ r r þ r r the critical shearing stress in Eq. (2), so the rupture value X Y Y Z Z X X Y Z ð6Þ 2 2 2 (I) is introduced in order to measure the probability of þ 21ðÞ þ v s þ s þ s =2E XY YZ ZX rock’s rupture according to Ding et al. (1998): where U is the strain energy density, v is Poisson’s ratio, I ¼ s =½ s ð5Þ r , r and r are the normal stress components in x, y and X Y Z The possibility of rock’s failure is very small when the z directions, respectively, and s , s and s are the XY YZ ZX rupture value (I) is far smaller than 1, whereas the possi- shearing stress components in the corresponding directions. bility is relatively larger when the rupture value (I) exceeds Strain energy density (U) could be utilized to indicate the 1. The fracture density (f) and the rupture value (I) may fracture distribution. have a positive correlation, so the rupture value (I)isan Rupture value (I) stands for the possibility of rock effective index for fracture prediction through empirical failure, whereas the strain energy density (U) stands for the formulas established between them. developing ability of structural fractures. In this study, 34°N 38°N 42°N 34°N 38°N 42°N 34°N 38°N 42°N 34°N 38°N 42°N 12 Pet. Sci. (2017) 14:1–23 105°E 109°E 113°E 105°E 109°E 113°E (a) (b) Late Mesozoic Cenozoic Hangjinqi Dongsheng Dongsheng Hangjinqi 31.5-50.9 MPa (Zhang et al., 2014) 84.2-85.6 MPa Wushenqi (Zhou et al., 2009b) Wushenqi 84.4-97.9 MPa 89.4-103.9 MPa (Zhou et al., 2009b) Jingbian (Zhou et al., 2009b) Jingbian 22.3-55.1 MPa Dingbian Dingbian (Zhou et al., 2009a) 79.1-90.0 MPa 30.0-50.0 MPa 40.0-70.0 MPa (Zhou et al., 2009b) (Wang et al., 2014c) (Wang et al., 2014c) Wuqi Wuqi Yanchang Yanchang 57.4-58.5 MPa (Zhou et al., 2009a) Qingyang Qingyang Zhenyuan Zhenyuan Tongchuan Tongchuan Xi’an Xi’an 040 80 km 040 80 km 105°E 109°E 113°E 105°E 109°E 113°E 0 13.3 26.7 40.0 53.3 66.7 80.0 93.3 107.0 120.0 MPa Fig. 10 Maximum principal stress distribution of the two models in a the Late Mesozoic and b the Cenozoic eras. Red frames denote the areas where the calculated stress magnitudes match well with the Acoustic Emission paleo-stress magnitudes in earlier literature (Zhou et al. 2009a,b; Wang et al. 2014a; Zhang et al. 2014) syntheses of the rupture value and the strain energy density, necessary to verify the correctness of the two models namely the two-factor method, are applied, in order to proposed in this paper, including the Late Mesozoic and build ﬁnite element models for fracture prediction in the the Cenozoic ones, by comparing the results of ﬁnite ele- Ordos Basin (Ding et al. 1998). ment modeling with earlier published data. The calculated displacement directions reveal that the relative rotation directions in these periods are (1) anti- 5 Results and analyses clockwise from the Early Jurassic to the Cretaceous and (2) clockwise in the Cenozoic era (Fig. 9). These results are in Because the orientation and the distribution of structural good agreement with earlier ﬁndings (e.g., Pei et al. 2011; fractures are the key elements in fracture prediction, the Li et al. 2014; Yang et al. 2014). fracture orientation and the estimated density have been Acoustic Emission (AE) is an important technique in calculated with the ﬁnite element modeling and will be rock mechanics and experimental seismology, which can compared with the observed data in outcrops and cores. offer rock mechanical parameters, such as the maximum With the two-factor method, modeling results, including principal stress magnitudes generated in the geological the principal compressive stress orientations, the rupture history. The maximum principal stress magnitudes of the values, the strain energy density and the fracture density, Late Mesozoic era after pore-pressure correction range are presented as maps, which can imply the relative from 40.0 to 103.9 MPa in the Yanhewan, the Dingbian, degrees of fracture development in the Longdong area. the Dongsheng areas, etc. (Fig. 10a). The Cenozoic stress magnitudes remain in a limited range of 22.3–58.5 MPa 5.1 Validity of models within the Wuqi-Yanhewan, the