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Features and genesis of Paleogene high-quality reservoirs in lacustrine mixed siliciclastic–carbonate sediments, central Bohai Sea, China

Features and genesis of Paleogene high-quality reservoirs in lacustrine mixed... Pet. Sci. (2017) 14:50–60 DOI 10.1007/s12182-016-0147-9 OR IGINAL PAPER Features and genesis of Paleogene high-quality reservoirs in lacustrine mixed siliciclastic–carbonate sediments, central Bohai Sea, China 1,2 1 1 1 • • • • Zheng-Xiang Lu¨ Shun-Li Zhang Chao Yin Hai-Long Meng 1 1 Xiu-Zhang Song Jian Zhang Received: 25 February 2016 / Published online: 21 January 2017 The Author(s) 2017. This article is published with open access at Springerlink.com Abstract The characteristics and formation mechanisms early stage, and oil emplacement has further led to the of the mixed siliciclastic–carbonate reservoirs of the preservation of good reservoir quality. Paleogene Shahejie Formation in the central Bohai Sea were examined based on polarized light microscopy and Keywords High-quality reservoirs  Mixed sediments scanning electron microscopy observations, X-ray diffrac- Paleogene Bohai Sea tometry, carbon and oxygen stable isotope geochemistry, and integrated fluid inclusion analysis. High-quality reservoirs are mainly distributed in Type I and Type II 1 Introduction mixed siliciclastic–carbonate sediments, and the dominant pore types include residual primary intergranular pores and In addition to carbonate and clastic reservoir rock types, intrafossil pores, feldspar dissolution pores mainly devel- magmatic, metamorphic, shale and mixed siliciclastic– oped in Type II sediments. Type I mixed sediments are carbonate sedimentary reservoirs can also be considered as characterized by precipitation of early pore-lining dolo- important targets for oil and gas exploration and develop- mite, relatively weak mechanical compaction during deep ment (Ge et al. 2011; Tong et al. 2012; Xiao et al. 2015; burial, and the occurrence of abundant oil inclusions in Palermo et al. 2008). The concept of ‘‘mixed sediments’’ high-quality reservoirs. Microfacies played a critical role in was firstly proposed by Mount (1984) and is commonly the formation of the mixed reservoirs, and high-quality referred to as sediments that are composed of mixtures of reservoirs are commonly found in high-energy environ- siliciclastic and carbonate material (including allochemical ments, such as fan delta underwater distributary channels, particles) (Lubeseder et al. 2009; Brandano et al. 2010;Xu mouth bars, and submarine uplift beach bars. Abundant et al. 2014). Many Chinese and foreign scholars have made intrafossil pores were formed by bioclastic decay, and in-depth studies of the formation mechanisms of this type secondary pores due to feldspar dissolution further enhance of sediment and suggested that it can be developed in both reservoir porosity. Mechanical compaction was inhibited marine and lacustrine environments. Influenced by sea by the precipitation of pore-lining dolomite formed during (lake)-level fluctuations, structural changes, storm, current and tidal actions, mixed siliciclastic–carbonate sediments are widely distributed in transitional marine-terrestrial, continental shelf, and slope environments (Garcıa-Hidalgo & Shun-Li Zhang et al. 2007; Zonneveld et al. 2012). Under certain condi- 1205799554@qq.com tions, mixed siliciclastic–carbonate sediments may be rich College of Energy Resources, Chengdu University of in oil and gas. For example, hydrocarbon accumulations Technology, Chengdu 610059, Sichuan, China have been discovered in the high-quality mixed siliciclas- State Key Laboratory of Oil-Gas Reservoirs Geology and tic–carbonate reservoirs in China, such as the Bohai Bay Exploitation, Chengdu University of Technology, Basin, the Qaidam Basin and the Sichuan Basin (Feng et al. Chengdu 610059, Sichuan, China 2011a, b, 2013; Zhang et al. 2006; Liu et al. 2011; Garcı ´a- Hidalgo et al. 2007). Although carbonate and clastic Edited by Jie Hao 123 Pet. Sci. (2017) 14:50–60 51 reservoirs have been the subject of intensive study by a Kongdian Formation (E k) and underlies the Dongying large number of researchers, mixed siliciclastic–carbonate Formation (E d). Because economically significant hydro- reservoirs have received less attention. Previously, studies carbon accumulations have been found in the mixed of mixed siliciclastic–carbonate reservoirs have mainly reservoirs, the mixed siliciclastic–carbonate reservoirs focused on petrography, structure, classification, the have been the focus of study in recent years (Wang et al. establishment of depositional models (Caracciolo et al. 2015). 2012; Sha 2001; Zand-Moghadam et al. 2013; Zonneveld et al. 2012; Ma and Liu 2003), and the reconstruction of the sedimentary environment on the basis of sequence 3 Samples and experimental methodology stratigraphy, sea level change and paleoclimate (Anan 2014; Campbell 2005; Moissette et al. 2010). However, the In this study, 240 mixed siliciclastic–carbonate sediment microscopic features and the formation mechanisms of samples from 12 wells in the Shijiutuo Uplift in the central high-quality reservoirs have not been well investigated, Bohai Bay Basin, such as Well HD2 and Well HD5, were which has restricted the exploration and development of selected for porosity and permeability measurements. The mixed siliciclastic–carbonate reservoirs. locations of sample wells are shown in Fig. 1. The The Bohai Bay Basin is an important petroliferous basin microscopic features, such as petrology, pore space types, in North China. In the Paleogene, steep slope zones were and diagenesis, were obtained from 122 thin sections with well developed and are represented by a series of high different physical properties. Multi-purpose thin sections steep fault noses (Lu 2005; Guan et al. 2012). The tec- were prepared with blue-dyed epoxy impregnation and tonically induced physiographic changes controlled the double-sided polishing. The mineral composition was distribution and areal extension of mixed siliciclastic–car- identified by polarized light microscopy, and X-ray bonate sediments. For example, typical mixed sediments diffraction (XRD) analyses were carried out on twenty-two composed of lacustrine carbonate and siliciclastic material bulk samples and \2 lm size fractions using a Rigaku are widely distributed in the Shijiutuo Uplift in the central DMAX-3C diffractometer. The chemical composition of basin, and a large number of high-quality reservoirs are grain-coating and dissolved minerals was determined developed in them (Liu et al. 2011; Song et al. 2013). quantitatively by electron microprobe analysis (EMPA) Statistics suggest that reservoir quality is one of the key using a Shimadzu EPMA-1720 and a JEOL JXA-8100 controls on prospectivity during petroleum exploration and electron microprobes (operating conditions: 15 kV accel- production. The study of the characteristics and formation erating voltage, 10 mA current, 1 lm beam diameter). mechanisms of mixed siliciclastic–carbonate reservoirs is Fifteen double-thickness polished thin sections were therefore of significant importance for guiding the oil and selected for microthermometric measurements. Homoge- gas exploration and production in the Bohai Sea. The nization temperatures were measured using a Linkam purpose of this paper is to compare different types of mixed THMS-600 heating/cooling stage. Only primary fluid siliciclastic–carbonate reservoirs, to describe the main inclusions with both aqueous and hydrocarbon phases were features of high-quality reservoirs, and to determine the selected from authigenic minerals to determine their