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Architecture mode, sedimentary evolution and controlling factors of deepwater turbidity channels: A case study of the M Oilfield in West Africa

Architecture mode, sedimentary evolution and controlling factors of deepwater turbidity channels:... Pet. Sci. (2017) 14:493–506 DOI 10.1007/s12182-017-0181-2 ORIGINAL PAPER Architecture mode, sedimentary evolution and controlling factors of deepwater turbidity channels: A case study of the M Oilfield in West Africa 1 1 1 1 • • • • Wen-Biao Zhang Tai-Zhong Duan Zhi-Qiang Liu Yan-Feng Liu 1 1 Lei Zhao Rui Xu Received: 25 October 2016 / Published online: 27 July 2017 The Author(s) 2017. This article is an open access publication Abstract Turbidity channels have been considered as one geological modeling in reservoir development and con- of the important types of deepwater reservoir, and the study tribute to efficient development of such reservoirs. of their architecture plays a key role in efficient develop- ment of an oil field. To better understand the reservoir Keywords Reservoir architecture  Turbidity channel architecture of the lower Congo Basin M oilfield, semi- Sedimentary evolution  Deep water  Shallow seismic quantitative–quantitative study on turbidity channel depo- Controlling factors sitional architecture patterns in the middle to lower slopes was conducted with the aid of abundant high quality materials (core, outcrop, logging and seismic data), 1 Introduction employing seismic stratigraphy, seismic sedimentology and sedimentary petrography methods. Then, its sedimen- Deep water channels are considered one of the significant tary evolution was analyzed accordingly. The results types of reservoir, sometimes containing rich oil and gas indicated that in the study area, grade 3 to grade 5 archi- resources. There have been studies abroad on their depo- tecture units were single channel, complex channel and sitional architecture, mainly focusing on architecture ele- channel systems, respectively. Single channel sinuosity is ment recognition and description through core and other negatively correlated with the slope, as internal grains observations. Achievements have been made by indepen- became finer and thickness became thinner from bottom to dent oil companies (IOCs) and institutions, with the top, axis to edge. The migration type of a single channel assistance of the rapid development of deepwater drilling, within one complex channel can be lateral migration and geophysics theory and sonar scanning, on sedimentary along paleocurrent migration horizontally, and lateral, configuration of deepwater turbidite channels. Domestic indented and swing stacking in section view. Based on researchers are doing similar studies using deepwater external morphological characteristics and boundaries, sediments in the South China Sea area. These studies and channel systems are comprised of a weakly confining type related achievements contribute a lot to reducing risks in and a non-confining type. The O73 channel system can be early-stage deepwater exploration (Heinio¨ and Davies divided into four complex channels named S1–S4, from 2007; Posamentier and Kolla 2003; Slatt 2006; Menard bottom to top, with gradually less incision and more 1955; Li et al. 2008; Wang et al. 2009; Deng et al. 2008;Lu¨ accretion. The study in this article will promote deeper et al. 2008). For continental slope areas in the Lower understanding of turbidity channel theory, guide 3D Congo Basin, West Africa, there have been studies of deepwater channel sedimentary patterns and the factors controlling them in terms of sedimentology and sequence & Wen-Biao Zhang stratigraphy. zwb.syky@sinopec.com However, with the gradual development of more deep- Petroleum Exploration and Production Research Institute, water oilfields, plenty of dynamic data reveal complex SINOPEC, Beijing 100083, China superimposition of single sand bodies inside channel sys- tems and various high-heterogeneity rock types in single Edited by Jie Hao 123 494 Pet. Sci. (2017) 14:493–506 sand bodies. This impedes further and more effective limited deepwater seismic resolution, making full-scale development of deepwater oilfields. Therefore, it is nec- analysis of channel architecture type and scale possible; essary to analyze configurations of both complex and sin- core data provide complementary information for research gle sand bodies to clarify single sand body distribution into inner filling models. Comprehensive use of available patterns, quantitative scales, superimposition and lithology information is a key method to study sedimentary inside single sand bodies. architecture. We took the deepwater M oilfield in West Africa as an example to study semi-quantitative–quantitative sedimen- tary configuration patterns and their spatial evolution in 3 Turbidite channels hierarchical division this article. Based on abundant drilling, core and high- quality seismic data, the study was carried out at three Several criteria were proposed for the hierarchical division different levels, including channel system, complex chan- of turbidite channel architecture. For example, Mutti and nel and single channel levels, employing methods such as Normark proposed a five-turbidite-facies scheme in 1987, core description, log recognition and seismic attribute sli- mainly based on the genetic type of sand bodies. Later, a ces. The study in this article may promote turbidite channel seven-turbidite-facies scheme was put forward by Zhao theory understanding and benefit 3-D geomodeling, mak- et al. (2012a, b) and Lin et al. (2013). In this division, they ing it useful in developing this type oilfield more gave more thought to hydrodynamic characteristics, sedi- efficiently. mentation and contact relations of the formation of channel sand bodies. In this article, we adopted the seven-turbidite- facies scheme to study sedimentary architecture patterns on 2 Geological background three levels, which are single channel, complex channel and channel system levels, to demonstrate the influence on The M oilfield is one of the most favorable exploration and reservoir distribution from macro perspectives (Table 1). development petroliferous basins located in the lower There are certain genetic connections between units of Congo Basin, with typical passive continental margin channels at different levels (Zhao et al. 2012a, b). The characteristics (Liu and Li 2009; Xiong et al. 2005; Kolla channel system is characterized by complex channels of et al. 2001). The research zone lies in the middle–lower different periods, while the formation of complex channels slope, between compressive and extensional zones, 186 km is subject to the migration patterns of single channels. away from Luanda, Angola. The architecture is not There are differences in the scale of architectural units at severely impaired by Cretaceous gypsum activities, and the different levels, thereby influencing the data type required main target layer is Oligocene. Its sedimentary type is for study. The architectural units of large-scale channel considered to be a deep water turbidite channel system systems are recognized mainly through seismic data under a regressive background with a water depth of (comprising of seismic facies and seismic reflection 1400–1800 m (Fig. 1). There are 18 wells in the research structures), while that of the intermediate scale (complex zone of the Oligocene O73 reservoir, and they are char- channel) can be recognized also through seismic data acterized by a core depth of 168 m; average well distance (strata slicing). With regard to the small scale (single more than 1000 m; dominant seismic frequency of 35 Hz; channels), drilling data (coring and well logging data) and and sand recognition 15–20 m. The Pleistocene layer of the high-resolution 3D seismic data near sea bottom are near-seabed area has turbidite channel sediments under a applied. The relations among various hierarchies are shown regressive background as well. The provenance is from the in Fig. 2, and higher level sedimentary characteristics are Eastern Congo River. The dominant seismic frequency usually subjected to the perturbation and sedimentology of reaches 65 Hz, and vertical sand recognition is 6 m. lower level architectural units. Therefore, it is reasonable to make an analogy between the target layer (deep zone) and the near-seabed layer (shallow zone), due to the similar turbidite channel sediments. The 4 Sedimentary architectural patterns of turbidite basic data used in this article include core data, logging channel data, high-density seismic acquisition, and shallow channel high-quality seismic data (single channel sand recogniz- It is the best to study the architectural patterns through able) and onsite outcrop measurement data. These types of detailed investigation of each hierarchy’s characteristics information can be crosschecked and used in complemen- and its origin. Nevertheless, considering the impact of tary manners in geological research models, e.g., the reservoir distribution on real well development and pro- combination of shallow channel high-frequency seismic duction, we did the research from a 3-level perspective, and data and onsite outcrop data serves to complement the this was the channel system, complex channel and single 123 1500 Compressional zone Transitional zone Extensional zone M oilfield Congo River Pet. Sci. (2017) 14:493–506 495 Sea-Level Chronostratigraphic Age Sedimentary Democratic Republic of the Congo Reser- Group Lithology Curves Oilfield voir Ma Low High Environments Erathem System Series Stage Pliocene Miocene Paloukou Oligocene Eocene Angola Madingo Paleocene MAAS. CAMP. 80 Lower Congo basin SANT.-CON. TURON.