Zhenyuan, the Wushengqi areas, etc. (Zhou et al. 2009a, b; Wang et al. 2014a; Zhang Since reliable numerical models are the basis of further et al. 2014) (Fig. 10b). The calculated maximum principal study on the fracture prediction in the Longdong area, it is stress magnitudes in the Late Mesozoic and the Cenozoic 34°N 38°N 42°N 34°N 38°N 42°N 34°N 38°N 42°N 34°N 38°N 42°N Pet. Sci. (2017) 14:1–23 13 105°E 109°E 113°E 105°E 109°E 113°E (a) (b) Late Mesozoic Cenozoic Hangjinqi Dongsheng Hangjinqi Dongsheng Wushenqi Wushenqi Dingbian Jingbian Jingbian Dingbian Yanchang Wuqi Yanchang Wuqi Zhenyuan Qingyang Qingyang Zhenyuan Tongchuan Ruishuihe Tongchuan Ruishuihe Xi’an Xi’an 040 80 km 040 80 km 105°E 109°E 113°E 105°E 109°E 113°E 1 2 3 4 5 6 7 8 9 10 11 12 Fig. 11 Maximum principal compressive stress trajectory maps of the two models for the Ordos Basin in the a Late Mesozoic and b Cenozoic eras. Green and red arrows represent the two major orientations of horizontal maximum principal stress (S ) through conjugate joint Hmax measurements. Short black bars indicate the calculated S , and bars in other colors represent the observed S in previous literature. 1 Late Hmax Hmax Mesozoic S from Wan (1994), 2 Late Mesozoic S from Hou et al. (2010), 3 Late Mesozoic S from Sun et al. (2014), 4 Late Hmax Hmax Hmax Mesozoic S from Zhou et al. (2009b), 5 Late Mesozoic S deduced from conjugate joints in our ﬁeld measurements, 6 Cenozoic S Hmax Hmax Hmax from Wang et al. (2008), 7 Cenozoic S from Xie et al. (2011), 8 Cenozoic S from Sun et al. (2014), 9 Cenozoic S from Yang et al. Hmax Hmax Hmax (2014), 10 Cenozoic S from Zhou et al. (2009a), 11 Cenozoic S deduced from conjugate joints in our ﬁeld measurements, 12 City Hmax Hmax are in agreement with the range of stress magnitudes compressive stress and the measured ones, including the measured by AE technology (Fig. 10). The above-men- stress orientations in previous literature (e.g., Wan 1994; tioned evidence strengthens the validity of our calculated Hou et al. 2010; Sun et al. 2014) and the measured data in results in the models. the present study, are in general less than 5, proving the In addition, earlier published stress orientation data reliability of the Late Mesozoic and Cenozoic models (Wan 1994; Hou et al. 2010; Sun et al. 2014) are also used (Fig. 11). as evidence to substantiate our models (Fig. 11a). These Evidence including the rotation directions, the measured stress orientation data suggest that the dominant orientation maximum principal stress magnitudes and the previous of maximum principal compressive stress in the Late stress data is gathered to prove the authenticity of the two Mesozoic is WNW. Current stress ﬁeld data can also be stress ﬁelds in the Late Mesozoic–Cenozoic models, and it utilized to interpret the Cenozoic stress ﬁelds because the is found that the calculated results are reliable. Despite basin has been stable during this period (Wang et al. 2008; slight differences between the calculated and observed Xie et al. 2011; Sun et al. 2014; Yang et al. 2014). Based maximum principal compressive stress, the modeling on the borehole collapse and multiple strain analyses in the results of the Late Mesozoic stress ﬁelds indicate that the Yanhewan area, it can be inferred that the dominant ori- orientation of the maximum principal compressive stress in entation of maximum principle compressive stress in the the Ordos Basin is WNW, whereas in the Cenozoic model, Cenozoic is NE (Zhou et al. 2009a). All these orientations the orientation is NE. Based on the above-mentioned are presented in the stereonets (Fig. 11). The differences proofs, the validity of the two models in the Late Mesozoic between the calculated orientations of maximum and the Cenozoic can be corroborated. 