min- formation mechanisms of high-quality reservoirs by inte- imum precipitation temperatures (Liu et al. 2005;Lu ¨ et al. grating geological and geochemical data. 2015; Guo et al. 2012; Tian et al. 2016). In-situ carbon and oxygen isotope analysis was performed using an Nd:YAG laser microprobe. Laser probe microsampling of C and O 2 Geological setting from carbonate cements for isotopic analysis was achieved by focusing a laser beam with a wavelength of 1064 nm The Bohai Bay Basin is a Cenozoic rift basin superimposed and a diameter of 20 lm onto a sample situated in a vac- on the Paleozoic basement of the North China platform (Lu uum chamber to ablate a small area on the sample and 2005). The study area is located in the Shijiutuo Uplift in liberate CO gas. After purification, the CO gas was led 2 2 the central Bohai Bay Basin and bounded by two large directly into a Finnigan MAT 252 mass spectrometer for hydrocarbon generation sags—Bozhong Sag and Qin’nan isotopic analysis. After obtaining the isotopic values, the Sag (Fig. 1). The hydrocarbon accumulation condition is dolomite formation temperature (T) was calculated using excellent with high-quality source rocks and a series of the empirical formula proposed by Hu et al. (2012): Paleogene high steep fault nose traps developed (Guan et al. 2012). The mixed sedimentary reservoirs in the study T ¼ 16:5  4:3ðdC  dWÞþ 0:14ðdC  dWÞðdC  dWÞ area are mainly developed in the Paleogene Shahejie For- where dC is the d O of a dolomite precipitate, and dW is mation (E s). The Shahejie Formation is 300–400 m thick the d O of parent water. with burial depths [3000 m and conformably overlies the 123 52 Pet. Sci. (2017) 14:50–60 0 30 km Bohai Bay Basin China Qinnan Uplift Qinnan Sag Matouying Uplift HE 5 HE 2 HD 33 HE 3 Z13 HD 2 Shijiutuo Uplift HD 5 Nanbao Sag BZ 3 Shaleitian Uplift Shanan Sag HD 5 Uplift Coastline Major Fault Study Area Well Fig. 1 Location map and tectonic elements of the central Bohai Sea The timing of feldspar dissolution was mainly deter- Siliciclastic mined based on the fluid inclusion temperatures of the dissolution products (authigenic quartz). Sixty-two samples were observed under a DM4500P fluorescence microscope in order to identify possible petroleum inclusions. Fifteen 2 3 inclusions were also examined using a Renishaw inVia 50 50 laser Raman microprobe with a wavelength of 514.5 nm to document the existence of hydrocarbons. 5 6 The mixed siliciclastic–carbonate sediments were deposited in a fan delta environment (Guan et al. 2012; 7 10 Zhang et al. 2015; Ni et al. 2013). In order to illustrate the relationship between petrophysical properties and sedi- mentary microfacies, the sedimentary facies were identified Carbonate Mudstone by analyzing rock textures and well log data for Well HD2 (micritic carbonate) and Well HD5, in which core porosity and permeability Fig. 2 Rock types of E s mixed siliciclastic–carbonate sediments. 1: were measured. sand (gravel) rock, 2: carbonate siliciclastic mixed sedimentary rocks, 3: carbonate-bearing siliciclastic mixed sedimentary rocks, 4: silici- clastic carbonate mixed sedimentary rocks, 5: carbonate/siliciclastic mixed sedimentary rocks, 6: carbonate-bearing argillaceous silici- clastic mixed sedimentary rocks, 7: siliciclastic-bearing carbonate 4 Results mixed sedimentary rocks, 8: siliciclastic-bearing micrite carbonate mixed sedimentary rocks, 9: carbonate, 10: mudstone (micritic 4.1 Rock types carbonate) The E s mixed sediments are composed of siliciclastic and 2 mixed sediments, the content of the former two was, lacustrine carbonate rocks. For the siliciclastic grains, respectively, not less than 10%, while the latter two carbonate grains, matrix, and micrite that constituted the accounted for less than 50%. The identification results of Xihe Uplift Xinanzhuang Uplift Qikou Sag Liaoxinan Uplift Liaoxi Uplift Liaozhong Sag Bozhong Sag Bodong Lower Uplift Bodong Sag Miaoxibei Uplift Pet. Sci. (2017) 14:50–60 53 122 thin sections show that (Fig. 2)E s mixed siliciclastic– distributed in the pores in the form of small crystals carbonate sediments were divided into three classes. Class I (Fig. 3f), and pyrite can be occasionally seen. was mainly composed of siliciclastic carbonate mixed sedimentary rocks and siliciclastic-bearing carbonate 4.2.3 Dissolution mixed sedimentary rocks. It represented up to 55% with carbonate particles content of more than 50% (4, 7 area in Dissolution was well developed in the E s mixed silici- Fig. 2). Carbonate grains were mainly bioclasts, account- clastic–carbonate sediments, and it effectively improved ing for 65% (103 sampling points), followed by oolites and the quality of reservoirs with high proportion of siliciclastic arenes; Class II was mainly composed of carbonate silici- rocks. The dissolved minerals were mainly feldspar, clastic mixed sedimentary rocks and carbonate-bearing especially albite and K-feldspar (Fig. 3d, e). A small siliciclastic mixed sedimentary rocks, accounting for 30%, amount of carbonate minerals, such as dolomite and with siliciclastic particles content of more than 50% (2, 3 ankerite, were dissolved but this made little contribution to area in Fig. 2); Class III was uniformly with less than 50% pores. of siliciclastic grains and of carbonate grains (5, 6 and 8 area in Fig. 2). It was in the lowest content, only 4.3 Reservoir space features accounting for 16%. The interstitial material was mainly dolomite, followed by calcite and small amounts of The reservoir space of E s mixed siliciclastic–carbonate argillaceous matrix, which was well-sorted and sub-roun- sediments was dominated by residual primary intergranular ded to rounded. pores and dissolved pores, with minor intercrystalline porosity. Primary pores mainly included residual primary 4.2 Diagenetic features intergranular pores and intrafossil pores (Fig. 3a). Dis- solved pores mainly included intergranular dissolved pores 4.2.1 Compaction in feldspars and rock fragments (Fig. 3d) and intercrys- talline pores mainly included intercrystalline pores in From the contact relationship of grains in E s mixed sili- kaolinite (Fig. 3f). ciclastic–carbonate sediments, it showed that the com- paction was not strong, mainly composed of point-line 4.4 Petrophysical features contact (Fig. 3a, b). The porosity of E s mixed siliciclastic–carbonate sedi- 4.2.2 Precipitation of authigenic minerals ments ranged between 0.45% and 36%. In the 240 samples, 76% samples had a porosity of over 15% (Fig. 4). Per- There were numerous types of authigenic minerals formed meability mainly ranged between 0.014 and 11259 mD. in E s mixed siliciclastic–carbonate sediments. As with the Most samples had a permeability of over 10 mD, different proportions of siliciclastics and carbonate, it led accounting for 53% of the total samples (Fig. 5). to the differences of authigenic mineral content in the mixed siliciclastic–carbonate sediments. In the mixed sili- 4.5 Features of sedimentary microfacies ciclastic–carbonate sediments with a high proportion of carbonate, authigenic dolomite, calcite and other carbonate The E s in the study area was deposited in a continental minerals were in high proportions and authigenic clay was offshore lacustrine and near-source fan delta depositional in small proportions. However, in the mixed siliciclastic– environment (Guan et al. 2012; Zhang et al. 2015; Ni et al. carbonate sediments with a high proportion of siliciclastic 2013). The mixed siliciclastic–carbonate sediments were rocks, the authigenic minerals were dominated by kaolin- mainly developed in delta front sandbars and shallow ite, illite, and quartz, and the authigenic carbonate minerals lacustrine underwater uplift beach bars, followed by delta were in minor amounts. front underwater distributary channels. Front sandbars Among authigenic carbonate minerals, dolomite made were divided into mouth bar and distal bar microfacies with up the largest share, followed by calcite; in addition, there reverse grain size grading and funnel-shaped gamma-ray were minor amounts of ankerite and ferroan calcite. The (GR) curves, but the former showed lower GR curves. occurrence states of dolomites were a pore liner (Fig. 3a), Underwater uplift beach bars were characterized by fine pore fillings (Fig. 3a, c) and replacement particles (Fig. 3d, grain size, good sorting, low content of matrix and micrite e). Calcite mainly occurred as local replacement particles. and a box-shaped GR curve. Underwater distributary Authigenic clay minerals included kaolinite (Fig. 3f) and a channels abruptly contacted with underlying strata, with small amount of illite (Fig. 3f). Authigenic quartz was coarse grain size at the bottom and minor gravel (Fig. 6). 