- Licouala CENOM. Sendji ALBIAN Loeme Evaporites APTIAN Shallow-Lacustrine Chela Deep- Marnes shallow BARR. 130 lacustrine Nories Lacustrine Djeno NEOCOMIAN deposit Kwanza basin 0 0 0 20 km Sialivakou Vandji 150 Jurassic TITH. Fluvial-lacustrine Base Block area Precambrian system M oilfield Water depth contour/m River Regional structure boundary Basin boundary Marine Turbidite Fluvial- Carbonate Precambrian- Shallow- mudstone lacustrine base lacustrine Fig. 1 Location and comprehensive stratigraphic column of study area Table 1 Comparison of different schemes of hierarchical division of sedimentary configuration of turbidity channels (Lin et al. 2013, revised) Mutti and Normark (1987) Lamb (2003) Lin et al. (2013) 1 Basin filling, fan 6 Complete set of strata 7 Submarine fan complex complex 2 Single fan 5 Composition of several 4-level configurational units, 6 Single submarine fan which are distinguishable 3 Fan development stage 4 Sedimentation products of various sedimentary 5 Channel system environments and patterns of flow 4 Complex channel 4 Natural levee 3 Products under the same genetic mechanism 3 Single channel microrelief of channel 5 Lithofacies, bedding 2 Single sedimentation unit 2 Sedimentation unit inside single channel microrelief (e.g., Bouma sequence) 1 Further segmentation of a single sedimentation unit 1 Rhythmical layers inside the sedimentation unit channel levels. We believed that the channel system the architectural patterns and internal filling features in influenced the vertical development layer selection, while order to understand reservoir development. the scale of the complex channel and the relations of its inner single channels played a key role in determining well 4.1.1 Geometrical morphology characteristics spacing. Elements of the geometrical channel morphology usually 4.1 Hierarchical architectural patterns of single include the channel widths, depth, sinuosity, arc, length of channels curve, wave length (Wood and Mize-Spansky 2009). Based on studies of modern and ancient submarine fans, scholars Single channels are formed by repeated gravity-flow found that due to the combined effects of ancient sedi- deposits along one channel over a period of time, a major mentary environments, tectonic subsidence and eustatic sea origin unit in turbidite channels. So it is important to study level changes, there were telling differences in the Cenozoic Mesozoic Cretaceous system Palaeogene Neogene Lower Cretaceous Upper Cretaceous Restricted Marine Pelagic sediment marine Open marine deposit deposit Carbonate Turbidite-delta system Marginal marine platform Continental Fluvial Fluvial system Continental margin 496 Pet. Sci. (2017) 14:493–506 1000 m Sand filled channel Silted channel Inter-channel filled Channel-lag deposit Fig. 2 Configuration unit sedimentary pattern of turbidity channels geometrical morphology characteristics of turbidite chan- slope. Against the backdrop of similar sedimentary origin nels. Because of the relative small scale of single channels and sedimentation background, we considered the channels (min. depth being 10 meters), while dominant frequency of located near the sea bottom due to their being less impacted M oilfield seismic data is 40 Hz, it is very difficult to by tectonic movement. Then, analysis of the relationship recognize a single channel. However, the M oilfield near- between single channel sinuosity and topographic slope seabed area high-frequency seismic data show a dominant (h = arctan(h/l), h being slope, h being the width, l being frequency of 70 Hz and similar sedimentary background the length, see Fig. 4a) was done, which shows a negative and type, which makes a critical complement to single correlation between the two and a correlation coefficient of channel studies. Since it is hard to extract ideal single 0.8. See Fig. 4b for image. As the slope becomes steep, the channels from real oilfield seismic data, we analyze geo- downcutting enhances while lateral migration weakens. metrical morphology and characteristics of single channels Whereas when the slope is gentle, provenance supply drops with the aid of shallow high-frequency seismic data. It has off and sedimentation becomes finer, enabling weaker been found that in seismic profiles, single channels are U or downcutting and increased lateral migration. Thus, high- V shaped, presenting medium-strong amplitude inner side, sinuosity single channels are formed. This analysis can also parallel or wave-like reflection, and good consistency as support estimating paleotopography slope based on the depicted by Fig. 3. current single channel sinuosity. As for single channel Sinuosity is one essential parameter in the study width and depth (thickness), they are obtained from shal- (k = h /l , k being sinuosity, while h being the winding low high-frequency seismic data coupled with outcrop a a a length and l being the valley length, see Fig. 3a). It is measurements, on account of sparse well distribution and measured through samples extracted from shallow seismic difficulty to determine single sand body boundaries from data. Statistical analysis revealed that the sinuosity distri- such well spacing. The result shows that in the study area, bution of single channels ranges between 1.0 and 5.4, the depth of single channels (d) ranged between 10 and averaging 1.87. As a result, single channels are classified as 35 m, and width (w) generally between 150 and 450 m low-sinuosity channels and high-sinuosity channels. The (Fig. 3b). average sinuosity of low-sinuosity channels was 1.2, while that of high-sinuosity channels was 1.8. Such difference is 4.1.2 Lithofacies filling model due to various geological factors (Wynn et al. 2007; Deptuck et al. 2003; Peakall et al. 2000) and may have Lithofacies directly reflect the nature of the sedimentary much to do with the gradient of the ancient continental environment. Different lithofacies have different genetic 50 m 100 m Pet. Sci. (2017) 14:493–506 497 (a) m’ m’ ha la (b) A Am mplitude plitude Wavelength Fig. 3 Geometric elements of turbidity channels (the shallow seismic data in the study area) Steep slope 4.0 Along the source direction Gentle slope Steep slop 0.31 3.5 k=1.5667θ R²=0.8089 3.0 High sinuosity 2.5 Low sinuosity 2.0 θ=4.74° 1.5 θ=2.64° θ=2.59° 1.0 θ=3.68° θ=4.02° 0.5 300 m Gradient θ, ° Fig. 4 Correlation of tortuosity and slope gradient (the shallow seismic data in the study area) mechanisms, indicating different permeable capacity also be seen on some sites. Floating gravels are visible (Bouma 1985; Habgood et al. 2003). Therefore, exact inside the massive sandstones (Fig. 5). The sand bodies are identification of lithofacies types is required for studies on generally fining upward. That is, lithofacies inside the the genetic mechanism of turbidite channels and analysis channel, from bottom upward, form a configuration pattern on permeable discrepancy. The coring data show that of retention sediment * massive gravelly coarse sand- obvious turbidite channel sedimentary characteristics can stones (which may contain mud-sized grain) * massive be found in the M oilfield. Lithologically, turbidite chan- middle-fine sandstone * interlaced bedded sand- nels in the area are mainly composed of massive sand- stone * fine-grained sediment, with sand bodies becoming stones, mixed with a little fine-grained sediment. Based on thinner from bottom to top. sedimentary tectonics, they have Bouma sequence features The specific sedimentary sequences inside the channels and are mostly blocky structure. Cross and parallel bed- produce corresponding logging responses. Single channel dings can be seen on the top. Erosive bases are generally responses for individual wells are as follows. Natural developed, mixed with retention sediments (mudstone gammas are mainly bell-wise and nearly box shaped, while fragments) at the bottom of the channel. Multiple washings the resistivity curve is slightly funnel shaped (Zhao et al. can be seen in the main body of the channel (secondary 2010). Finally, based on such information from field out- erosion). Mudstone interlayers and argillaceous slumps can crops, cores of the target stratum, etc., the internal filling Sinuosity k 5 cm 5 cm 5 cm 5 cm 5 cm 5 cm 5 cm 5 cm 5 cm 5 cm 5 cm 498 Pet. Sci. (2017) 14:493–506 (a) (b) (c) (d) (e) Td Tc Tb (f) (g) (h) (i) (j) (k) Ta Fig. 5 Internal lithofacies characteristics of single channels in the M well-1A, 3205.0 m; g corrugated-bedding siltstone, well-1A, oilfield, West Africa (well position as shown in Fig. 9). From top left 3206.12 m; h massive gravelly medium-coarse sand facies (floating a massive gravelly coarse sandstone, well-1A, 3196.25 m; b massive boulder clays on the top), well-2B, 3202.08 m; i corrugated-bedding mud-sized grain—coarse sandstone, well-1A, 3197.16 m; c retention siltstone, well-2G, 3209.26 m; j massive medium-fine-grained sand sediments at the bottom, well-1A, 3199.06 m; d massive gravelly facies, well-2B, 3213.67 m; k Bouma sequence, well-2B, 3217 m coarse sand facies, well-2B, 3202.29 m; e massive sandstone (see the erosion surface), well-2B, 3213.03 m; f thin-layer muddy silt facies, 100 m Well_2B Gamma Density Resistivity Depth, Core 20 90 2.4 2.1 0.5 4 m rhythm GM/CC Ω·m API Silty fine-grained Blocked medium- Muddy interlayer sediment fine sandstone Pebbly Medium-coarse Fine Siltstone Shale Massive gravelly- Retention deposit sandstone sandstone sandstone coarse sandstone Fig. 6 Logging and lithofacies characteristics of single channel (well position as shown in Fig. 9) 10 m Pet. Sci. (2017) 14:493–506 499 patterns of single channels are summarized (Fig. 6). The classified into lateral and vertical migrations (Labourdette inner part of single channels is filled in ‘‘bundles,’’ with 2007; Hubbard et al. 2009). The lateral migration causes sand bodies becoming thinner and finer from axis to the single channels joining together in the lateral direction edge. Vertically, retention gravels are usually deposited at horizontally, so the sand body thickness of complex the bottom of the channel. Sand bodies of the channel channels is very close to that of a single channel (Fig. 8a). change progressively, mixed with thin muddy or clay As such, the vertical heterogeneity is relatively