34°N 38°N 42°N 34°N 38°N 42°N 34°N 38°N 42°N 34°N 38°N 42°N 14 Pet. Sci. (2017) 14:1–23 0 50 100 200 km Cenozoic Late Mesozoic (a) (b) B504 B504 Huanxian B286 L124 Huanxian L124 B286 B478 L89 Huachi Huachi B270 B270 L179 C77 X251 Z17 Z17 Ze130 X251 Laocheng Laocheng Qingcheng Qingcheng X266 Z166 X266 Ze265 Ze265 Z197 X46 Z197 X46 N68 N68 Heshui Qingyang Qingyang Heshui X93 Ningxian Ningxian Zhengning X66 Zhengning Fig. 12 Calculated maximum principal compressive stress orientations in a the Late Mesozoic and b the Cenozoic within the Longdong area are compared with the observed fracture orientations in 19 wells. The observed orientations are obtained from imaging logging (FMI technology) data, which are shown as rose diagrams in the ﬁgures. The green rose diagrams denote the structural fractures in NW–EW trends, whereas the red ones denote those in NNE–ENE trends 5.2 Maximum principal stress orientations utilized to indicate the stress orientations in the Late Mesozoic–Cenozoic. From numerical modeling, the ori- Tectonic events of different episodes have distinct effects entations of calculated maximum compressive stress in the on the principal stress orientations in the Ordos Basin. Late Mesozoic are mainly WNW, while those in the Since there is little difference between the Chang 7 and 7 Cenozoic are mainly NE (Zhao et al. 2013, 2016) (Fig. 12). 1 2 members except for lithology and layer thickness, the In outcrops and cores, the observed fractures developed in pattern of principal stress orientation during the same the Late Mesozoic are chieﬂy in NW–EW trends and those period is similar in each layer of the Longdong area. Thus, in the Cenozoic are chieﬂy in NNE–ENE trends (Fig. 8). the Chang 7 member is taken as an example to demon- Our ﬁeld measurements also corroborate that the ENE- strate the distribution of maximum principal compressive trending structural fractures developed later than the NW- stress in the study area (Fig. 12). trending ones. Therefore, it can be concluded that the NW On the basis of paleo-magnetic evidence in earlier to EW fractures were developed in a Late Mesozoic stress studies, although the Ordos Basin experienced rotation in ﬁeld, whereas the NNE to ENE ones were developed in a different directions from the Late Mesozoic to the Ceno- Cenozoic stress ﬁeld. Despite tiny differences between the zoic, the rotation angle of the basin is less than 5 in the calculated and the observed data, in general, modeling Late Mesozoic–Cenozoic eras (e.g., Huang et al. 2005). results ﬁt well with the dominant orientations of observed Therefore, the present stress data, including fracture trends fractures which are obtained from the FMI technology and Formation Microscanner Image (FMI) data, can also be (Fig. 12). 123 Pet. Sci. (2017) 14:1–23 15 Structural fractures in the Ordos Basin were developed d). The distribution of sand bodies and the thickness of in multiple orientations under different stress ﬁelds, pri- sandstone layers have a distinct impact on the distribution marily in the Late Mesozoic and the Cenozoic episodes, of rupture values within the Longdong area. Both in the and this intersection pattern will contribute to wider Chang 7 and 7 members, the rupture values are rela- 1 2 opening and better connectivity of the fractures. The tively higher where sand bodies are developed and the formed fracture networks provide a path for ﬂuid trans- thickness of sandstone layers is relatively larger, due to mission and enhance the permeability, which will have the brittleness of sandstones (Fig. 4). The regional stress notably improved the fractured tight-oil reservoirs in the ﬁelds during different periods also inﬂuence the rupture Ordos Basin (e.g., Izadi and Elsworth 2014). values, resulting in the Cenozoic rupture values being smaller than the Late Mesozoic ones. However, the 5.3 Rupture values inﬂuence of regional stress ﬁelds is not as remarkable as that of lithology, because regional stress ﬁelds determine Since the rupture value is an important parameter to indi- only the magnitudes, not the distribution of rupture values cate the fracture development in the study area, comparison in the Chang 7 and 7 members within the study area 1 2 between the calculated rupture values and the observed (Fig. 13). core fracture density is informative to help analyze the reliability of the models (Figs. 13, 14). 