123 54 Pet. Sci. (2017) 14:50–60 (a) (b) Dol Dol Dol P -Primary intergranular pore P -Intrafossil pores 1 2 Dol -Pore-lining dolomite Dol -Pore filling dolomite F -Yellow fluorescence F -Green fluorescence 1 2 1 2 (c) (d) Dol Dol Dol P-Dissolution pores f-Dissolution feldspar residual Dol-Pore filling dolomite Dol-Pore filling dolomite (e) (f) Kao Dol P-Dissolution pores f-Dissolution feldspar residual Dol-Pore filling dolomite Q-Authigenic quartz Kao-Kaolinite I-Illite Fig. 3 Photomicrographs of a residual intergranular primary pore dolomite replaces feldspar, feldspar (EPMA: Na O: 0.2%, K O: 2 2 and intrafossil pores, pore-lining dolomite and filling dolomite, point- 16.5%, Al O : 18.3%, SiO : 64.6%) dissolution, Well HD5, 2 3 2 line contact, Well HD2, 3762.6 m, polarized light. b Two phases of 3382.1 m, polarized light. e multiphased authigenic dolomite, hydrocarbon charging in intergranular dissolved pores and residual dolomite replaces feldspar, feldspar dissolution, Well HD5, intergranular primary pore, Well HD2, 3774.33 m, fluorescence 3382.1 m, Cathodoluminescence. f Kaolinite, authigenic quartz, illite, microscope. c Pore filling dolomite, pore is poorly developed, Well Well HD5, 3486.5 m, Scanning electron microscope HD2, 3774.33 m, polarized light. d Multiphased authigenic dolomite, 4.6 Features of high-quality reservoirs According to the sedimentary microfacies and the statistics of components in the 156 samples, the content of silici- clastic particles decreased from 83% to 18% from delta The reservoirs with porosity of [15% and permeability of front mouth bar—distal bar—shallow lake underwater [10 mD were generally referred to as high-quality reser- uplift beach bar facies. voirs in this paper. 123 Pet. Sci. (2017) 14:50–60 55 with minor authigenic calcite. The mixed siliciclastic– carbonate sediments with pore-filling dolomite (Fig. 3c) and calcite were poor in physical properties. Dissolution 30 was common in Class II mixed siliciclastic–carbonate sediments. Through comparing the reservoir space of high- quality reservoirs and poor-quality reservoirs, it can be 20 seen that Class I mixed siliciclastic–carbonate sediments were dominated by primary porosity, such as intrafossil pores, followed by residual intergranular primary pores (Fig. 3a), while Class II mixed siliciclastic–carbonate sediments were dominated by residual intergranular pri- mary and dissolved porosity. <10 <15 <25 <30 Porosity, % 5 Discussion Fig. 4 Porosity distribution histogram of E s mixed siliciclastic– 5.1 Genesis of primary pore development carbonate sediments 30 Primary pores were pervasive in high-quality mixed sili- ciclastic–carbonate sedimentary reservoirs, especially in the reservoirs of Class I mixed siliciclastic–carbonate sediments. According to statistics of the microscopic pore type and plane porosity of the 87 cast thin sections of Class I mixed siliciclastic–carbonate sediments and 20 cast thin sections of Class II mixed siliciclastic–carbonate sedi- ments, the primary plane porosity of Class I accounted for 90% of the total, while the primary plane porosity of Class II accounted for 42% of the total. The sedimentary microfacies of different types of mixed siliciclastic–car- bonate sediments indicate that rocks were formed due to K<0.1 K<1 K<10 K<50 K<500 K<2000 the mixed deposition of the siliciclastic grains and matrix Permeability, mD in fan delta facies, and the carbonate particles and micrite deposited in the lacustrine facies. Carbonate particles Fig. 5 Permeability distribution histogram of E s mixed siliciclastic– mainly occurred in lacustrine high-energy underwater carbonate sediments beach bars far away from terrigenous provenance, so that The sedimentary microfacies and the corresponding 106 Class I mixed sediments with low micrite content and high groups of petrophysical data of the coring interval in Well primary intergranular porosity were well developed, HD2 and HD5, as well as the petrophysical data of the 50 whereas terrigenous clastics were common in the mouth sidewall cores of the other wells showed that the mixed bars near terrigenous provenance, so that Class II mixed siliciclastic–carbonate sediments developed in mouth bar, sediments with low matrix content were developed. distributary river channel, and underwater beach bar Sixty-four percentage of carbonate particles were bio- microfacies had good physical properties, but high-quality clasts. The bioclastic content and reservoir porosity showed reservoirs were basically not developed in the other a positive correlation in the study area (Fig. 8), since a microfacies (Fig. 7). Seventy-six percentage of the high- large amount of intrafossil pores were formed due to bio- quality reservoirs were developed in Class I mixed silici- logical decay. Most intrafossil pores were well preserved clastic–carbonate sediments, and their porosity and bio- during burial process, so intrafossil pores are well devel- clastic content had good positive correlation (Fig. 8). oped in rocks (Fig. 3a). Thus, the bioclasts in the high- Seventeen percentage of the high-quality reservoirs quality reservoirs in Class I mixed sediments contributed occurred in Class II mixed siliciclastic–carbonate sedi- largely to the primary porosity. ments, and only 7% high-quality reservoirs occurred in Pore-lining dolomites were common in high-quality Class III mixed siliciclastic–carbonate sediments. For dia- reservoirs, and they represented the formation features of genetic features, the vast majority of high-quality reser- vadose zone–phreatic zone as indicated by blade- and voirs were composed of pore-lining dolomite (Fig. 3a), overhang-shaped distribution features. The analytical Frequency, % Frequency, % 56 Pet. Sci. (2017) 14:50–60 GR LLD 0 180 Porosity Lithological Reservoir 0.2 100 Permeability Lithology Depth, m Microfacies description types DEN LLS 040 0.1 1000 2.2 2.7 0.2 100 Sand-bearing Beach bar bioclastic dolomite Dolomitic Distal bar fine sandstone Sand-bearing bioclastic Beach bar dolomite Dolomitic Mouth bar moderate sandstone Underwater Dolomitic distributary coarse sandstone channel Dolomitic moderate Mouth bar sandstone Dolomitic Distal bar fine sandstone Fig. 6 Sedimentary microfacies of HD5 mixed siliciclastic–carbonate sedimentary interval (3360–3430 m) results of isotopic temperatures (Table 1) showed that depth of less than 1700 m and the pore-lining dolomites pore-lining dolomite was formed at a temperature of were formed early. It is indicated by the microscopic 29–83C; together with the geothermal gradient of this observation of early pore-lining dolomite development in region (Liu et al. 2012), it is inferred that the pore-lining reservoirs that the compaction was not strong. Under the dolomite in stage 1 (the earliest stage) was formed at a burial condition of 4000 m, the point-line contact in grains paleoburial depth of less than 150 m and the liner dolomite was ubiquitously seen and the primary pores were well in stage 3 (the latest stage) was formed at a paleoburial developed (Fig. 3a). By comparing the mixed siliciclastic– 123 Pet. Sci. (2017) 14:50–60 57 5.2 Genesis of dissolved pore development The dissolved pores were well developed in E s Class II mixed siliciclastic–carbonate sediments. The statistics of the pore types and content of 20 cast thin sections showed that the plane porosity of dissolved pores accounted for 58%. The crystal optical features of dissolved minerals showed that the dissolved minerals were mainly feldspar. 