weak. Also, layers. From the bottom upward, the pattern of sand tends the vertical migration can be categorized into two types: to be thinner and finer. The sedimentary tectonics of thin indented and swing (Fig. 8b, c), causing channels to sand-shale interbed at the top of the channel form parallel superimpose in the vertical direction. The thicknesses of bedding-corrugated bedding. sand bodies are generally larger than the depth of single channels. Moreover, as retention slump-block deposits 4.2 Hierarchical architecture patterns of complex (inferior filter beds) usually occur at the bottom of the channels channel, the vertical heterogeneity of such compound sand bodies is relatively strong. Results show that the depths of A complex channel consists of single channels that are complex channels in the study area lie between 20 and laterally or vertically superimposed, mainly controlled by 185 m. As affected by profile migration features and the an autogenetic cycle. It is a medium-sized architectural unit lithological changes inside, there are obvious well-seismic (Mutti and Normark 1987; Kane et al. 2007; Abreu et al. response characteristics in the boundary regions. The 2003; Anka et al. 2009). We studied the internal architec- mudstone content rises at the boundary, making the sand tural patterns using both shallow high-frequency and deep body thinner, mostly manifested by weakened amplitude well-to-seismic integration data. intensity. In addition, the lateral migration of channels can create imbricate responses on the seismic profile (Fig. 8). 4.2.1 Horizontal migration pattern 4.3 Hierarchical architecture pattern of channel By drawing the RMS amplitude attributes slice of channel systems deposits in the M oilfield, we can see that there are two migration types in single channels: in lateral and paleocurrent The channel system (canyon or large incised waterway) is direction (Zhao et al. 2012a, b) from the channel plan view. So regarded as a large-scale unit, superimposed by various we built the migration pattern of single channels inside the complex channels (Xie et al. 2012; Yu et al. 2012). complex channel according to the seismic response features, Affected by erosive power, the reservoir architecture models of the same deepwater turbidity channel system as shown in Fig. 7. Lateral migration causes sand body dis- tribution more continuous, which expands the connected differ greatly in different deposit locations. Spatial struc- range of sand bodies. With migration in the paleocurrent ture characteristics can be represented by the seismic direction, since the single channels are vertically superposed, information. Researchers categorize channel systems into the sand bodies show great heterogeneity in the vertical restricted (with incised valley), weakly restricted (with direction. In plan view, channels migrate along the prove- incised valley) and non-restricted (without incised valley) nance direction, presenting wide stripes for sand bodies (Liu according to geomorphic features (Hubbard et al. 2009; et al. 2013). The width of complex channels ranges between Zhao et al. 2012a, b; Lin et al. 2013; Clark and Pickering 300 and 1500 m in the study area. Due to different migration 1996; Deptuck et al. 2003; Sun et al. 2014; Chen et al. patterns of different channel segments, there are ambiguous 2015). relations between the width and depth for complex channels in Different superimposition patterns of complex channels our study. Another important factor to describe migration lead to different architectures inside the channel system. patterns in complex channels is sinuosity, which is due to Inside restricted channel systems, deeply incised indented single channel migration. It is measured to be 1.0–1.8, aver- and swing complex channels are the major part and barely aging at 1.3 in our study, using the central line in the boundary develop large natural levees. With weakly restricted area of the complex channel as baseline. This is much smaller channel system, there are deeply incised indented and than that of single channels. swing complex channels combined with weakly incised horizontal migration patterns, with deposit overflow along 4.2.2 Profile migration pattern incised valleys, developing large natural levees on both sides. Non-restricted channel system features weakly The profile migration patterns of complex channels are also incised indented and swing channels, along with horizontal subjected to single channel migrations. As we know, the migration patterns. There are occasional deeply incised profile migration pattern of single channels can be channels at the bottom of non-restricted channel systems, 123 500 Pet. Sci. (2017) 14:493–506 Channel 1 Horizontal migration pattern Channel 2 Lateral swing Palaeocurrent-oriented sweep 1 km RMS horizon-oriented slice Fig. 7 Plane migration patterns of single channel based on layer slicing in M oilfield (slice position as shown in Fig. 1) with no evident natural levee development. Especially in in inconspicuous incised valleys. Results show widths of the late stage of channel system development, it is difficult 1000–3000 m and depth of 80–280 m for the O73 channel to distinguish between fine-particle-filled channel deposits system, a large-scale one. and natural levee deposits. To sum up, the spatial patterns of channel system vary significantly due to deposition patterns of complex channels. 5 Sedimentary evolution of turbidity channels Weakly restricted and non-restricted channel systems are considered as major categories developed in the O73 It is through analyzing the evolution of channel systems that oilfield in the study area, and their seismic response char- we understand the deposition processes and architectural acteristics are shown in Fig. 9. There show certain evolu- genesis. More importantly, it will strengthen the credibility tion trends in the plane. The closer it is to the provenance of inter-well prediction for the architectural characterization direction, the stronger the channels incision. Weakly based on well-log and seismic data under wide spacing. restricted channel systems have distinct incised sections, and wedges (large natural levee deposit) are developed on 5.1 Sedimentary evolution characteristics both sides. The further it is from the provenance direction, the weaker the incision, and the stronger the aggradation Through well-to-seismic calibration, we make a compre- and lateral migration. Non-restricted channel systems hensive explanation of S1-S4 (Fig. 9) complex channel develop with no evident boundary features (without sediments in the O73 channel system of the M oilfield. developing large incised valleys). Typical vertical evolu- Seismic attributes demonstrating sand body distributions, tion patterns can be found inside non-restricted channel such as RMS amplitude, were extracted (Fig. 10). Com- systems, depicted in four stages. Great incision in the early- bined with core data, we have managed to explain evolu- stage deposits and large incise valleys are developed. But tion characteristics of each stage (Fig. 11). in later stages, because of rising sea level, incision abates The O73 reservoir of the M oilfield shows noticeable while aggradation and lateral migration enhances, resulting sequence sedimentary characteristics. The early 123 20m 20m 100m 100m 20m 100m Pet. Sci. (2017) 14:493–506 501 Well_6ST 300m 400m 400m 400m a Horizontal b Indented c Swing Lateral migration Vertical migration Fig. 8 Profile migration patterns and vertical evolution of single channels of zone O73 in the M oilfield Well_5 A’ Well_5 A’ Weakly restricted channel 800m Well_6ST B B Well_2B Well_6ST B’ Non-restricted channel 2km 800m Fig. 9 Plane evolution relation of the O73 channel system (slice position as shown in Fig. 1) 123 502 Pet. Sci. (2017) 14:493–506 RMS RMS RMS RMS 1000 1000 1000 800 800 800 600 600 600 400 400 400 200 200 200 0 0 0 Well_2B Well_2B Well_2B Well_2B 1 km 0 1 km 0 0 1 km 0 1 km O73_S4 O73_S3 O73_S2 O73_S1 Fig. 10 Plane characteristics of O73 channel system in the M oilfield (slice position as shown in Fig. 9) sedimentary stage (S1) belongs to deepwater stratified period channel sand bodies results in expanded distribution sediments. It shows distinct restricted characteristics with of sand bodies. For well-2B, the meander section of the abundant supply and strong erosive power, widely dis- outer S3 sequence channel complex was drilled, so the core tributed in the riverbed at the bottom of the large sub- only represents a partial sedimentary filling sequence. marine canyon of the O73 reservoir. Well-2B coring shows There is a layer of gravelly sediments (about 4 m in that the lowermost part of the sequence mainly consists of a thickness) at the bottom of the coring section, which is coarse sandstone stratigraphic unit, which often contains covered by a hard sand clastic layer (about 2 m in thick- coarse to boulder-level conglomerates and mudstone frag- ness) and then comes a 6-m-thick muddy siltstone (the top ments. It can be deduced that its high sedimentary energy sediment of the S3 sequence is draped by the mudstone enables it to downcut the older conglomerate layer and layer). For well-2G, drilling of the meander section of the clastic layer. The middle part of the sequence is a mixed interior S3 sequence channel revealed that S3 sequence sedimentary unit of thicker coarse sand and medium sand, channel intensively eroded S2 sequence sediments probably high-density turbidite sediment, and it progres- (Fig. 11e), which is possibly related to weak consolidation sively changed into a finer-grained Tb and Tc type low- of the early channel sediment and the supply channel density turbidite layer in an upward direction. The upper- formed due to the negative topography on the edge of the most part is a mudstone layer, indicating gradually waning channel. energy. The S4 sequence is the last layer of the O73 channel During the S2 stage, sediment supply is still sufficient, system, belonging to the sediment shrinkage stage when but the sea level began to rise forming a slightly restricted the sea level reached a peak. The channel is still highly channel. Here the single channel shows lateral migration, sinuous, but the scale is much smaller than that of the S3 as a weakly restricted channel, and thus widely distributed. sequence. The downcutting depth decreases, with As shown in Fig. 11, the S2 sequence channel sediment strengthened lateral restriction. Core analysis indicates that presented a transition trend from the main channel axis to the bottom of the sequence is well-sorted medium-fine the edge, with muddy interlayers gradually developing in massive sandstone, with good consistency in seismic the edge. Core analysis indicates that the lithology of S2 is response within the whole oil field (Fig. 10). It is the final mainly thick massive sandstone, partially interbedded with product of channel filling, and there will be the abandon- thin mudstone layers. The top layer gradually changed to ment stage of the O73 channel system afterward. siltstone and mudstone layers, with occasional ripple bed- ding, showing gradual abandonment characteristics. 