5.4 Strain energy density In the maps of rupture values in the Chang 7 member during the Late Mesozoic–Cenozoic era, the highest Because rocks with higher strain energy density are more rupture values are situated in the east and center of the likely to form structural fractures than those with a lower study area, mainly concentrated in the Qingyang, the one, the strain energy density can be used as another Laocheng and the Zhengning areas (Fig. 13a, b), while parameter to predict the fracture density. the highest rupture values in the Chang 7 member are Similar to the rupture value, there is obvious positive chieﬂy situated in the mid-southern area, particularly in correlation between the strain energy density and the the Qingyang-Heshui and the Ningxian areas (Fig. 13c, thickness of sandstone layers. The strain energy density is 0 50 100 200 km Chang 7 1 in Late Mesozoic Chang 7 1 in Cenozoic N N (a) (b) Huanxian Huanxian Huachi Huachi Laocheng Qingcheng Laocheng Qingcheng Heshui Qingyang Heshui Qingyang Ningxian Ningxian Zhengning Zhengning 0.95 0.98 1.01 1.04 1.07 1.10 1.13 1.16 1.19 1.22 0.95 0.98 1.01 1.04 1.07 1.10 1.13 1.16 1.19 1.22 Chang 7 2 in Late Mesozoic Chang 7 2 in Cenozoic N N (c) (d) Huanxian Huachi Huanxian Huachi Laocheng Laocheng Qingcheng Qingcheng Heshui Heshui Qingyang Qingyang Ningxian Ningxian Zhengning Zhengning 0.95 0.98 1.01 1.04 1.07 1.10 1.13 1.16 1.19 1.22 0.95 0.98 1.01 1.04 1.07 1.10 1.13 1.16 1.19 1.22 Fig. 13 Distribution of rupture value of the Late Mesozoic and the Cenozoic in the Chang 7 and 7 members within the Longdong area. 1 2 a Rupture values of the Late Mesozoic in the Chang 7 member, b rupture values of the Cenozoic in the Chang 7 member, c rupture values of 1 1 the Late Mesozoic in the Chang 7 member and d rupture values of the Cenozoic in the Chang 7 member 2 2 123 16 Pet. Sci. (2017) 14:1–23 02 50 100 00 km Chang 7 1 in Late Mesozoic Chang 7 1 in Cenozoic (a) (b) N N Huanxian Huanxian Huachi Huachi Laocheng Qingcheng Qingcheng Laocheng Heshui Heshui Qingyang Qingyang Ningxian Ningxian Zhengning Zhengning 7.0 7.3 7.6 7.9 8.2 8.5 8.8 9.1 9.4 9.7 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5 5.7 5.9 Chang 7 2 in Late Mesozoic Chang 7 2 in Cenozoic (c) (d) N N Huanxian Huanxian Huachi Huachi Laocheng Laocheng Qingcheng Qingcheng Heshui Heshui Qingyang Qingyang Ningxian Ningxian Zhengning Zhengning 7.0 7.3 7.6 7.9 8.2 8.5 8.8 9.1 9.4 9.7 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5 5.7 5.9 4 3 Fig. 14 Distribution of strain energy density (10 J/m ) of the Late Mesozoic and the Cenozoic in Chang 7 and 7 members within the 1 2 Longdong area. a Strain energy density of the Late Mesozoic in the Chang 7 member, b strain energy density of the Cenozoic in the Chang 7 1 1 member, c strain energy density of the Late Mesozoic in the Chang 7 member and d strain energy density of the Cenozoic in the Chang 7 2 2 member Table 3 Curve-ﬁtting Layers Curve-ﬁtting relations Correlation coefﬁcient relationships of the measured 2 2 fracture densities, the calculated Chang 7 D = 3.493 I - 0.049 U - 6.241 I ? 0.695 U ? 0.270 0.947 1 M rupture values and the strain 2 2 D = 3.581 I - 0.123 U - 7.105 I ? 3.240 U ? 1.054 0.904 energy densities of Chang 7 2 2 Chang 7 D = 48.429 I - 0.039 U - 100.308 I ? 0.734 U ? 48.585 0.871 2 M and 7 members in the 2 2 Longdong area D = 22.944 I ? 0.450 U - 46.876 I - 4.094 U ? 33.241 0.941 -1 -1 D (m ) and D (m ) represent the measured fracture densities in cores of the Late Mesozoic and the M C Cenozoic periods, respectively. I and U denote the calculated rupture values and the strain energy densities 4 3 (10 J/m ), respectively higher where the sand bodies are developed as a whole 5.5 Predicted fracture distribution (Figs. 4, 14). Although the Cenozoic stress ﬁeld of the Ordos Basin is strikingly different from the Late Mesozoic In order to predict the fracture distribution in the Yanchang Formation within the Longdong area, connection between one, the impact of regional stress is mainly limited to the magnitudes, not the distribution of strain energy density in the calculated and the measured fracture density