10 Furthermore, the microprobe component analysis results of erosion remnants confirmed that the dissolved minerals were mainly albite and K-feldspar (Table 2). In addition, the authigenic clay minerals were kaolinite and illite, Natural levee Crevasse Mouth bar Distributary Underwater indicating that dissolution took place under a K-rich con- splay channel beach bar dition, resulting in the further transformation from kaolinite Fig. 7 Physical properties of different sedimentary microfacies of to illite (Zhang et al. 2007). The inclusion temperature of E s mixed siliciclastic–carbonate sediments the authigenic quartz in mixed sedimentary reservoirs ranged between 122–143C, and the authigenic quartz was formed due to the dissolution of feldspar. Thus, it is 35 inferred that the dissolution of feldspar took place from late middle diagenetic stage to early epidiagenetic stage. The temperature ranges coincided with the temperature ranges of organic matter maturity stage, indicating that the abun- dant acidic fluids were discharged during organic matter evolution which had created conditions for the formation of dissolved pores in feldspar (Meng et al. 2010; Cao et al. 2014). 5.3 Early phase and multiphased hydrocarbon charging on pores 0 102030405060 Bioclastic content, % The microscopic fluorescence features of E s mixed sedi- ments reflected multistage hydrocarbon charging features. Fig. 8 Relation between the bioclastic content and porosity of For example, residual primary pores and intragranular E s mixed siliciclastic–carbonate sediments dissolved pores had two types of completely different flu- orescence, indicating at least two stages of hydrocarbon carbonate sediments with and without pore-lining in early stage, it can be seen that the rocks without development of charging. The early stage residual intergranular primary pore showed yellow fluorescence, and the late stage pore-lining in early stage mostly represented line contact, showed green fluorescence, which was mainly from the with low primary porosity (Fig. 3c). Therefore, the for- mation of the pore-lining dolomites in early stage effec- dissolved pores in oolite (Fig. 3b). Hydrocarbon compo- nents were detected in the inclusions in temperature range tively weakened the destruction of compaction on pores and was favorable to the preservation of intergranular of 73–87C and 119–129C with laser Raman (Table 3), indicating at least two stages of hydrocarbon charging. primary pores. Table 1 C and O isotope distribution of the pore-lining dolomite in E s mixed siliciclastic–carbonate sediments 13 18 Well Well depth, m Sample attribute d C PDB, % d O PDB, % Formation Formation buried temperature, C depth, m HD2 3382.1 Pore-lining dolomite in Stage 1 4.7 -0.76 29.4 126.70 HD2 3762.6 Pore-lining dolomite in Stage 2 5.41 -3.99 47.3 636.58 HD5 3375.06 Pore-lining dolomite in Stage 2 1.88 -4.01 47.4 639.99 HD5 3380.25 Pore-lining dolomite in Stage 2 2.02 -4.97 53.3 807.78 HD5 3375.65 Pore-lining dolomite in Stage 3 -0.42 -9.35 83.3 1666.86 Porosity, % Porosity, % 58 Pet. Sci. (2017) 14:50–60 Table 2 Probe composition distribution of the dissolved feldspar remnants in E s mixed siliciclastic–carbonate sediments Well Well depth, m NaOKOCr O Al O CaO MnO MgO SiO FeO NiO TiO Mineral 2 2 2 3 2 3 2 2 HD5 3340.8 0.6 14.0 0.0 19.1 0.4 0.8 0.8 62.0 2.0 0.1 0.2 K-feldspar HD5 3367.5 0.3 16.1 0.0 18.7 0.0 0.1 0.1 64.4 0.3 0.0 0.0 K-feldspar HE3 3321.4 0.8 15.7 0.1 17.9 0.0 0.0 0.0 65.4 0.1 0.0 0.0 K-feldspar HE3 3321.4 3.5 12.0 0.0 18.3 0.1 0.0 0.0 65.9 0.2 0.0 0.0 K-feldspar HE3 3320.8 0.4 16.9 0.0 18.5 0.0 0.0 0.0 64.1 0.1 0.0 0.0 K-feldspar HD2 3324.4 0.9 15.8 0.0 19.4 0.0 0.0 0.0 63.9 0.0 0.0 0.0 K-feldspar HD2 3324.4 0.8 15.9 0.0 19.2 0.0 0.0 0.0 64.0 0.0 0.1 0.0 K-feldspar HD2 3326 0.4 16.3 0.0 18.0 0.0 0.0 0.0 65.2 0.1 0.0 0.0 K-feldspar Z13 3762.6 11.6 0.0 0.0 19.2 0.0 0.0 0.0 69.2 0.0 0.0 0.0 Albite Z13 3762.6 11.8 0.0 0.1 19.2 0.0 0.0 0.0 68.9 0.0 0.0 0.0 Albite Z13 3762.6 11.4 0.1 0.0 19.1 0.0 0.0 0.0 69.4 0.0 0.0 0.0 Albite Z13 3762.6 11.6 0.1 0.0 19.2 0.0 0.0 0.0 69.1 0.0 0.0 0.0 Albite BZ3 3779.2 11.7 0.1 0.0 19.3 0.2 0.0 0.0 68.6 0.1 0.0 0.0 Albite BZ3 3779.2 11.5 0.0 0.0 19.1 0.2 0.0 0.0 69.2 0.0 0.0 0.0 Albite BZ3 3779.2 11.6 0.0 0.0 19.4 0.1 0.0 0.0 68.9 0.0 0.0 0.0 Albite Table 3 Gas phase components of the inclusions in E s mixed siliciclastic–carbonate sedimentary reservoirs Well Well depth, m Gas phase, % Host minerals Homogenization temperature, C CO HSCH N H Total 2 2 4 2 2 HD5 3382.1 0 0 35.7 0 64.3 100.0 Pore-lining dolomite of Stage 3 119 HD5 3370.05 0 16.1 20.8 63.1 0 100.0 Filling dolomite within oolite 73 HD5 3382.1 0 0 9.5 90.5 0 100.0 Pore-lining dolomite within 87 intergranular pores HD5 3383.1 78.1 0 21.9 0 0 100.0 Filling dolomite within 129 intergranular pores HD2 3454.98 51.2 0 7.3 41.5 0 100.0 Quartz enlarging 122 Combined with the paleogeothermal gradient in the study sedimentary reservoir rocks are mainly developed in Class area, it is inferred that the reservoirs were buried at less I, followed by Class II. The development of the high- than 1500 m when there was hydrocarbon charging at the quality reservoirs of Class I siliciclastic–carbonate sedi- earliest time. Generally, the early hydrocarbon charging ments was mainly controlled by a high-energy depositional can inhibit cementation and also reduced further com- environment, high bioclastic content and pore-lining paction. Thus, the pores in reservoirs were effectively dolomite and hydrocarbon charging in the early stage. preserved (Meng et al. 2010; Cao et al. 2014). Primary pores were developed in the underwater uplift beach bars with strong hydrodynamic conditions and low micrite content. Intrafossil pores were common due to soft 6 Conclusions biological decay, forming the main reservoir space of the high-quality reservoir rocks of Class I. The development of The E s high-quality mixed siliciclastic–carbonate sedi- early stage pore-lining dolomite effectively weakened the mentary reservoirs in the central Bohai Sea were deposited destruction of mechanical compaction on pores. The in a fan delta-lacustrine environment. The rocks were hydrocarbon charging in the early stage effectively pre- formed due to the mixed deposition of the siliciclastic served reservoir pores. The development of the high- material in fan deltas and carbonate particles deposited in quality reservoirs of Class II mixed siliciclastic–carbonate lacustrine environments. The mixed sediment content of sediments was mainly controlled by high-energy deposi- the carbonates gradually increased from a near provenance tional environments, feldspar dissolution, pore-lining region to lacustrine underwater high-energy beach bars. dolomite and hydrocarbon charging in the early stage. 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Features and genesis of Paleogene high-quality reservoirs in lacustrine mixed siliciclastic–carbonate sediments, central Bohai Sea, China