5.2 Sedimentary controlling factors and their During the S3 stage, the sea level continued to rise, and evolution sediment supply started to decrease. Channel sediments show distinct non-restricted characteristics. Lateral migra- 5.2.1 Sedimentary controlling factors tion and vertical downcutting are both strong for single channels, as well as high sinuosity. The channel on the Deepwater detrital deposits are controlled by autogenetic planar graph (Fig. 10) is very clear. Superposition of multi- cycles and allogenetic cycles. The controlling factors 123 Pet. Sci. (2017) 14:493–506 503 S4 Abandonment S3 Abandonment S4 S2 Abandonment 1c S3 S2 (i) Late stage channel abandonment coating deep S1 water mudstone S3 Abandonment S4 S2 Abandonment 1c S3 S2 (h) Late stage S4 high- curvature constructive S1 channel sediments S3 Abandonment S2 Abandonment 1c S3 S2 (g) S3 late stage sediments abandonment S1 with pelitic filling S2 Abandonment S3 S2 (f) S3 constructive channel sediments S1 changing original erosion interface S2 Abandonment S2 (e) S3 form deep incised S1 valley before sediment and eroded S2 channel sand S2 Abandonment S2 (d) S2 sedimentary S1 weakening and channel abandonment S2 (c) S2 erosion constructive channel S1 with pelitic laminate (b) S1 coarse erosion channel and top pelitic S1 fine-grained sediments (a) O73 channel system incised valley formation Fig. 11 Vertical evolutionary model of O73 channel system in the M oilfield 123 504 Pet. Sci. (2017) 14:493–506 include eustacy, basin tectonic movement, sediment types and thick. Because of the contemporaneous falling sea and supply rates. Moreover, events such as earthquakes and level and continent uplifting, it reduces the distance tsunamis may also allow the clastic particles to reach the between provenance and deepwater sedimentary supply, deep sea after traversing the continental shelf and slope which is beneficial to form sediments. Therefore, the above valley, forming deepwater sediments (Stow et al. 1996; analysis indicates that sediment type of the O73 channel Shanmugam 2008). The combination of many controlling system is subjected to controlling factors including sedi- factors causes the difference in the erosive power of ment supply, deepwater gravitational flow and density. channels, resulting in the complex and diverse superposed relationship of sand bodies. These controlling factors 5.2.2 Evolution discussion include the provenance distance, provenance types, climate in the provenance area, sea level eustacy, topographic Although models of the development of the channel system slope. Under normal circumstances, the closer to the are affected by multiple factors, they follow certain evo- provenance, the greater the topographic slope and the more lutionary trends (Posamentier and Kolla 2003; Prather sea level drops, the more abundant the sediment supply, the 2003; Liu et al. 2008). Horizontally, the development is greater the load density, the higher the deposit velocity, and mainly manifested as the evolution of different channel the stronger the erosive power (He et al. 2011; Zhuo et al. system types; and vertically, the development is primarily 2013; Li et al. 2011). These factors carry various weights in represented by the evolution of its internal complex influencing channel systems, and they correspond to vari- channels. ous types of channel systems. For instance, fast sedimen- Horizontally, along the provenance direction, there are tary flow and powerful erosion in steep slopes favor certain trends in changes on account of differences in restricted or weakly restricted channel systems, whereas erosion of sediments. As the root of the channel system is non-restricted channel systems are often observed in gentle nearer to the sediment source, large size, high flow rate and slope areas. Likewise, when sea level falls, there is ample strong erosive power, large incised valleys can be formed sediment supply and all kinds of channel systems can form. and restricted channel systems that focus on transporting Otherwise, in times of rising sea level where sedimentary sediments were mainly developed, leading to a large supply is scarce, turbidite channel systems are seldom amount of fragmental flow, turbidite and slump sediments developed. Furthermore, allogenetic cycles are more evi- developed in it. In the middle of the channel system, sed- dent in high sea level periods while autogenetic cycles iments become finer with decreased flow rate, resulting in dominate in low sea level periods (Posamentier and Kolla weakened downcutting and strengthened aggradation, so 2003; Prather 2003). For the O73 channel system of the M the weakly restricted channel system (e.g., O73 channel system as shown in Fig. 9) is mainly developed at this oilfield, eustacy, tectonic movement and topographic slope play a key controlling role in reservoir architecture and point, in which channels with some degree of bending are distribution, and abundant sediment from the Congo River developed and filled with an amount of fragmental flow thanks to the moist climate in the Oligocene is also another and slump substances. While at the distal end of the vital factor in turbidite channel formation (Booth et al. channel system, the supply energy wanes and sediments are 2003; Violet et al. 2005; Beydoun et al. 2002). Tectonic of the smallest size and lowest flow rate. At this point, the movements such as differential uplift keep modifying both sediments downcutting capacity is weak, but the lateral the macro- and micro-topography, which alters the energy migration capacity is strong, developing non-restricted of gravitational flow, and then alters development location channel systems mainly in which the single channels are and distribution of deepwater sedimentary units. From a mostly moderately to highly bent. macro-perspective, the differential uplift of the Congo Vertically, influenced by eustacy and delivery rate of Basin in the Angola area causes the sedimentary center to sediments, the development of internal complex channels move north. Meanwhile, from a micro-perspective, tec- inside the channel system also follows evolutionary tonics like salt diapir accompanied by partial salt rock trends. In the early development period of the channel movement also greatly affects deepwater channel systems system (S1 stage), high flow rate and abundant supply of (Anka et al. 2009; Broucke et al. 2004; Kolla 2007; Pirmez sediments with strong erosion mostly contribute to form and Imran 2003). Eustacy influences the development of deep downcutting complex channels. They, as erosive deepwater channel systems too. Deepwater sediments in channels, mainly transport sediments. In the middle West Africa developed in the Upper Cretaceous when development period of the channel system (S2 stage), the global sea level fell. At that time, the scale of deepwater sea level begins to rise. The sediment supply is still rich, channel systems expanded as sea level fell and they but the slowing flow rate leads to its slightly weakened advanced toward the sea. Furthermore, the planar features downcutting capacity and strengthened aggradation, with of deepwater channels turned from wide and thin to narrow mixed development of aggradational channels and erosion 123 Pet. Sci. (2017) 14:493–506 505 Acknowledgements This paper is supported by the National Major channels mainly under the effect of sedimentation. In the Scientific and Technological Special Project during the Thirteenth middle to late development period of the channel system Five-year Plan Period (2016ZX05033-003-002) and the Project of (S3 stage), with the continual rise of the sea level, the Sinopec Science and Technology Development Department (G5800- sediment supply falls gradually (except in tsunamis, 15-ZS-KJB016). earthquakes and other unexpected events). The channel’s Open Access This article is distributed under the terms of the downcutting capacity weakens, but the lateral accretion Creative Commons Attribution 4.0 International License (http://crea capacity becomes increasingly stronger, with aggrading tivecommons.org/licenses/by/4.0/), which permits unrestricted use, highly sinuous channels mainly developed. In the late distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a development period of the channel system (S4 stage), the link to the Creative Commons license, and indicate if changes were sea level reached a high level and the sediment supply made. was the weakest, pointing to a sediment shrinkage stage where only a small number of highly sinuous aggraded channels and even isolated mudstone-filled single chan- References nels were developed. Abreu V, Sullivan M, Pirmez C, et al. 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Architecture mode, sedimentary evolution and controlling factors of deepwater turbidity channels: A case study of the M Oilfield in West Africa