in cores the Longdong area. The distribution of strain energy den- must be established to study their relationship. In this sity in the Late Mesozoic and the Cenozoic periods is paper, the two-factor method is utilized to compare the similar, but the Late Mesozoic strain energy density is calculated data (including the rupture value and the strain larger than the Cenozoic one both in the Chang 7 and 7 energy density) and the measured fracture density (Ding 1 2 members, implying that the strain energy density is more et al. 1998). Since structural fractures in the Ordos Basin were chieﬂy developed during two stages of stress ﬁelds, inﬂuenced by the movement in the Late Mesozoic than that in the Cenozoic (Fig. 14). namely the Late Mesozoic and the Cenozoic ones, two 123 Pet. Sci. (2017) 14:1–23 17 Table 4 Overview of predicted and measured fracture densities in the Chang 7 member in the Longdong area Well Measured Predicted Absolute Relative Well Measured Predicted Absolute Relative -1 -1 -1 -1 -1 -1 name density, m density, m error, m error, % name density, m density, m error, m error, % B117 0.020 0.045 0.025 122 W98 0.000 0.000 0.000 – B146 0.000 0.005 0.005 – X140 0.000 0.000 0.000 – B170 0.000 0.008 0.008 – X195 0.028 0.015 0.013 46 B456 0.000 0.012 0.012 – X233 0.000 0.000 0.000 – B478 0.059 0.085 0.026 44 X259 0.000 0.027 0.027 – Ban12 0.320 0.305 0.015 5 X261 0.000 0.005 0.005 – C87 0.070 0.058 0.012 18 X263 0.000 0.000 0.000 – Hua56 0.000 0.007 0.007 – X67 0.015 0.008 0.007 47 L189 0.068 0.069 0.001 2 X73 0.026 0.057 0.031 118 L47 0.018 0.042 0.023 128 Y433 0.085 0.052 0.033 39 L79 0.033 0.023 0.010 29 Z124 0.000 0.013 0.013 – L96 0.000 0.008 0.008 – Z148 0.000 0.020 0.020 – M28 0.000 0.005 0.005 – Z15 0.068 0.099 0.031 46 M40 0.036 0.022 0.014 39 Z172 0.038 0.048 0.010 26 N43 0.062 0.090 0.029 46 Z186 0.344 0.372 0.028 8 N51 0.137 0.110 0.027 20 Z200 0.122 0.112 0.009 8 N57 0.055 0.080 0.025 45 Z230 0.046 0.054 0.009 19 N75 0.037 0.047 0.010 27 Z233 0.080 0.084 0.004 5 N76 0.155 0.155 0.000 0 Z24 0.061 0.086 0.025 41 N78 0.210 0.176 0.033 16 Z47 0.199 0.132 0.067 34 N81 0.057 0.084 0.027 47 Z57 0.151 0.121 0.030 20 S142 0.164 0.136 0.028 17 Z78 0.316 0.231 0.085 27 S160 0.000 0.000 0.000 – Z79 0.071 0.036 0.035 49 T15 0.113 0.097 0.016 14 Z87 0.201 0.234 0.033 16 T2 0.055 0.063 0.008 14 Ze220 0.050 0.026 0.024 48 W47 0.053 0.079 0.026 49 Ze97 0.000 0.000 0.000 – W67 0.000 0.024 0.024 – Zeg70 0.180 0.132 0.048 27 -1 ‘‘–’’ means that the relative errors do not exist in these wells because the corresponding measured fracture densities are 0 m , and large errors, including absolute and relative errors, are denoted in bold type in this table episodes of fractures should be ﬁtted separately and then be D D p m e ¼ 100% ð8Þ added up by weight. By multiple regression analyses, bi-quadratic rela- D denotes the absolute error and e denotes the relative tionships between the rupture values, the strain energy error. D and D represent the predicted and the measured P M density and the measured fracture density in the Chang fracture densities, respectively. Generally, when e is less 7 and 7 members of different episodes have been built 1 2 than 50%, we can consider that the predicted data match and the empirical formulas are shown in Table 3.Cor- the measured ones and the modeling results are reliable to a relation coefﬁcients in all curve-ﬁtting relationships are certain extent. larger than 0.87, which means that there is a signiﬁcant The differences between the measured and the predicted correlation between the calculated and the measured fracture densities are shown in Tables 4 and 5. For most of data. the wells in the Chang 7 member, the predicted and the To further illustrate the reliability of our models, error measured data match quite well. In the 54 measured wells, analyses are carried out as follows. Both the absolute error -1 only 2 wells exceed 0.05 m in absolute errors and 3 and the relative error were applied to reﬂect the accuracy of wells exceed 50% in relative errors (Table 4). The differ- fracture prediction. Absolute error is