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Earth Sciences; Mineral Resources; Industrial Chemistry/Chemical Engineering; Industrial and Production Engineering; Energy Economics
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Pet. Sci. (2017) 14:50–60 DOI 10.1007/s12182-016-0147-9 OR IGINAL PAPER Features and genesis of Paleogene high-quality reservoirs in lacustrine mixed siliciclastic–carbonate sediments, central Bohai Sea, China 1,2 1 1 1 • • • • Zheng-Xiang Lu¨ Shun-Li Zhang Chao Yin Hai-Long Meng 1 1 Xiu-Zhang Song Jian Zhang Received: 25 February 2016 / Published online: 21 January 2017 The Author(s) 2017. This article is published with open access at Springerlink.com Abstract The characteristics and formation mechanisms early stage, and oil emplacement has further led to the of the mixed siliciclastic–carbonate reservoirs of the preservation of good reservoir quality. Paleogene Shahejie Formation in the central Bohai Sea were examined based on polarized light microscopy and Keywords High-quality reservoirs  Mixed sediments scanning electron microscopy observations, X-ray diffrac- Paleogene Bohai Sea tometry, carbon and oxygen stable isotope geochemistry, and integrated fluid inclusion analysis. High-quality reservoirs are mainly distributed in Type I and Type II 1 Introduction mixed siliciclastic–carbonate sediments, and the dominant pore types include residual primary intergranular pores and In addition to carbonate and clastic reservoir rock types, intrafossil pores, feldspar dissolution pores mainly devel- magmatic, metamorphic, shale and mixed siliciclastic– oped in Type II sediments. Type I mixed sediments are carbonate sedimentary reservoirs can also be considered as characterized by precipitation of early pore-lining dolo- important targets for oil and gas exploration and develop- mite, relatively weak mechanical compaction during deep ment (Ge et al. 2011; Tong et al. 2012; Xiao et al. 2015; burial, and the occurrence of abundant oil inclusions in Palermo et al. 2008). The concept of ‘‘mixed sediments’’ high-quality reservoirs. Microfacies played a critical role in was firstly proposed by Mount (1984) and is commonly the formation of the mixed reservoirs, and high-quality referred to as sediments that are composed of mixtures of reservoirs are commonly found in high-energy environ- siliciclastic and carbonate material (including allochemical ments, such as fan delta underwater distributary channels, particles) (Lubeseder et al. 2009; Brandano et al. 2010;Xu mouth bars, and submarine uplift beach bars. Abundant et al. 2014). Many Chinese and foreign scholars have made intrafossil pores were formed by bioclastic decay, and in-depth studies of the formation mechanisms of this type secondary pores due to feldspar dissolution further enhance of sediment and suggested that it can be developed in both reservoir porosity. Mechanical compaction was inhibited marine and lacustrine environments. Influenced by sea by the precipitation of pore-lining dolomite formed during (lake)-level fluctuations, structural changes, storm, current and tidal actions, mixed siliciclastic–carbonate sediments are widely distributed in transitional marine-terrestrial, continental shelf, and slope environments (Garcıa-Hidalgo & Shun-Li Zhang et al. 2007; Zonneveld et al. 2012). Under certain condi- 1205799554@qq.com tions, mixed siliciclastic–carbonate sediments may be rich College of Energy Resources, Chengdu University of in oil and gas. For example, hydrocarbon accumulations Technology, Chengdu 610059, Sichuan, China have been discovered in the high-quality mixed siliciclas- State Key Laboratory of Oil-Gas Reservoirs Geology and tic–carbonate reservoirs in China, such as the Bohai Bay Exploitation, Chengdu University of Technology, Basin, the Qaidam Basin and the Sichuan Basin (Feng et al. Chengdu 610059, Sichuan, China 2011a, b, 2013; Zhang et al. 2006; Liu et al. 2011; Garcı ´a- Hidalgo et al. 2007). Although carbonate and clastic Edited by Jie Hao 123 Pet. Sci. (2017) 14:50–60 51 reservoirs have been the subject of intensive study by a Kongdian Formation (E k) and underlies the Dongying large number of researchers, mixed siliciclastic–carbonate Formation (E d). Because economically significant hydro- reservoirs have received less attention. Previously, studies carbon accumulations have been found in the mixed of mixed siliciclastic–carbonate reservoirs have mainly reservoirs, the mixed siliciclastic–carbonate reservoirs focused on petrography, structure, classification, the have been the focus of study in recent years (Wang et al. establishment of depositional models (Caracciolo et al. 2015). 2012; Sha 2001; Zand-Moghadam et al. 2013; Zonneveld et al. 2012; Ma and Liu 2003), and the reconstruction of the sedimentary environment on the basis of sequence 3 Samples and experimental methodology stratigraphy, sea level change and paleoclimate (Anan 2014; Campbell 2005; Moissette et al. 2010). However, the In this study, 240 mixed siliciclastic–carbonate sediment microscopic features and the formation mechanisms of samples from 12 wells in the Shijiutuo Uplift in the central high-quality reservoirs have not been well investigated, Bohai Bay Basin, such as Well HD2 and Well HD5, were which has restricted the exploration and development of selected for porosity and permeability measurements. The mixed siliciclastic–carbonate reservoirs. locations of sample wells are shown in Fig. 1. The The Bohai Bay Basin is an important petroliferous basin microscopic features, such as petrology, pore space types, in North China. In the Paleogene, steep slope zones were and diagenesis, were obtained from 122 thin sections with well developed and are represented by a series of high different physical properties. Multi-purpose thin sections steep fault noses (Lu 2005; Guan et al. 2012). The tec- were prepared with blue-dyed epoxy impregnation and tonically induced physiographic changes controlled the double-sided polishing. The mineral composition was distribution and areal extension of mixed siliciclastic–car- identified by polarized light microscopy, and X-ray bonate sediments. For example, typical mixed sediments diffraction (XRD) analyses were carried out on twenty-two composed of lacustrine carbonate and siliciclastic material bulk samples and \2 lm size fractions using a Rigaku are widely distributed in the Shijiutuo Uplift in the central DMAX-3C diffractometer. The chemical composition of basin, and a large number of high-quality reservoirs are grain-coating and dissolved minerals was determined developed in them (Liu et al. 2011; Song et al. 2013). quantitatively by electron microprobe analysis (EMPA) Statistics suggest that reservoir quality is one of the key using a Shimadzu EPMA-1720 and a JEOL JXA-8100 controls on prospectivity during petroleum exploration and electron microprobes (operating conditions: 15 kV accel- production. The study of the characteristics and formation erating voltage, 10 mA current, 1 lm beam diameter). mechanisms of mixed siliciclastic–carbonate reservoirs is Fifteen double-thickness polished thin sections were therefore of significant importance for guiding the oil and selected for microthermometric measurements. Homoge- gas exploration and production in the Bohai Sea. The nization temperatures were measured using a Linkam purpose of this paper is to compare different types of mixed THMS-600 heating/cooling stage. Only primary fluid siliciclastic–carbonate reservoirs, to describe the main inclusions with both aqueous and hydrocarbon phases were features of high-quality reservoirs, and to determine