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Earth Sciences; Mineral Resources; Industrial Chemistry/Chemical Engineering; Industrial and Production Engineering; Energy Economics
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Abstract

Pet. Sci. (2017) 14:493–506 DOI 10.1007/s12182-017-0181-2 ORIGINAL PAPER Architecture mode, sedimentary evolution and controlling factors of deepwater turbidity channels: A case study of the M Oilfield in West Africa 1 1 1 1 • • • • Wen-Biao Zhang Tai-Zhong Duan Zhi-Qiang Liu Yan-Feng Liu 1 1 Lei Zhao Rui Xu Received: 25 October 2016 / Published online: 27 July 2017 The Author(s) 2017. This article is an open access publication Abstract Turbidity channels have been considered as one geological modeling in reservoir development and con- of the important types of deepwater reservoir, and the study tribute to efficient development of such reservoirs. of their architecture plays a key role in efficient develop- ment of an oil field. To better understand the reservoir Keywords Reservoir architecture  Turbidity channel architecture of the lower Congo Basin M oilfield, semi- Sedimentary evolution  Deep water  Shallow seismic quantitative–quantitative study on turbidity channel depo- Controlling factors sitional architecture patterns in the middle to lower slopes was conducted with the aid of abundant high quality materials (core, outcrop, logging and seismic data), 1 Introduction employing seismic stratigraphy, seismic sedimentology and sedimentary petrography methods. Then, its sedimen- Deep water channels are considered one of the significant tary evolution was analyzed accordingly. The results types of reservoir, sometimes containing rich oil and gas indicated that in the study area, grade 3 to grade 5 archi- resources. There have been studies abroad on their depo- tecture units were single channel, complex channel and sitional architecture, mainly focusing on architecture ele- channel systems, respectively. Single channel sinuosity is ment recognition and description through core and other negatively correlated with the slope, as internal grains observations. Achievements have been made by indepen- became finer and thickness became thinner from bottom to dent oil companies (IOCs) and institutions, with the top, axis to edge. The migration type of a single channel assistance of the rapid development of deepwater drilling, within one complex channel can be lateral migration and geophysics theory and sonar scanning, on sedimentary along paleocurrent migration horizontally, and lateral, configuration of deepwater turbidite channels. Domestic indented and swing stacking in section view. Based on researchers are doing similar studies using deepwater external morphological characteristics and boundaries, sediments in the South China Sea area. These studies and channel systems are comprised of a weakly confining type related achievements contribute a lot to reducing risks in and a non-confining type. The O73 channel system can be early-stage deepwater exploration (Heinio¨ and Davies divided into four complex channels named S1–S4, from 2007; Posamentier and Kolla 2003; Slatt 2006; Menard bottom to top, with gradually less incision and more 1955; Li et al. 2008; Wang et al. 2009; Deng et al. 2008;Lu¨ accretion. The study in this article will promote deeper et al. 2008). For continental slope areas in the Lower understanding of turbidity channel theory, guide 3D Congo Basin, West Africa, there have been studies of deepwater channel sedimentary patterns and the factors controlling them in terms of sedimentology and sequence & Wen-Biao Zhang stratigraphy. zwb.syky@sinopec.com However, with the gradual development of more deep- Petroleum Exploration and Production Research Institute, water oilfields, plenty of dynamic data reveal complex SINOPEC, Beijing 100083, China superimposition of single sand bodies inside channel sys- tems and various high-heterogeneity rock types in single Edited by Jie Hao 123 494 Pet. Sci. (2017) 14:493–506 sand bodies. This impedes further and more effective limited deepwater seismic resolution, making full-scale development of deepwater oilfields. Therefore, it is nec- analysis of channel architecture type and scale possible; essary to analyze configurations of both complex and sin- core data provide complementary information for research gle sand bodies to clarify single sand body distribution into inner filling models. Comprehensive use of available patterns, quantitative scales, superimposition and lithology information is a key method to study sedimentary inside single sand bodies. architecture. We took the deepwater M oilfield in West Africa as an example to study semi-quantitative–quantitative sedimen- tary configuration patterns and their spatial evolution in 3 Turbidite channels hierarchical division this article. Based on abundant drilling, core and high- quality seismic data, the study was carried out at three Several criteria were proposed for the hierarchical division different levels, including channel system, complex chan- of turbidite channel architecture. For example, Mutti and nel and single channel levels, employing methods such as Normark proposed a five-turbidite-facies scheme in 1987, core description, log recognition and seismic attribute sli- mainly based on the genetic type of sand bodies. Later, a ces. The study in this article may promote turbidite channel seven-turbidite-facies scheme was put forward by Zhao theory understanding and benefit 3-D geomodeling, mak- et al. (2012a, b) and Lin et al. (2013). In this division, they ing it useful in developing this type oilfield more gave more thought to hydrodynamic characteristics, sedi- efficiently. mentation and contact relations of the formation of channel sand bodies. In this article, we adopted the seven-turbidite- facies scheme to study sedimentary architecture patterns on 2 Geological background three levels, which are single channel, complex channel and channel system levels, to demonstrate the influence on The M oilfield is one of the most favorable exploration and reservoir distribution from macro perspectives (Table 1). development petroliferous basins located in the lower There are certain genetic connections between units of Congo Basin, with typical passive continental margin channels at different levels (Zhao et al. 2012a, b). The characteristics (Liu and Li 2009; Xiong et al. 2005; Kolla channel system is characterized by complex channels of et al. 2001). The research zone lies in the middle–lower different periods, while the formation of complex channels slope, between compressive and extensional zones, 186 km is subject to the migration patterns of single channels. away from Luanda, Angola. The architecture is not There are differences in the scale of architectural units at severely impaired by Cretaceous gypsum activities, and the different levels, thereby influencing the data type required main target layer is Oligocene. Its sedimentary type is for study. The architectural units of large-scale channel considered to be a deep water turbidite channel system systems are recognized mainly through seismic data under a regressive background with a water depth of (comprising of seismic facies and seismic reflection 1400–1800 m (Fig. 1). There are 18 wells in the research structures), while that of the intermediate scale (complex zone of the Oligocene O73 reservoir, and they are char- channel) can be recognized also through seismic data acterized by a core depth of 168 m; average well distance (strata slicing). With regard to the small scale (single more than 1000 m; dominant seismic frequency of 35 Hz; channels), drilling data (coring and well logging data) and and sand recognition 15–20 m. The Pleistocene layer of the high-resolution 3D seismic data near sea bottom are near-seabed area has turbidite channel sediments under a applied. The relations among various hierarchies are shown regressive background as well. The provenance is from the in Fig. 2, and higher level sedimentary characteristics are Eastern Congo River. The dominant seismic frequency usually subjected to the perturbation and sedimentology of reaches 65 Hz, and vertical sand recognition is 6 m. lower level architectural units. Therefore, it is reasonable to make an analogy between the target layer (deep zone) and the near-seabed layer (shallow zone), due to the similar turbidite channel sediments. The 4 Sedimentary architectural patterns of turbidite basic data used in this article include core data, logging channel data, high-density seismic acquisition, and shallow channel high-quality seismic data (single channel sand recogniz- It is the best to study the architectural patterns through able) and onsite outcrop measurement data. These types of detailed investigation of each hierarchy’s characteristics information can be crosschecked and used in complemen- and its origin. Nevertheless, considering the impact of tary manners in geological research models, e.g., the reservoir distribution on real well development and pro- combination of shallow channel high-frequency seismic duction, we did the research from a 3-level perspective, and data and onsite outcrop data serves to complement the this was the channel system, complex channel and single 123 1500 Compressional zone Transitional zone Extensional zone M oilfield Congo River Pet. Sci. (2017) 14:493–506 495 Sea-Level Chronostratigraphic Age Sedimentary Democratic Republic of the Congo Reser- Group Lithology Curves Oilfield voir Ma Low High Environments Erathem System Series Stage Pliocene Miocene Paloukou Oligocene Eocene Angola Madingo Paleocene MAAS. CAMP. 80 Lower Congo basin SANT.-CON. TURON.