calculated by: ences between them may be caused by the stress concen- D ¼ D D ð7Þ p m tration in some areas, such as Well Z47 and Z78, where numerous fractures are found. As for the Chang 7 And relative error can be described as follows: 123 18 Pet. Sci. (2017) 14:1–23 Table 5 Overview of predicted and measured fracture densities in the Chang 7 member in the Longdong area Well Measured Predicted Absolute Relative Well Measured Predicted Absolute Relative -1 -1 -1 -1 -1 -1 name density, m density, m error, m error, % name density, m density, m error, m error, % B117 0.062 0.083 0.021 34 X233 0.000 0.049 0.049 – B146 0.000 0.033 0.033 – X263 0.000 0.004 0.004 – B36 0.000 0.000 0.000 – X270 0.000 0.041 0.041 – B401 0.000 0.026 0.026 – X65 0.053 0.040 0.013 25 B456 0.000 0.000 0.000 – X67 0.026 0.031 0.005 20 Ban12 0.918 0.903 0.016 2 X69 0.157 0.080 0.077 49 C87 0.043 0.036 0.007 15. Z172 0.025 0.022 0.003 12 Hua312 0.000 0.000 0.000 – Z230 0.064 0.037 0.027 42 L189 0.052 0.037 0.015 29 Z233 0.271 0.109 0.162 60 L47 0.000 0.009 0.009 – Z78 0.464 0.464 0.000 0 L79 0.047 0.039 0.008 16 Z79 0.072 0.000 0.072 100 L96 0.000 0.000 0.000 – Ze118 0.000 0.045 0.045 – N43 0.000 0.000 0.000 – Ze284 0.000 0.023 0.023 – N51 0.385 0.337 0.048 12 Ze298 0.000 0.097 0.097 – N55 0.000 0.009 0.009 – Ze362 0.000 0.131 0.131 – N57 0.137 0.100 0.036 27 Ze77 0.129 0.072 0.057 44 N75 0.036 0.149 0.113 317 Ze95 0.052 0.036 0.016 30 N76 0.093 0.038 0.055 59 Ze97 0.000 0.029 0.029 – N78 0.024 0.056 0.032 132 Zeg70 0.050 0.036 0.014 27 W98 0.000 0.044 0.044 – -1 ‘‘–’’ means that the relative errors do not exist in these wells because the corresponding measured fracture densities are 0 m , and large errors, including absolute and relative errors, are denoted in bold type in this table member, predicted data of only 8 wells in the 39 measured the Cenozoic ones are NNE–ENE (Fig. 12b) which are -1 wells are more than 0.05 m in absolute errors, and data consistent with the regional stress ﬁelds of the Ordos Basin of only 5 wells are more than 50% in relative errors in the corresponding periods (e.g., Zhang et al. 2003). In (Table 5). Most of these wells are with extraordinarily high the maps of predicted fracture density in different periods, fracture density, which results in large errors between the the average density of the Cenozoic fractures is larger than predicted and the measured fracture densities. Some large that of the Late Mesozoic ones (Fig. 15). By comparison errors may be caused by non-structural factors, such as between the distribution maps of predicted total fracture various sedimentary phenomena. Cross bedding and len- densities in the Chang 7 and 7 members within the study 1 2 ticular bedding appeared widely in Well Ze77, etc., which area (Fig. 16), the predicted fracture density in each may lead to the difference between the predicted and member is alike as a whole; however, their fracture dis- measured data. Despite these differences, the tendency of tributions are signiﬁcantly distinct. In the Chang 7 mem- predicted fracture distribution is still in accordance with the ber, the maximum fracture density is located in the center measured one. In short, the errors between the predicted and the east of the Longdong area (Fig. 16a), while in the and the measured fracture densities are within accept- Chang 7 member, the maximum density is situated in the able limits, implying that the modeling results are suit- southern-central section of the study area (Fig. 16b). able for the fracture prediction in the Yanchang Formation In addition, by comparing the predicted fracture density of the Ordos Basin. with the distribution of sand bodies, their similarity reveals that the lithology is a key factor in controlling the fracture distribution in the Ordos Basin. Structural fractures are 6 Discussion more likely to be developed in the sandstones rather than in the mudstones. Where thicker sandstone layers are devel- As is shown in the maps of maximum principal compres- oped, the fracture density is relatively higher than other sive stress orientations in the Chang 7 and 7 