the selected from authigenic minerals to determine their min- formation mechanisms of high-quality reservoirs by inte- imum precipitation temperatures (Liu et al. 2005;Lu ¨ et al. grating geological and geochemical data. 2015; Guo et al. 2012; Tian et al. 2016). In-situ carbon and oxygen isotope analysis was performed using an Nd:YAG laser microprobe. Laser probe microsampling of C and O 2 Geological setting from carbonate cements for isotopic analysis was achieved by focusing a laser beam with a wavelength of 1064 nm The Bohai Bay Basin is a Cenozoic rift basin superimposed and a diameter of 20 lm onto a sample situated in a vac- on the Paleozoic basement of the North China platform (Lu uum chamber to ablate a small area on the sample and 2005). The study area is located in the Shijiutuo Uplift in liberate CO gas. After purification, the CO gas was led 2 2 the central Bohai Bay Basin and bounded by two large directly into a Finnigan MAT 252 mass spectrometer for hydrocarbon generation sags—Bozhong Sag and Qin’nan isotopic analysis. After obtaining the isotopic values, the Sag (Fig. 1). The hydrocarbon accumulation condition is dolomite formation temperature (T) was calculated using excellent with high-quality source rocks and a series of the empirical formula proposed by Hu et al. (2012): Paleogene high steep fault nose traps developed (Guan et al. 2012). The mixed sedimentary reservoirs in the study T ¼ 16:5  4:3ðdC  dWÞþ 0:14ðdC  dWÞðdC  dWÞ area are mainly developed in the Paleogene Shahejie For- where dC is the d O of a dolomite precipitate, and dW is mation (E s). The Shahejie Formation is 300–400 m thick the d O of parent water. with burial depths [3000 m and conformably overlies the 123 52 Pet. Sci. (2017) 14:50–60 0 30 km Bohai Bay Basin China Qinnan Uplift Qinnan Sag Matouying Uplift HE 5 HE 2 HD 33 HE 3 Z13 HD 2 Shijiutuo Uplift HD 5 Nanbao Sag BZ 3 Shaleitian Uplift Shanan Sag HD 5 Uplift Coastline Major Fault Study Area Well Fig. 1 Location map and tectonic elements of the central Bohai Sea The timing of feldspar dissolution was mainly deter- Siliciclastic mined based on the fluid inclusion temperatures of the dissolution products (authigenic quartz). Sixty-two samples were observed under a DM4500P fluorescence microscope in order to identify possible petroleum inclusions. Fifteen 2 3 inclusions were also examined using a Renishaw inVia 50 50 laser Raman microprobe with a wavelength of 514.5 nm to document the existence of hydrocarbons. 5 6 The mixed siliciclastic–carbonate sediments were deposited in a fan delta environment (Guan et al. 2012; 7 10 Zhang et al. 2015; Ni et al. 2013). In order to illustrate the relationship between petrophysical properties and sedi- mentary microfacies, the sedimentary facies were identified Carbonate Mudstone by analyzing rock textures and well log data for Well HD2 (micritic carbonate) and Well HD5, in which core porosity and permeability Fig. 2 Rock types of E s mixed siliciclastic–carbonate sediments. 1: were measured. sand (gravel) rock, 2: carbonate siliciclastic mixed sedimentary rocks, 3: carbonate-bearing siliciclastic mixed sedimentary rocks, 4: silici- clastic carbonate mixed sedimentary rocks, 5: carbonate/siliciclastic mixed sedimentary rocks, 6: carbonate-bearing argillaceous silici- clastic mixed sedimentary rocks, 7: siliciclastic-bearing carbonate 4 Results mixed sedimentary rocks, 8: siliciclastic-bearing micrite carbonate mixed sedimentary rocks, 9: carbonate, 10: mudstone (micritic 4.1 Rock types carbonate) The E s mixed sediments are composed of siliciclastic and 2 mixed sediments, the content of the former two was, lacustrine carbonate rocks. For the siliciclastic grains, respectively, not less than 10%, while the latter two carbonate grains, matrix, and micrite that constituted the accounted for less than 50%. The identification results of Xihe Uplift Xinanzhuang Uplift Qikou Sag Liaoxinan Uplift Liaoxi Uplift Liaozhong Sag Bozhong Sag Bodong Lower Uplift Bodong Sag Miaoxibei Uplift Pet. Sci. (2017) 14:50–60 53 122 thin sections show that (Fig. 2)E s mixed siliciclastic– distributed in the pores in the form of small crystals carbonate sediments were divided into three classes. Class I (Fig. 3f), and pyrite can be occasionally seen. was mainly composed of siliciclastic carbonate mixed sedimentary rocks and siliciclastic-bearing carbonate 4.2.3 Dissolution mixed sedimentary rocks. It represented up to 55% with carbonate particles content of more than 50% (4, 7 area in Dissolution was well developed in the E s mixed silici- Fig. 2). Carbonate grains were mainly bioclasts, account- clastic–carbonate sediments, and it effectively improved ing for 65% (103 sampling points), followed by oolites and the quality of reservoirs with high proportion of siliciclastic arenes; Class II was mainly composed of carbonate silici- rocks. The dissolved minerals were mainly feldspar, clastic mixed sedimentary rocks and carbonate-bearing especially albite and K-feldspar (Fig. 3d, e). A small siliciclastic mixed sedimentary rocks, accounting for 30%, amount of carbonate minerals, such as dolomite and with siliciclastic particles content of more than 50% (2, 3 ankerite, were dissolved but this made little contribution to area in Fig. 2); Class III was uniformly with less than 50% pores. of siliciclastic grains and of carbonate grains (5, 6 and 8 area in Fig. 2). It was in the lowest content, only 4.3 Reservoir space features accounting for 16%. The interstitial material was mainly dolomite, followed by calcite and small amounts of The reservoir space of E s mixed siliciclastic–carbonate argillaceous matrix, which was well-sorted and sub-roun- sediments was dominated by residual primary intergranular ded to rounded. pores and dissolved pores, with minor intercrystalline porosity. Primary pores mainly included residual primary 4.2 Diagenetic features intergranular pores and intrafossil pores (Fig. 3a). Dis- solved pores mainly included intergranular dissolved pores 4.2.1 Compaction in feldspars and rock fragments (Fig. 3d) and intercrys- talline pores mainly included intercrystalline pores in From the contact relationship of grains in E s mixed sili- kaolinite (Fig. 3f). ciclastic–carbonate sediments, it showed that the com- paction was not strong, mainly composed of point-line 4.4 Petrophysical features contact (Fig. 3a, b). The porosity of E s mixed siliciclastic–carbonate sedi- 4.2.2 Precipitation of authigenic minerals ments ranged between 0.45% and 36%. In the 240 samples, 76% samples had a porosity of over 15% (Fig. 4). Per- There were numerous types of authigenic minerals formed meability mainly ranged between 0.014 and 11259 mD. in E s mixed siliciclastic–carbonate sediments. As with the Most samples had a permeability of over 10 mD, different proportions of siliciclastics and carbonate, it led accounting for 53% of the total samples (Fig. 5). to the differences of authigenic mineral content in the mixed siliciclastic–carbonate sediments. In the mixed sili- 4.5 Features of sedimentary microfacies ciclastic–carbonate sediments with a high proportion of carbonate, authigenic dolomite, calcite and other carbonate The E s in the study area was deposited in a continental minerals were in high proportions and authigenic clay was offshore lacustrine and near-source fan delta depositional in small proportions. However, in the mixed siliciclastic– environment (Guan et al. 2012; Zhang et al. 2015; Ni et al. carbonate sediments with a high proportion of siliciclastic 2013). The mixed siliciclastic–carbonate sediments were rocks, the authigenic minerals were dominated by kaolin- mainly developed in delta front sandbars and shallow ite, illite, and quartz, and the authigenic carbonate minerals lacustrine underwater uplift beach bars, followed by delta were in minor amounts. front underwater distributary channels. Front sandbars Among authigenic carbonate minerals, dolomite made were divided into mouth bar and distal bar microfacies with up the largest share, followed by calcite; in addition, there reverse grain size grading and funnel-shaped gamma-ray were minor amounts of ankerite and ferroan calcite. The (GR) curves, but the former showed lower GR curves. occurrence states of dolomites were a pore liner (Fig. 3a), Underwater uplift beach bars were characterized by fine pore fillings (Fig. 3a, c) and replacement particles (Fig. 3d, grain size, good sorting, low content of matrix and micrite e). Calcite mainly occurred as local replacement particles. and a box-shaped GR curve. Underwater distributary Authigenic clay minerals included kaolinite (Fig. 3f) and a channels abruptly contacted with underlying strata, with small amount of illite (Fig. 3f). Authigenic quartz was coarse grain size at the bottom and minor gravel (Fig. 6). 