- Licouala CENOM. Sendji ALBIAN Loeme Evaporites APTIAN Shallow-Lacustrine Chela Deep- Marnes shallow BARR. 130 lacustrine Nories Lacustrine Djeno NEOCOMIAN deposit Kwanza basin 0 0 0 20 km Sialivakou Vandji 150 Jurassic TITH. Fluvial-lacustrine Base Block area Precambrian system M oilfield Water depth contour/m River Regional structure boundary Basin boundary Marine Turbidite Fluvial- Carbonate Precambrian- Shallow- mudstone lacustrine base lacustrine Fig. 1 Location and comprehensive stratigraphic column of study area Table 1 Comparison of different schemes of hierarchical division of sedimentary configuration of turbidity channels (Lin et al. 2013, revised) Mutti and Normark (1987) Lamb (2003) Lin et al. (2013) 1 Basin filling, fan 6 Complete set of strata 7 Submarine fan complex complex 2 Single fan 5 Composition of several 4-level configurational units, 6 Single submarine fan which are distinguishable 3 Fan development stage 4 Sedimentation products of various sedimentary 5 Channel system environments and patterns of flow 4 Complex channel 4 Natural levee 3 Products under the same genetic mechanism 3 Single channel microrelief of channel 5 Lithofacies, bedding 2 Single sedimentation unit 2 Sedimentation unit inside single channel microrelief (e.g., Bouma sequence) 1 Further segmentation of a single sedimentation unit 1 Rhythmical layers inside the sedimentation unit channel levels. We believed that the channel system the architectural patterns and internal filling features in influenced the vertical development layer selection, while order to understand reservoir development. the scale of the complex channel and the relations of its inner single channels played a key role in determining well 4.1.1 Geometrical morphology characteristics spacing. Elements of the geometrical channel morphology usually 4.1 Hierarchical architectural patterns of single include the channel widths, depth, sinuosity, arc, length of channels curve, wave length (Wood and Mize-Spansky 2009). Based on studies of modern and ancient submarine fans, scholars Single channels are formed by repeated gravity-flow found that due to the combined effects of ancient sedi- deposits along one channel over a period of time, a major mentary environments, tectonic subsidence and eustatic sea origin unit in turbidite channels. So it is important to study level changes, there were telling differences in the Cenozoic Mesozoic Cretaceous system Palaeogene Neogene Lower Cretaceous Upper Cretaceous Restricted Marine Pelagic sediment marine Open marine deposit deposit Carbonate Turbidite-delta system Marginal marine platform Continental Fluvial Fluvial system Continental margin 496 Pet. Sci. (2017) 14:493–506 1000 m Sand filled channel Silted channel Inter-channel filled Channel-lag deposit Fig. 2 Configuration unit sedimentary pattern of turbidity channels geometrical morphology characteristics of turbidite chan- slope. Against the backdrop of similar sedimentary origin nels. Because of the relative small scale of single channels and sedimentation background, we considered the channels (min. depth being 10 meters), while dominant frequency of located near the sea bottom due to their being less impacted M oilfield seismic data is 40 Hz, it is very difficult to by tectonic movement. Then, analysis of the relationship recognize a single channel. However, the M oilfield near- between single channel sinuosity and topographic slope seabed area high-frequency seismic data show a dominant (h = arctan(h/l), h being slope, h being the width, l being frequency of 70 Hz and similar sedimentary background the length, see Fig. 4a) was done, which shows a negative and type, which makes a critical complement to single correlation between the two and a correlation coefficient of channel studies. Since it is hard to extract ideal single 0.8. See Fig. 4b for image. As the slope becomes steep, the channels from real oilfield seismic data, we analyze geo- downcutting enhances while lateral migration weakens. metrical morphology and characteristics of single channels Whereas when the slope is gentle, provenance supply drops with the aid of shallow high-frequency seismic data. It has off and sedimentation becomes finer, enabling weaker been found that in seismic profiles, single channels are U or downcutting and increased lateral migration. Thus, high- V shaped, presenting medium-strong amplitude inner side, sinuosity single channels are formed. This analysis can also parallel or wave-like reflection, and good consistency as support estimating paleotopography slope based on the depicted by Fig. 3. current single channel sinuosity. As for single channel Sinuosity is one essential parameter in the study width and depth (thickness), they are obtained from shal- (k = h /l , k being sinuosity, while h being the winding low high-frequency seismic data coupled with outcrop a a a length and l being the valley length, see Fig. 3a). It is measurements, on account of sparse well distribution and measured through samples extracted from shallow seismic difficulty to determine single sand body boundaries from data. Statistical analysis revealed that the sinuosity distri- such well spacing. The result shows that in the study area, bution of single channels ranges between 1.0 and 5.4, the depth of single channels (d) ranged between 10 and averaging 1.87. As a result, single channels are classified as 35 m, and width (w) generally between 150 and 450 m low-sinuosity channels and high-sinuosity channels. The (Fig. 3b). average sinuosity of low-sinuosity channels was 1.2, while that of high-sinuosity channels was 1.8. Such difference is 4.1.2 Lithofacies filling model due to various geological factors (Wynn et al. 2007; Deptuck et al. 2003; Peakall et al. 2000) and may have Lithofacies directly reflect the nature of the sedimentary much to do with the gradient of the ancient continental environment. Different lithofacies have different genetic 50 m 100 m Pet. Sci. (2017) 14:493–506 497 (a) m’ m’ ha la (b) A Am mplitude plitude Wavelength Fig. 3 Geometric elements of turbidity channels (the shallow seismic data in the study area) Steep slope 4.0 Along the source direction Gentle slope Steep slop 0.31 3.5 k=1.5667θ R²=0.8089 3.0 High sinuosity 2.5 Low sinuosity 2.0 θ=4.74° 1.5 θ=2.64° θ=2.59° 1.0 θ=3.68° θ=4.02° 0.5 300 m Gradient θ, ° Fig. 4 Correlation of tortuosity and slope gradient (the shallow seismic data in the study area) mechanisms, indicating different permeable capacity also be seen on some sites. Floating gravels are visible (Bouma 1985; Habgood et al. 2003). Therefore, exact inside the massive sandstones (Fig. 5). The sand bodies are identification of lithofacies types is required for studies on generally fining upward. That is, lithofacies inside the the genetic mechanism of turbidite channels and analysis channel, from bottom upward, form a configuration pattern on permeable discrepancy. The coring data show that of retention sediment * massive gravelly coarse sand- obvious turbidite channel sedimentary characteristics can stones (which may contain mud-sized grain) * massive be found in the M oilfield. Lithologically, turbidite chan- middle-fine sandstone * interlaced bedded sand- nels in the area are mainly composed of massive sand- stone * fine-grained sediment, with sand bodies becoming stones, mixed with a little fine-grained sediment. Based on thinner from bottom to top. sedimentary tectonics, they have Bouma sequence features The specific sedimentary sequences inside the channels and are mostly blocky structure. Cross and parallel bed- produce corresponding logging responses. Single channel dings can be seen on the top. Erosive bases are generally responses for individual wells are as follows. Natural developed, mixed with retention sediments (mudstone gammas are mainly bell-wise and nearly box shaped, while fragments) at the bottom of the channel. Multiple washings the resistivity curve is slightly funnel shaped (Zhao et al. can be seen in the main body of the channel (secondary 2010). Finally, based on such information from field out- erosion). Mudstone interlayers and argillaceous slumps can crops, cores of the target stratum, etc., the internal filling Sinuosity k 5 cm 5 cm 5 cm 5 cm 5 cm 5 cm 5 cm 5 cm 5 cm 5 cm 5 cm 498 Pet. Sci. (2017) 14:493–506 (a) (b) (c) (d) (e) Td Tc Tb (f) (g) (h) (i) (j) (k) Ta Fig. 5 Internal lithofacies characteristics of single channels in the M well-1A, 3205.0 m; g corrugated-bedding siltstone, well-1A, oilfield, West Africa (well position as shown in Fig. 9). From top left 3206.12 m; h massive gravelly medium-coarse sand facies (floating a massive gravelly coarse sandstone, well-1A, 3196.25 m; b massive boulder clays on the top), well-2B, 3202.08 m; i corrugated-bedding mud-sized grain—coarse sandstone, well-1A, 3197.16 m; c retention siltstone, well-2G, 3209.26 m; j massive medium-fine-grained sand sediments at the bottom, well-1A, 3199.06 m; d massive gravelly facies, well-2B, 3213.67 m; k Bouma sequence, well-2B, 3217 m coarse sand facies, well-2B, 3202.29 m; e massive sandstone (see the erosion surface), well-2B, 3213.03 m; f thin-layer muddy silt facies, 100 m Well_2B Gamma Density Resistivity Depth, Core 20 90 2.4 2.1 0.5 4 m rhythm GM/CC Ω·m API Silty fine-grained Blocked medium- Muddy interlayer sediment fine sandstone Pebbly Medium-coarse Fine Siltstone Shale Massive gravelly- Retention deposit sandstone sandstone sandstone coarse sandstone Fig. 6 Logging and lithofacies characteristics of single channel (well position as shown in Fig. 9) 10 m Pet. Sci. (2017) 14:493–506 499 patterns of single channels are summarized (Fig. 6). The classified into lateral and vertical migrations (Labourdette inner part of single channels is filled in ‘‘bundles,’’ with 2007; Hubbard et al. 2009). The lateral migration causes sand bodies becoming thinner and finer from axis to the single channels joining together in the lateral direction edge. Vertically, retention gravels are usually deposited at horizontally, so the sand body thickness of complex the bottom of the channel. Sand bodies of the channel channels is very close to that of a single channel (Fig. 8a). change progressively, mixed with thin muddy or clay As such, the vertical heterogeneity is relatively weak. Also, layers. From