Table of Content

28 December 2021, Volume 42 Issue 6

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Petroleum Geology

Petroleum systems and exploration potential in deep Paleogene of the Dongying Sag, Bohai Bay Basin

Mingshui Song, Yongshi Wang, Xuefeng Hao, Tianxia An

2021, 42(6):  1243-1254.  doi:10.11743/ogg20210601

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The Dongying Sag is a representative non-marine faulted depression in the Bohai Bay Basin. Two wells (FSh1 1 and FShX 101) drilled into the deep of the sag were tested with high rates of oil flow, revealing great exploration potential of deeper layers that have been relatively overlooked during years of intensive oil and gas exploration and development in the basin. This study, taking the deep Paleogene strata in the sag as an example, aims to assess the potential through oil and gas accumulation models established on analyses of source rocks and genesis of high-quality reservoirs in the sag. The results show that the Ek (Kongdian Formation) and lower Es⁴ layers in the deep Paleogene constitute on their own an independent petroleum system. The source rocks in the Ek² and lower Es⁴, developed in a saline lake (salinized) environment are characterized by an early, long and efficient hydrocarbon expulsion and a long hydrocarbon generation-expulsion cycle, providing key components for the formation of a deep petroleum system. The multi-type sedimentary facies of the saline lake have experienced alternative modification of acidic and alkaline diagenetic fluids, and formed high-quality reservoirs mainly composed of secondary dissolution pores, which provided good reservoir conditions for the deep hydrocarbon accumulation. The distribution of oil/gas reservoirs in the deep layers corresponds well to the structure of faulted depression, the development of sedimentary reservoirs and the dynamic system of hydrocarbon accumulation. The "indigenous" and "non-indigenous" hydrocarbon accumulation patterns are orderly distributed. Different types of oil/gas reservoirs, such as red beds and beach bar sandstone in gentle slope zone, as well as sandy conglomerate in steep slope zone, are the focus of exploration in the deep Paleogene of Dongying Sag.

Differences in hydrocarbon accumulation stages and main controlling factors in the Bohai Bay Basin

Youlu Jiang, Shengmin Su, Hua Liu, Honghao Zhao

2021, 42(6):  1255-1264.  doi:10.11743/ogg20210602

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The Bohai Bay Basin hosts dozens of sags with relatively independent petroleum systems, in which large-scale hydrocarbon migration and accumulation occurred at various geologic times and stages. A systematic investigation is conducted on the timing of hydrocarbon accumulation as well as the main controlling factors on the differences in geologic times and stages of hydrocarbon accumulation in the major reservoirs of these sags in the basin through combining a large number of statistical data and the analyses of hydrocarbon generation history of source rocks with previous studies. Results show that hydrocarbons generated by the Paleogene source rocks probably accumulated in two stages, namely the end of Paleogene (early stage) and middle Neogene-Quaternary (late stage but more important), respectively. During the whole process, the predominance of early accumulation has been gradually taken over by the late accumulation from the basin edge sag inward to the Bozhong sag, and the accumulation stages feature in changing from single stage of early accumulation to two stages with early accumulation predominance, to two stages with late accumulation predominance, and to single stage of late accumulation. Vertically from deep to middle and further to shallow reservoirs, hydrocarbon accumulation stages change from single stage of early accumulation to two stages and further to single stage of late accumulation. The subsidence patterns of the sags during the Cenozoic are suggested to be the major control over the thermal evolution of source rocks, which in turn controls the differences of hydrocarbon accumulation stages, resulting in a good correspondence between the subsidence patterns and the dominant accumulation stages. The denuded thickness at the end of Paleogene and the sedimentary thickness of Neogene-Quaternary are believed to be the determining factor on which accumulation stage being predominant. Observation shows that the greater the compensation thickness, the more obvious the characteristics of late accumulation stage. The main hydrocarbon generation periods determine the main hydrocarbon accumulation stages. Hydrocarbons sourced from the Kongdian Formation source rocks is likely to accumulate mainly in the early stage, while that from the source rocks in the first member of Shahejie Formation to the third member of the Dongying Formation is more likely to be related to the late accumulation. Hydrocarbons from source rocks in the upper fourth and third members of Shahejie Formation shows characteristics of two stage accumulation.