123 54 Pet. Sci. (2017) 14:50–60 (a) (b) Dol Dol Dol P -Primary intergranular pore P -Intrafossil pores 1 2 Dol -Pore-lining dolomite Dol -Pore filling dolomite F -Yellow fluorescence F -Green fluorescence 1 2 1 2 (c) (d) Dol Dol Dol P-Dissolution pores f-Dissolution feldspar residual Dol-Pore filling dolomite Dol-Pore filling dolomite (e) (f) Kao Dol P-Dissolution pores f-Dissolution feldspar residual Dol-Pore filling dolomite Q-Authigenic quartz Kao-Kaolinite I-Illite Fig. 3 Photomicrographs of a residual intergranular primary pore dolomite replaces feldspar, feldspar (EPMA: Na O: 0.2%, K O: 2 2 and intrafossil pores, pore-lining dolomite and filling dolomite, point- 16.5%, Al O : 18.3%, SiO : 64.6%) dissolution, Well HD5, 2 3 2 line contact, Well HD2, 3762.6 m, polarized light. b Two phases of 3382.1 m, polarized light. e multiphased authigenic dolomite, hydrocarbon charging in intergranular dissolved pores and residual dolomite replaces feldspar, feldspar dissolution, Well HD5, intergranular primary pore, Well HD2, 3774.33 m, fluorescence 3382.1 m, Cathodoluminescence. f Kaolinite, authigenic quartz, illite, microscope. c Pore filling dolomite, pore is poorly developed, Well Well HD5, 3486.5 m, Scanning electron microscope HD2, 3774.33 m, polarized light. d Multiphased authigenic dolomite, 4.6 Features of high-quality reservoirs According to the sedimentary microfacies and the statistics of components in the 156 samples, the content of silici- clastic particles decreased from 83% to 18% from delta The reservoirs with porosity of [15% and permeability of front mouth bar—distal bar—shallow lake underwater [10 mD were generally referred to as high-quality reser- uplift beach bar facies. voirs in this paper. 123 Pet. Sci. (2017) 14:50–60 55 with minor authigenic calcite. The mixed siliciclastic– carbonate sediments with pore-filling dolomite (Fig. 3c) and calcite were poor in physical properties. Dissolution 30 was common in Class II mixed siliciclastic–carbonate sediments. Through comparing the reservoir space of high- quality reservoirs and poor-quality reservoirs, it can be 20 seen that Class I mixed siliciclastic–carbonate sediments were dominated by primary porosity, such as intrafossil pores, followed by residual intergranular primary pores (Fig. 3a), while Class II mixed siliciclastic–carbonate sediments were dominated by residual intergranular pri- mary and dissolved porosity. <10 <15 <25 <30 Porosity, % 5 Discussion Fig. 4 Porosity distribution histogram of E s mixed siliciclastic– 5.1 Genesis of primary pore development carbonate sediments 30 Primary pores were pervasive in high-quality mixed sili- ciclastic–carbonate sedimentary reservoirs, especially in the reservoirs of Class I mixed siliciclastic–carbonate sediments. According to statistics of the microscopic pore type and plane porosity of the 87 cast thin sections of Class I mixed siliciclastic–carbonate sediments and 20 cast thin sections of Class II mixed siliciclastic–carbonate sedi- ments, the primary plane porosity of Class I accounted for 90% of the total, while the primary plane porosity of Class II accounted for 42% of the total. The sedimentary microfacies of different types of mixed siliciclastic–car- bonate sediments indicate that rocks were formed due to K<0.1 K<1 K<10 K<50 K<500 K<2000 the mixed deposition of the siliciclastic grains and matrix Permeability, mD in fan delta facies, and the carbonate particles and micrite deposited in the lacustrine facies. Carbonate particles Fig. 5 Permeability distribution histogram of E s mixed siliciclastic– mainly occurred in lacustrine high-energy underwater carbonate sediments beach bars far away from terrigenous provenance, so that The sedimentary microfacies and the corresponding 106 Class I mixed sediments with low micrite content and high groups of petrophysical data of the coring interval in Well primary intergranular porosity were well developed, HD2 and HD5, as well as the petrophysical data of the 50 whereas terrigenous clastics were common in the mouth sidewall cores of the other wells showed that the mixed bars near terrigenous provenance, so that Class II mixed siliciclastic–carbonate sediments developed in mouth bar, sediments with low matrix content were developed. distributary river channel, and underwater beach bar Sixty-four percentage of carbonate particles were bio- microfacies had good physical properties, but high-quality clasts. The bioclastic content and reservoir porosity showed reservoirs were basically not developed in the other a positive correlation in the study area (Fig. 8), since a microfacies (Fig. 7). Seventy-six percentage of the high- large amount of intrafossil pores were formed due to bio- quality reservoirs were developed in Class I mixed silici- logical decay. Most intrafossil pores were well preserved clastic–carbonate sediments, and their porosity and bio- during burial process, so intrafossil pores are well devel- clastic content had good positive correlation (Fig. 8). oped in rocks (Fig. 3a). Thus, the bioclasts in the high- Seventeen percentage of the high-quality reservoirs quality reservoirs in Class I mixed sediments contributed occurred in Class II mixed siliciclastic–carbonate sedi- largely to the primary porosity. ments, and only 7% high-quality reservoirs occurred in Pore-lining dolomites were common in high-quality Class III mixed siliciclastic–carbonate sediments. For dia- reservoirs, and they represented the formation features of genetic features, the vast majority of high-quality reser- vadose zone–phreatic zone as indicated by blade- and voirs were composed of pore-lining dolomite (Fig. 3a), overhang-shaped distribution features. The analytical Frequency, % Frequency, % 56 Pet. Sci. (2017) 14:50–60 GR LLD 0 180 Porosity Lithological Reservoir 0.2 100 Permeability Lithology Depth, m Microfacies description types DEN LLS 040 0.1 1000 2.2 2.7 0.2 100 Sand-bearing Beach bar bioclastic dolomite Dolomitic Distal bar fine sandstone Sand-bearing bioclastic Beach bar dolomite Dolomitic Mouth bar moderate sandstone Underwater Dolomitic distributary coarse sandstone channel Dolomitic moderate Mouth bar sandstone Dolomitic Distal bar fine sandstone Fig. 6 Sedimentary microfacies of HD5 mixed siliciclastic–carbonate sedimentary interval (3360–3430 m) results of isotopic temperatures (Table 1) showed that depth of less than 1700 m and the pore-lining dolomites pore-lining dolomite was formed at a temperature of were formed early. It is indicated by the microscopic 29–83C; together with the geothermal gradient of this observation of early pore-lining dolomite development in region (Liu et al. 2012), it is inferred that the pore-lining reservoirs that the compaction was not strong. Under the dolomite in stage 1 (the earliest stage) was formed at a burial condition of 4000 m, the point-line contact in grains paleoburial depth of less than 150 m and the liner dolomite was ubiquitously seen and the primary pores were well in stage 3 (the latest stage) was formed at a paleoburial developed (Fig. 3a). By comparing the mixed siliciclastic– 123 Pet. Sci. (2017) 14:50–60 57 5.2 Genesis of dissolved pore development The dissolved pores were well developed in E s Class II mixed siliciclastic–carbonate sediments. The statistics of the pore types and content of 20 cast thin sections showed that the plane porosity of dissolved pores accounted for 58%. The crystal optical features of dissolved minerals showed that the dissolved minerals were mainly feldspar. 