the bottom upward, the pattern of sand tends the vertical migration can be categorized into two types: to be thinner and finer. The sedimentary tectonics of thin indented and swing (Fig. 8b, c), causing channels to sand-shale interbed at the top of the channel form parallel superimpose in the vertical direction. The thicknesses of bedding-corrugated bedding. sand bodies are generally larger than the depth of single channels. Moreover, as retention slump-block deposits 4.2 Hierarchical architecture patterns of complex (inferior filter beds) usually occur at the bottom of the channels channel, the vertical heterogeneity of such compound sand bodies is relatively strong. Results show that the depths of A complex channel consists of single channels that are complex channels in the study area lie between 20 and laterally or vertically superimposed, mainly controlled by 185 m. As affected by profile migration features and the an autogenetic cycle. It is a medium-sized architectural unit lithological changes inside, there are obvious well-seismic (Mutti and Normark 1987; Kane et al. 2007; Abreu et al. response characteristics in the boundary regions. The 2003; Anka et al. 2009). We studied the internal architec- mudstone content rises at the boundary, making the sand tural patterns using both shallow high-frequency and deep body thinner, mostly manifested by weakened amplitude well-to-seismic integration data. intensity. In addition, the lateral migration of channels can create imbricate responses on the seismic profile (Fig. 8). 4.2.1 Horizontal migration pattern 4.3 Hierarchical architecture pattern of channel By drawing the RMS amplitude attributes slice of channel systems deposits in the M oilfield, we can see that there are two migration types in single channels: in lateral and paleocurrent The channel system (canyon or large incised waterway) is direction (Zhao et al. 2012a, b) from the channel plan view. So regarded as a large-scale unit, superimposed by various we built the migration pattern of single channels inside the complex channels (Xie et al. 2012; Yu et al. 2012). complex channel according to the seismic response features, Affected by erosive power, the reservoir architecture models of the same deepwater turbidity channel system as shown in Fig. 7. Lateral migration causes sand body dis- tribution more continuous, which expands the connected differ greatly in different deposit locations. Spatial struc- range of sand bodies. With migration in the paleocurrent ture characteristics can be represented by the seismic direction, since the single channels are vertically superposed, information. Researchers categorize channel systems into the sand bodies show great heterogeneity in the vertical restricted (with incised valley), weakly restricted (with direction. In plan view, channels migrate along the prove- incised valley) and non-restricted (without incised valley) nance direction, presenting wide stripes for sand bodies (Liu according to geomorphic features (Hubbard et al. 2009; et al. 2013). The width of complex channels ranges between Zhao et al. 2012a, b; Lin et al. 2013; Clark and Pickering 300 and 1500 m in the study area. Due to different migration 1996; Deptuck et al. 2003; Sun et al. 2014; Chen et al. patterns of different channel segments, there are ambiguous 2015). relations between the width and depth for complex channels in Different superimposition patterns of complex channels our study. Another important factor to describe migration lead to different architectures inside the channel system. patterns in complex channels is sinuosity, which is due to Inside restricted channel systems, deeply incised indented single channel migration. It is measured to be 1.0–1.8, aver- and swing complex channels are the major part and barely aging at 1.3 in our study, using the central line in the boundary develop large natural levees. With weakly restricted area of the complex channel as baseline. This is much smaller channel system, there are deeply incised indented and than that of single channels. swing complex channels combined with weakly incised horizontal migration patterns, with deposit overflow along 4.2.2 Profile migration pattern incised valleys, developing large natural levees on both sides. Non-restricted channel system features weakly The profile migration patterns of complex channels are also incised indented and swing channels, along with horizontal subjected to single channel migrations. As we know, the migration patterns. There are occasional deeply incised profile migration pattern of single channels can be channels at the bottom of non-restricted channel systems, 123 500 Pet. Sci. (2017) 14:493–506 Channel 1 Horizontal migration pattern Channel 2 Lateral swing Palaeocurrent-oriented sweep 1 km RMS horizon-oriented slice Fig. 7 Plane migration patterns of single channel based on layer slicing in M oilfield (slice position as shown in Fig. 1) with no evident natural levee development. Especially in in inconspicuous incised valleys. Results show widths of the late stage of channel system development, it is difficult 1000–3000 m and depth of 80–280 m for the O73 channel to distinguish between fine-particle-filled channel deposits system, a large-scale one. and natural levee deposits. To sum up, the spatial patterns of channel system vary significantly due to deposition patterns of complex channels. 5 Sedimentary evolution of turbidity channels Weakly restricted and non-restricted channel systems are considered as major categories developed in the O73 It is through analyzing the evolution of channel systems that oilfield in the study area, and their seismic response char- we understand the deposition processes and architectural acteristics are shown in Fig. 9. There show certain evolu- genesis. More importantly, it will strengthen the credibility tion trends in the plane. The closer it is to the provenance of inter-well prediction for the architectural characterization direction, the stronger the channels incision. Weakly based on well-log and seismic data under wide spacing. restricted channel systems have distinct incised sections, and wedges (large natural levee deposit) are developed on 5.1 Sedimentary evolution characteristics both sides. The further it is from the provenance direction, the weaker the incision, and the stronger the aggradation Through well-to-seismic calibration, we make a compre- and lateral migration. Non-restricted channel systems hensive explanation of S1-S4 (Fig. 9) complex channel develop with no evident boundary features (without sediments in the O73 channel system of the M oilfield. developing large incised valleys). Typical vertical evolu- Seismic attributes demonstrating sand body distributions, tion patterns can be found inside non-restricted channel such as RMS amplitude, were extracted (Fig. 10). Com- systems, depicted in four stages. Great incision in the early- bined with core data, we have managed to explain evolu- stage deposits and large incise valleys are developed. But tion characteristics of each stage (Fig. 11). in later stages, because of rising sea level, incision abates The O73 reservoir of the M oilfield shows noticeable while aggradation and lateral migration enhances, resulting sequence sedimentary characteristics. The early 123 20m 20m 100m 100m 20m 100m Pet. Sci. (2017) 14:493–506 501 Well_6ST 300m 400m 400m 400m a Horizontal b Indented c Swing Lateral migration Vertical migration Fig. 8 Profile migration patterns and vertical evolution of single channels of zone O73 in the M oilfield Well_5 A’ Well_5 A’ Weakly restricted channel 800m Well_6ST B B Well_2B Well_6ST B’ Non-restricted channel 2km 800m Fig. 9 Plane evolution relation of the O73 channel system (slice position as shown in Fig. 1) 123 502 Pet. Sci. (2017) 14:493–506 RMS RMS RMS RMS 1000 1000 1000 800 800 800 600 600 600 400 400 400 200 200 200 0 0 0 Well_2B Well_2B Well_2B Well_2B 1 km 0 1 km 0 0 1 km 0 1 km O73_S4 O73_S3 O73_S2 O73_S1 Fig. 10 Plane characteristics of O73 channel system in the M oilfield (slice position as shown in Fig. 9) sedimentary stage (S1) belongs to deepwater stratified period channel sand bodies results in expanded distribution sediments. It shows distinct restricted characteristics with of sand bodies. For well-2B, the meander section of the abundant supply and strong erosive power, widely dis- outer S3 sequence channel complex was drilled, so the core tributed in the riverbed at the bottom of the large sub- only represents a partial sedimentary filling sequence. marine canyon of the O73 reservoir. Well-2B coring shows There is a layer of gravelly sediments (about 4 m in that the lowermost part of the sequence mainly consists of a thickness) at the bottom of the coring section, which is coarse sandstone stratigraphic unit, which often contains covered by a hard sand clastic layer (about 2 m in thick- coarse to boulder-level conglomerates and mudstone frag- ness) and then comes a 6-m-thick muddy siltstone (the top ments. It can be deduced that its high sedimentary energy sediment of the S3 sequence is draped by the mudstone enables it to downcut the older conglomerate layer and layer). For well-2G, drilling of the meander section of the clastic layer. The middle part of the sequence is a mixed interior S3 sequence channel revealed that S3 sequence sedimentary unit of thicker coarse sand and medium sand, channel intensively eroded S2 sequence sediments probably high-density turbidite sediment, and it progres- (Fig. 11e), which is possibly related to weak consolidation sively changed into a finer-grained Tb and Tc type low- of the early channel sediment and the supply channel density turbidite layer in an upward direction. The upper- formed due to the negative topography on the edge of the most part is a mudstone layer, indicating gradually waning channel. energy. The S4 sequence is the last layer of the O73 channel During the S2 stage, sediment supply is still sufficient, system, belonging to the sediment shrinkage stage when but the sea level began to rise forming a slightly restricted the sea level reached a peak. The channel is still highly channel. Here the single channel shows lateral migration, sinuous, but the scale is much smaller than that of the S3 as a weakly restricted channel, and thus widely distributed. sequence. The downcutting depth decreases, with As shown in Fig. 11, the S2 sequence channel sediment strengthened lateral restriction. Core analysis indicates that presented a transition trend from the main channel axis to the bottom of the sequence is well-sorted medium-fine the edge, with muddy interlayers gradually developing in massive sandstone, with good consistency in seismic the edge. Core analysis indicates that the lithology of S2 is response within the whole oil field (Fig. 10). It is the final mainly thick massive sandstone, partially interbedded with product of channel filling, and there will be the abandon- thin mudstone layers. The top layer gradually changed to ment stage of the O73 channel system afterward. siltstone and mudstone layers, with occasional ripple bed- ding, showing gradual abandonment characteristics. 