Hydrothermal dolomitization and its role in improving Middle Permian reservoirs for hydrocarbon accumulation, Sichuan Basin

Shuangjian Li, Tianbo Yang, Yueqing Han, Ping Gao, Yujin Wo, Zhiliang He

2021, 42(6):  1265-1280.  doi:10.11743/ogg20210603

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The Middle Permian in the Sichuan Basin hosts various types of hydrothermal dolomites that are important to oil and gas exploration. Their mechanisms and occurring models as well as their contribution to reservoir quality enhancement have been the focus of study for years but a consensus is yet to be reached. This paper summarizes the mechanisms and their role in shaping reservoirs through detailed mineralogical, petrographic and geochemical analyses of hydrothermal dolomites from typical locations in the Sichuan Basin. The results show that the hydrothermal dolomitization in the Middle Permian of the Sichuan Basin is mainly related to contemporaneous fault activities and has no obvious correlation with the lithology and lithofacies of surrounding rocks. According to the relationship between hydrothermal dolomites and surrounding rocks, the hydrothermal dolomitization possibly occurs in three models: (1) hydrothermal dolomitization from modification of grain-beach facies, mainly occurring in the Qixia Formation of western and central Sichuan Basin and has no significant contribution to the improvement of reservoirs; (2) hydrothermal dolomitization from modification of karstified limestone, mainly occurring in the Maokou Formation of central Sichuan Basin and is insignificant in contributing to reservoir quality enhancement; and (3) hydrothermal dolomitization from modification of tight limestone, mainly occurring in the Maokou Formation of eastern Sichuan Basin and has contributed greatly to reservoir property improvement. Fault-related fractures, residual dissolution pores and dolomite intercrystalline pores constitute most storage space in reservoirs with hydrothermal dolomitization. Compared with reservoirs with relatively good initial physical properties, the reservoirs with poor initial physical properties are even more significant in terms of quality enhancement after hydrothermal dolomitization and relevant structural activities.

Development patterns and constraints of superimposed karst reservoirs in Sinian Dengying Formation, Anyue gas field, Sichuan Basin

Xinhua Ma, Haijun Yan, Jingyuan Chen, Dongbo He, Wei Xu

2021, 42(6):  1281-1294, 1333.  doi:10.11743/ogg20210604

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Comprehensive analyses of karst reservoir development characteristics and productivity testing of completed wells in the reservoirs of the Sinian Dengying Formation, Anyue gas field, Sichuan Basin, are combined with the development mechanisms for carbonate karst reservoirs to propose a concept and development model of superimposed karst and clarify their main controlling factors and effect on gas well performance. According to core, imaging data, reservoir space type, size and the configuration between fractures and cavities, the karst reservoirs are classified into three types: fracture-cavity type, pore-cavity type and pore type. High-quality reservoirs in the fourth member of the Dengying Formation in the field mostly belong to the first two types. Influenced by the differences in the initial lithology and physical properties of sedimentary strata and in paleogeomorphology characteristics during karst period, the karst reservoirs vary in terms of dissolution depth, vertical development locations and plane distribution range. A development model with differential dissolution for superimposed karst is thus proposed. It is also suggested that the sedimentary characteristics of microbial mounds and shoals, the overlapping two-stage weathering crust, the development scale of fault-fracture systems and the geological process of tectonic inversion control the development of superimposed karst reservoirs. Apart from that, superimposed karst development intensity also controls gas well performance. Based on these understandings, it is suggested a development priority on superimposed karst reservoirs in the basin. The study of development model and the controlling factors of these reservoirs is helpful in gas reservoir development evaluation, production area screening and well emplacement in the area.