10 Furthermore, the microprobe component analysis results of erosion remnants confirmed that the dissolved minerals were mainly albite and K-feldspar (Table 2). In addition, the authigenic clay minerals were kaolinite and illite, Natural levee Crevasse Mouth bar Distributary Underwater indicating that dissolution took place under a K-rich con- splay channel beach bar dition, resulting in the further transformation from kaolinite Fig. 7 Physical properties of different sedimentary microfacies of to illite (Zhang et al. 2007). The inclusion temperature of E s mixed siliciclastic–carbonate sediments the authigenic quartz in mixed sedimentary reservoirs ranged between 122–143C, and the authigenic quartz was formed due to the dissolution of feldspar. Thus, it is 35 inferred that the dissolution of feldspar took place from late middle diagenetic stage to early epidiagenetic stage. The temperature ranges coincided with the temperature ranges of organic matter maturity stage, indicating that the abun- dant acidic fluids were discharged during organic matter evolution which had created conditions for the formation of dissolved pores in feldspar (Meng et al. 2010; Cao et al. 2014). 5.3 Early phase and multiphased hydrocarbon charging on pores 0 102030405060 Bioclastic content, % The microscopic fluorescence features of E s mixed sedi- ments reflected multistage hydrocarbon charging features. Fig. 8 Relation between the bioclastic content and porosity of For example, residual primary pores and intragranular E s mixed siliciclastic–carbonate sediments dissolved pores had two types of completely different flu- orescence, indicating at least two stages of hydrocarbon carbonate sediments with and without pore-lining in early stage, it can be seen that the rocks without development of charging. The early stage residual intergranular primary pore showed yellow fluorescence, and the late stage pore-lining in early stage mostly represented line contact, showed green fluorescence, which was mainly from the with low primary porosity (Fig. 3c). Therefore, the for- mation of the pore-lining dolomites in early stage effec- dissolved pores in oolite (Fig. 3b). Hydrocarbon compo- nents were detected in the inclusions in temperature range tively weakened the destruction of compaction on pores and was favorable to the preservation of intergranular of 73–87C and 119–129C with laser Raman (Table 3), indicating at least two stages of hydrocarbon charging. primary pores. Table 1 C and O isotope distribution of the pore-lining dolomite in E s mixed siliciclastic–carbonate sediments 13 18 Well Well depth, m Sample attribute d C PDB, % d O PDB, % Formation Formation buried temperature, C depth, m HD2 3382.1 Pore-lining dolomite in Stage 1 4.7 -0.76 29.4 126.70 HD2 3762.6 Pore-lining dolomite in Stage 2 5.41 -3.99 47.3 636.58 HD5 3375.06 Pore-lining dolomite in Stage 2 1.88 -4.01 47.4 639.99 HD5 3380.25 Pore-lining dolomite in Stage 2 2.02 -4.97 53.3 807.78 HD5 3375.65 Pore-lining dolomite in Stage 3 -0.42 -9.35 83.3 1666.86 Porosity, % Porosity, % 58 Pet. Sci. (2017) 14:50–60 Table 2 Probe composition distribution of the dissolved feldspar remnants in E s mixed siliciclastic–carbonate sediments Well Well depth, m NaOKOCr O Al O CaO MnO MgO SiO FeO NiO TiO Mineral 2 2 2 3 2 3 2 2 HD5 3340.8 0.6 14.0 0.0 19.1 0.4 0.8 0.8 62.0 2.0 0.1 0.2 K-feldspar HD5 3367.5 0.3 16.1 0.0 18.7 0.0 0.1 0.1 64.4 0.3 0.0 0.0 K-feldspar HE3 3321.4 0.8 15.7 0.1 17.9 0.0 0.0 0.0 65.4 0.1 0.0 0.0 K-feldspar HE3 3321.4 3.5 12.0 0.0 18.3 0.1 0.0 0.0 65.9 0.2 0.0 0.0 K-feldspar HE3 3320.8 0.4 16.9 0.0 18.5 0.0 0.0 0.0 64.1 0.1 0.0 0.0 K-feldspar HD2 3324.4 0.9 15.8 0.0 19.4 0.0 0.0 0.0 63.9 0.0 0.0 0.0 K-feldspar HD2 3324.4 0.8 15.9 0.0 19.2 0.0 0.0 0.0 64.0 0.0 0.1 0.0 K-feldspar HD2 3326 0.4 16.3 0.0 18.0 0.0 0.0 0.0 65.2 0.1 0.0 0.0 K-feldspar Z13 3762.6 11.6 0.0 0.0 19.2 0.0 0.0 0.0 69.2 0.0 0.0 0.0 Albite Z13 3762.6 11.8 0.0 0.1 19.2 0.0 0.0 0.0 68.9 0.0 0.0 0.0 Albite Z13 3762.6 11.4 0.1 0.0 19.1 0.0 0.0 0.0 69.4 0.0 0.0 0.0 Albite Z13 3762.6 11.6 0.1 0.0 19.2 0.0 0.0 0.0 69.1 0.0 0.0 0.0 Albite BZ3 3779.2 11.7 0.1 0.0 19.3 0.2 0.0 0.0 68.6 0.1 0.0 0.0 Albite BZ3 3779.2 11.5 0.0 0.0 19.1 0.2 0.0 0.0 69.2 0.0 0.0 0.0 Albite BZ3 3779.2 11.6 0.0 0.0 19.4 0.1 0.0 0.0 68.9 0.0 0.0 0.0 Albite Table 3 Gas phase components of the inclusions in E s mixed siliciclastic–carbonate sedimentary reservoirs Well Well depth, m Gas phase, % Host minerals Homogenization temperature, C CO HSCH N H Total 2 2 4 2 2 HD5 3382.1 0 0 35.7 0 64.3 100.0 Pore-lining dolomite of Stage 3 119 HD5 3370.05 0 16.1 20.8 63.1 0 100.0 Filling dolomite within oolite 73 HD5 3382.1 0 0 9.5 90.5 0 100.0 Pore-lining dolomite within 87 intergranular pores HD5 3383.1 78.1 0 21.9 0 0 100.0 Filling dolomite within 129 intergranular pores HD2 3454.98 51.2 0 7.3 41.5 0 100.0 Quartz enlarging 122 Combined with the paleogeothermal gradient in the study sedimentary reservoir rocks are mainly developed in Class area, it is inferred that the reservoirs were buried at less I, followed by Class II. The development of the high- than 1500 m when there was hydrocarbon charging at the quality reservoirs of Class I siliciclastic–carbonate sedi- earliest time. Generally, the early hydrocarbon charging ments was mainly controlled by a high-energy depositional can inhibit cementation and also reduced further com- environment, high bioclastic content and pore-lining paction. Thus, the pores in reservoirs were effectively dolomite and hydrocarbon charging in the early stage. preserved (Meng et al. 2010; Cao et al. 2014). Primary pores were developed in the underwater uplift beach bars with strong hydrodynamic conditions and low micrite content. Intrafossil pores were common due to soft 6 Conclusions biological decay, forming the main reservoir space of the high-quality reservoir rocks of Class I. The development of The E s high-quality mixed siliciclastic–carbonate sedi- early stage pore-lining dolomite effectively weakened the mentary reservoirs in the central Bohai Sea were deposited destruction of mechanical compaction on pores. The in a fan delta-lacustrine environment. The rocks were hydrocarbon charging in the early stage effectively pre- formed due to the mixed deposition of the siliciclastic served reservoir pores. The development of the high- material in fan deltas and carbonate particles deposited in quality reservoirs of Class II mixed siliciclastic–carbonate lacustrine environments. The mixed sediment content of sediments was mainly controlled by high-energy deposi- the carbonates gradually increased from a near provenance tional environments, feldspar dissolution, pore-lining region to lacustrine underwater high-energy beach bars. dolomite and hydrocarbon charging in the early stage. 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