members in areas (Figs. 4, 16). However, there is still a difference 1 2 the Longdong area, the dominant orientations of the Late between the predicted fracture distribution and the outline Mesozoic fractures are NW–EW (Fig. 12a) and those of of sand bodies, indicating that the regional stress ﬁeld also 123 Pet. Sci. (2017) 14:1–23 19 0 50 100 200 km Chang 7 1 in Late Mesozoic Chang 7 1 in Cenozoic N N (a) (b) Huanxian Huanxian Huachi Huachi Laocheng Laocheng Qingcheng Qingcheng Heshui Heshui Qingyang Qingyang Ningxian Ningxian Zhengning Zhengning 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 Chang 7 2 in Late Mesozoic Chang 7 2 in Cenozoic (c) (d) N N Huanxian Huanxian Huachi Huachi Laocheng Qingcheng Laocheng Qingcheng Heshui Heshui Qingyang Qingyang Ningxian Ningxian Zhengning Zhengning 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 -1 Fig. 15 Distribution of predicted fracture density (m ) of the Late Mesozoic and the Cenozoic in the Chang 7 and 7 members within the 1 2 Longdong area. a Predicted fracture density of the Late Mesozoic in the Chang 7 member, b predicted fracture density of the Cenozoic in the Chang 7 member, c predicted fracture density of the Late Mesozoic in the Chang 7 member and d predicted fracture density of the Cenozoic in 1 2 the Chang 7 member plays a role in the fracture development, even though its act as a reference for future regional-scale petroleum inﬂuence is limited compared with the lithology and the exploration, while the method of fracture prediction, layer thickness. including the two-factor method and the empirical formulas In brief, the stress ﬁelds determine the overall fracture can be used at well scale. Structural fractures play an orientations, and the lithology distribution and the thick- important role in reconstructing the tight clastic reservoirs, ness of sandstone layers in the study area play a predom- especially in their permeability (Reda 2013). inant role in the distribution of predicted fracture density. The controlling factors of fracture development are Some potential factors which are not covered in these complex owing to the complicated geological background. numerical models may restrict the accuracy of predicted Fault systems can be a vital factor in developing fractures results, including: where tectonic movements are strong such as the Kuqa Depression of the northern Tarim Basin in the northwestern 1. The complicated heterogeneity of each layer; China (Ju et al. 2014b) and the Upper Rhine Graben in 2. The extreme stress in some areas; France and Germany (Johanna et al. 2015); ﬂow may 3. The interaction between the two episodes of structural notably promote fracture development where ﬂuid ﬂow or fractures; and lava ﬂow appears (e.g., Agosta et al. 2010). However, in 4. The inﬂuence of deep paleo-faults. the Ordos Basin, where the tectonic events are rather weak Since the modeling is on a relatively large-scale while the and the dips of the Mesozoic–Cenozoic strata are less than outline of sand bodies is depicted in considerable detail, the 3, the lithology and the layer thickness are the dominant modeling results, including the rupture values and the strain factors in governing the distribution of fracture density. energy density, can still be used to guide further exploration The relationship between the lithology and the fracture in spite of the four above-mentioned restrictions. Mean- density is still obscure, but it may be related to the dif- while, the qualitative fracture prediction obtained from the ference of rock physical parameters (Table 1) according to numerical modeling may also be applicable. These results previous study (e.g., Zeng et al. 2008). The different grain 123 20 Pet. Sci. (2017) 14:1–23 0 50 100 200 km Chang 7 1 Hua56 W98 (a) Y433 S142 L96 M28 B170 B146 Huanxian B478 L189 S160 L79 Huachi B456 W47 M40 L47 C87 B117 W67 X259 Z47 Z57 Z87 Ze97 Z78 X233 Z79 T2 Z15 Z186 Laocheng Z24 X261 X73 Z200 Ze220 T15 X263 Qingcheng Z148 Z230 X195 X140 Z172 Z233 Heshui Z124 Ban12 N76 Qingyang N43 N78 X67 N75 Ningxian N81 N57 Zeg70 N51 Zhengning 0.00 0.04 0.08 0.12 