Characteristics of fractures and their significance for reservoirs in Wufeng-Longmaxi shale, Sichuan Basin and its periphery

Ruyue Wang, Zongquan Hu, Tong Zhou, Hanyong Bao, Jing Wu, Wei Du, Jianhua He, Pengwei Wang, Qian Chen

2021, 42(6):  1295-1306.  doi:10.11743/ogg20210605

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An integration of core and scanning electron microscopy (SEM) observation, FMI logging, and physical property analysis, is applied to discuss the types, characteristics and distribution of fractures and their effects on gas enrichment and preservation in the shale of Wufeng Formation-Longmaxi Formations in the Sichuan Basin and on it speriphery. The results show that: (1) the development of shale fractures results from a combination of factors including sedimentation, diagenesis, tectonic and pressure evolution. Low angle slip fractures and bedding-parallel fractures at low level of filling are conducive to reservoir porosity and permeability development. High angle fractures and horizontal interlayer fractures are usually fully filled, leading to low effectiveness as fluid pathways. The modification of low angle fractures on the layer-transecting capacity of high angle fractures is of great significance to the enrichment and preservation of shale gas. (2) The micro-fractures of shale are mainly non-structural. The upper organic-lean interval with high clay mineral content, is of low developmental degree of bedding-parallel and large-scale inter-layer micro-fractures; instead, the small-scale inter-clay particle micro-fractures are well-developed. In this case, the new hychaulic fracturing technologies of dense cluster spacing and high proppant concentration should be adopted to improve fracture network-controlled reserves. However, the bedding-parallel fractures, interlayer micro-fractures and rigid grain-edge fractures with weak mechanical properties are abundant in the lower siliceous shale interval; in addition, the coexistence of intensive low angle fractures and small-scale high angle fractures results in a favorable natural fracture network system. (3) The permeability of the bedding-parallel fractures and interlayer micro-fractures in shale intervals of shallow to middle burial depth, is significantly higher than that of the matrix, favorable for reservoir stimulation and seepage of shale gas. In contrast, permeability of fractures and matrix are both low and roughly equivalent for shale intervals of deep burial. Therefore, the reservoir seepage ability and hychaulic fracturing effect are essential factors for efficient development of deep shale gas.

Isotope geochemical responses and their geological significance of Changxing-Feixianguan Formation carbonates, northeastern Sichuan Basin

Qingmin Dong, Zhonggui Hu, Shiyue Chen, Shilin Li, Jialan Cai, Yixin Zhu, Yuying Zhang

2021, 42(6):  1307-1320.  doi:10.11743/ogg20210606

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The carbon and oxygen isotopes are important geochemical indexes for the study of carbonate rocks and have been widely used in revealing global climate changes and sea level fluctuation as well as diagenesis and stratigraphic correlations. This study starts with outcrop observation, thin-section identification and trace element analysis, continues with establishing isotope evolution curves with emphasis on the regional comparative analysis of the geochemical response characteristics of carbon and oxygen isotopes based on analyses of carbon and oxygen isotope compositions of 241 carbonate samples from the Changxing-Feixianguan Formations in northeastern Sichuan Basin, and ends with a discussion on the geological significance of these characteristics. The results show that the carbon isotope evolution curves positively correlate with sea level changes in the Changxing period and correspond well with the system tracts in the sequence. The oxygen isotope composition shows a relatively stable trend during this period. Strong negative drift of carbon and oxygen isotope values occurs near the Permian/Triassic boundary while reaching their lowest values. After entering the Feixianguan period, the carbon isotope value keeps rising with a low-amplitude fluctuation until reaching the maximum value inside the fourth member of Feixianguan Formation. The oxygen isotope composition is stable during the Feixianguan period. It is also suggested that the strong negative drift of carbon and oxygen isotopes at the Permian/Triassic boundary may be closely related to methane release and volcanic eruption events.