5.2 Sedimentary controlling factors and their During the S3 stage, the sea level continued to rise, and evolution sediment supply started to decrease. Channel sediments show distinct non-restricted characteristics. Lateral migra- 5.2.1 Sedimentary controlling factors tion and vertical downcutting are both strong for single channels, as well as high sinuosity. The channel on the Deepwater detrital deposits are controlled by autogenetic planar graph (Fig. 10) is very clear. Superposition of multi- cycles and allogenetic cycles. The controlling factors 123 Pet. Sci. (2017) 14:493–506 503 S4 Abandonment S3 Abandonment S4 S2 Abandonment 1c S3 S2 (i) Late stage channel abandonment coating deep S1 water mudstone S3 Abandonment S4 S2 Abandonment 1c S3 S2 (h) Late stage S4 high- curvature constructive S1 channel sediments S3 Abandonment S2 Abandonment 1c S3 S2 (g) S3 late stage sediments abandonment S1 with pelitic filling S2 Abandonment S3 S2 (f) S3 constructive channel sediments S1 changing original erosion interface S2 Abandonment S2 (e) S3 form deep incised S1 valley before sediment and eroded S2 channel sand S2 Abandonment S2 (d) S2 sedimentary S1 weakening and channel abandonment S2 (c) S2 erosion constructive channel S1 with pelitic laminate (b) S1 coarse erosion channel and top pelitic S1 fine-grained sediments (a) O73 channel system incised valley formation Fig. 11 Vertical evolutionary model of O73 channel system in the M oilfield 123 504 Pet. Sci. (2017) 14:493–506 include eustacy, basin tectonic movement, sediment types and thick. Because of the contemporaneous falling sea and supply rates. Moreover, events such as earthquakes and level and continent uplifting, it reduces the distance tsunamis may also allow the clastic particles to reach the between provenance and deepwater sedimentary supply, deep sea after traversing the continental shelf and slope which is beneficial to form sediments. Therefore, the above valley, forming deepwater sediments (Stow et al. 1996; analysis indicates that sediment type of the O73 channel Shanmugam 2008). The combination of many controlling system is subjected to controlling factors including sedi- factors causes the difference in the erosive power of ment supply, deepwater gravitational flow and density. channels, resulting in the complex and diverse superposed relationship of sand bodies. These controlling factors 5.2.2 Evolution discussion include the provenance distance, provenance types, climate in the provenance area, sea level eustacy, topographic Although models of the development of the channel system slope. Under normal circumstances, the closer to the are affected by multiple factors, they follow certain evo- provenance, the greater the topographic slope and the more lutionary trends (Posamentier and Kolla 2003; Prather sea level drops, the more abundant the sediment supply, the 2003; Liu et al. 2008). Horizontally, the development is greater the load density, the higher the deposit velocity, and mainly manifested as the evolution of different channel the stronger the erosive power (He et al. 2011; Zhuo et al. system types; and vertically, the development is primarily 2013; Li et al. 2011). These factors carry various weights in represented by the evolution of its internal complex influencing channel systems, and they correspond to vari- channels. ous types of channel systems. For instance, fast sedimen- Horizontally, along the provenance direction, there are tary flow and powerful erosion in steep slopes favor certain trends in changes on account of differences in restricted or weakly restricted channel systems, whereas erosion of sediments. As the root of the channel system is non-restricted channel systems are often observed in gentle nearer to the sediment source, large size, high flow rate and slope areas. Likewise, when sea level falls, there is ample strong erosive power, large incised valleys can be formed sediment supply and all kinds of channel systems can form. and restricted channel systems that focus on transporting Otherwise, in times of rising sea level where sedimentary sediments were mainly developed, leading to a large supply is scarce, turbidite channel systems are seldom amount of fragmental flow, turbidite and slump sediments developed. Furthermore, allogenetic cycles are more evi- developed in it. In the middle of the channel system, sed- dent in high sea level periods while autogenetic cycles iments become finer with decreased flow rate, resulting in dominate in low sea level periods (Posamentier and Kolla weakened downcutting and strengthened aggradation, so 2003; Prather 2003). For the O73 channel system of the M the weakly restricted channel system (e.g., O73 channel system as shown in Fig. 9) is mainly developed at this oilfield, eustacy, tectonic movement and topographic slope play a key controlling role in reservoir architecture and point, in which channels with some degree of bending are distribution, and abundant sediment from the Congo River developed and filled with an amount of fragmental flow thanks to the moist climate in the Oligocene is also another and slump substances. While at the distal end of the vital factor in turbidite channel formation (Booth et al. channel system, the supply energy wanes and sediments are 2003; Violet et al. 2005; Beydoun et al. 2002). Tectonic of the smallest size and lowest flow rate. At this point, the movements such as differential uplift keep modifying both sediments downcutting capacity is weak, but the lateral the macro- and micro-topography, which alters the energy migration capacity is strong, developing non-restricted of gravitational flow, and then alters development location channel systems mainly in which the single channels are and distribution of deepwater sedimentary units. From a mostly moderately to highly bent. macro-perspective, the differential uplift of the Congo Vertically, influenced by eustacy and delivery rate of Basin in the Angola area causes the sedimentary center to sediments, the development of internal complex channels move north. Meanwhile, from a micro-perspective, tec- inside the channel system also follows evolutionary tonics like salt diapir accompanied by partial salt rock trends. In the early development period of the channel movement also greatly affects deepwater channel systems system (S1 stage), high flow rate and abundant supply of (Anka et al. 2009; Broucke et al. 2004; Kolla 2007; Pirmez sediments with strong erosion mostly contribute to form and Imran 2003). Eustacy influences the development of deep downcutting complex channels. They, as erosive deepwater channel systems too. Deepwater sediments in channels, mainly transport sediments. In the middle West Africa developed in the Upper Cretaceous when development period of the channel system (S2 stage), the global sea level fell. At that time, the scale of deepwater sea level begins to rise. The sediment supply is still rich, channel systems expanded as sea level fell and they but the slowing flow rate leads to its slightly weakened advanced toward the sea. Furthermore, the planar features downcutting capacity and strengthened aggradation, with of deepwater channels turned from wide and thin to narrow mixed development of aggradational channels and erosion 123 Pet. Sci. (2017) 14:493–506 505 Acknowledgements This paper is supported by the National Major channels mainly under the effect of sedimentation. In the Scientific and Technological Special Project during the Thirteenth middle to late development period of the channel system Five-year Plan Period (2016ZX05033-003-002) and the Project of (S3 stage), with the continual rise of the sea level, the Sinopec Science and Technology Development Department (G5800- sediment supply falls gradually (except in tsunamis, 15-ZS-KJB016). earthquakes and other unexpected events). The channel’s Open Access This article is distributed under the terms of the downcutting capacity weakens, but the lateral accretion Creative Commons Attribution 4.0 International License (http://crea capacity becomes increasingly stronger, with aggrading tivecommons.org/licenses/by/4.0/), which permits unrestricted use, highly sinuous channels mainly developed. In the late distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a development period of the channel system (S4 stage), the link to the Creative Commons license, and indicate if changes were sea level reached a high level and the sediment supply made. was the weakest, pointing to a sediment shrinkage stage where only a small number of highly sinuous aggraded channels and even isolated mudstone-filled single chan- References nels were developed. Abreu V, Sullivan M, Pirmez C, et al. 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Petroleum Science – Springer Journals

Published: Jul 27, 2017

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