0.16 0.20 0.24 0.28 0.32 0.36 Chang 7 2 Hua312 B401 (b) W98 B36 L96 B146 Huanxian L189 Huachi L79 B456 C87 L47 Ze77 B117 Ze97 Z78 Laocheng X270 X233 Z79 Ze298 Qingcheng X263 Ze362 Ze95 Heshui Z230 Z172 Z233 Ze284 N76 Ban12 Qingyang Ze118 N43 N78 X67 N75 X69 Ningxian N57 X65 Zeg70 N51 N55 Zhengning 0.00 0.04 0.08 0.12 0.16 0.20 0.24 0.28 0.32 0.36 -1 Fig. 16 Distribution of predicted total fracture density (m )in a the Chang 7 and b Chang 7 members within the Longdong area. Black solid 1 2 dots represent the measured wells in the study area 2. Two episodes of structural fractures have been devel- sizes in various clastic rocks may be the micro-mechanism that causes the distribution of fracture density in the Ordos oped since the Late Triassic: The dominant orienta- tions of the Late Mesozoic fractures in the Yanchang Basin (Zhao et al. 2013; Ju et al. 2015). Formation are NW–EW, whereas those of the Ceno- zoic fractures are NNE–ENE, both of which are in agreement with the modeling results. 7 Conclusions 3. Structural fractures in the Ordos Basin are controlled by the regional stress ﬁelds, and the lithology and the The predicted fracture distribution provides a clear view of layer thickness have a signiﬁcant impact on the the fracture concentration and fracture development. Sev- distribution of structural fractures, because the stress eral primary conclusions can be drawn from the modeling distribution will be affected by the inhomogeneity of results: lithology and layer thickness. This conclusion is shown 1. A ﬁnite element modeling technique, applying the two- in the similarity between the maps of predicted fracture factor method, is suitable for the fracture prediction of density and observed sand bodies in the Yanchang the Ordos Basin, based on comparison between the Formation within the study area. calculated and the measured fracture densities of the 4. The average fracture density is close in the Chang 7 Chang 7 and 7 members in the Longdong area. and 7 members, but there are obvious differences in 1 2 2 123 Pet. Sci. (2017) 14:1–23 21 Faure M, Lin W, Chen Y. Is the Jurassic (Mesozoic) intraplate their fracture distributions. In the Chang 7 member, tectonics of North China due to westward indentation of North the maximum fracture density is concentrated in the China block? Terra Nova. 2012;24(6):456–66. doi:10.1111/ter. center and the east of the Longdong area, particularly in the Qingyang, the Laocheng and the Zhengning Feng JP, Ouyang ZY, Huang ZL. The application of balance -1 geological section technology in the mid-south section of the areas (up to 1.5 m ), while in the Chang 7 member, western margin of Ordos Basin. Geotecton Metallog. the maximum value is located in the central and 2013;37(3):393–7 (in Chinese). southern part of the area. Fournier M, Jolivet L, Davy P, et al. Back-arc extension and collision: 5. The modeling results and the predicted fracture density an experimental approach to the tectonics of Asia. Geophys J Int. 2004;157:871–89. doi:10.1111/j.1365-246X.2004.02223.x. can be utilized to guide future regional exploration, Gilder SA, Gomez J, Chen Y, et al. A new paleogeographic and the method of fracture prediction, namely the two- conﬁguration of the Eurasian landmass resolves a paleomagnetic factor method, can be referred for further study of the paradox of the Tarim Basin (China). Tectonics. tight-sand reservoirs. 2008;27(1):1256. doi:10.1029/2007TC002155. Glukhmanchuk ED, Vasilevskiy AN. 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Fracture prediction in the tight-oil reservoirs of the Triassic Yanchang Formation in the Ordos Basin, northern China

Fracture prediction in the tight-oil reservoirs of the Triassic Yanchang Formation in the Ordos Basin, northern China

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Fracture prediction in the tight-oil reservoirs of the Triassic Yanchang Formation in the Ordos Basin, northern China

Fracture prediction in the tight-oil reservoirs of the Triassic Yanchang Formation in the Ordos Basin, northern China

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