Characteristics of and factors influencing organic pores in the Lower Silurian Longmaxi Formation, Fushun-Yongchuan area, Sichuan Basin

Xiya Yang, Chenglin Liu, Wenping Liu, Haolin Ren

2021, 42(6):  1321-1333.  doi:10.11743/ogg20210607

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The Lower Paleozoic marine shale sequences are widely developed in the southern part of Sichuan Basin. Among others, the Longmaxi Formation shale possesses high-quality source rocks favorable for shale gas enrichment. We discuss the characteristics of and factors influencing the organic pores in the Lower Silurian Longmaxi Formation, Fushun-Yongchuan area, based on coring data obtained from the drilling of organic-rich shale, as well as related analysis and tests. The research results indicate that the organic matter abundance of the Longmaxi Formation shale is high with a TOC content ranging from 2.45% to 5.59%, and its thermal evolution reaches a high-to-over-mature stage with a vitrinite reflectance (R_o) of 1.97% to 2.23%, providing a favorable basis for the growth of organic pores. As important reservoir space for shale gas, most organic pores are round-, oval- or concave-shaped under scanning electron microscopy (SEM) observation, and a few occur in strips and/or irregular polygons; all pores are smooth-edged. Besides, nitrogen adsorption and high pressure mercury intrusion porosimetry (MIP), show that these organic pores are relatively small in size, most of which are supposed to be ascribed to mesopores, accounting for about 57%. Then, we quantitatively analyze the organic pores across the study area by establishing a relationship model between porosity of organic pores and spectral gamma-ray (GR) logs (U), and the results show that the porosity of organic pores varies between 1.10% and 3.64%, with an average of 2.15%. There are multiple factors related to the development and preservation of organic pores. The correlation between organic pore porosity with contents of clay minerals, brittle minerals and TOC, is set up and shows that the porosity of organic pores is positively correlated with contents of brittle minerals and TOC but negatively with clay mineral content. Compaction has a destructive effect on the preservation of organic pores, and a higher formation pressure coefficient is conducive to the preservation of organic pores.

Microscopic formation mechanisms and distribution patterns of remaining oil in the marine clastic reservoirs of the Carboniferous, Tarim Basin

Tingbin Sun, Chengyan Lin, Ling Wang

2021, 42(6):  1334-1343.  doi:10.11743/ogg20210608

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Three experiments including CT scanning during flooding, post-flooding observation of oil-bearing thin sections under high temperature and pressure, and water flooding under high temperature and pressure by microscopic etching model, are utilized to disclose the formation mechanisms of remaining oil, their types and distribution at the end of water flooding in the marine A reservoir at the bottom of the Carboniferous system, western Tarim Basin, and strategies of enhanced oil recovery (EOR) are proposed. The results show that the remaining oil is in dispersed distribution as a whole and features patchy enrichment, due to the stripping, fingering, by-passing and capturing effects; accordingly, the remaining oil can be categorized into 6 types including pore throat filling, pore half-filling, oil film on pore walls, dispersed oil droplets, throat retention and pore corner retention. Among others, the pore half-filling type and dispersed oil droplet type in the flooded areas, account for 65.4% of the total, the highest in production potential, followed by pore throat filling type in non-swept zones, which features patchy enrichment, accounting for 21%, with lower production potential. In this regard, the surfactant or gas flooding is suggested to enhance oil recovery.

Spatial structure of Shunbei No. 5 strike-slip fault and its relationship with oil and gas reservoirs in the Tarim Basin

Bo Lin, Lu Yun, Haiying Li, Chongyang Xiao, Xu Zhang, Maohui Liao, Jun Han, Peng Wang, Xuechun Xu

2021, 42(6):  1344-1353, 1400.  doi:10.11743/ogg20210609

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Small-scale strike-slip faults developed in the Tarim Basin are spatially characterized by "small displacement, long extension, plane segmentation and longitudinal stratification". These geological characteristics, especially the longitudinal stratification, are believed to play an important role in hydrocarbon accumulation. Exploration activities in the Shunbei Oilfield in recent years have come across many fault-karst reservoirs and confirmed the impact of these faults on the pooling of oil and gas. However, how exactly the faults control the process of oil and gas accumulation is not fully clarified. The middle-north section of Shunbei No. 5 fault is selected to carry out a three dimensional assessment of the faults in terms of their role in connecting sources and accumulations as well as controlling, capping and improving reservoirs by using high-precision three-dimensional seismic fault analysis and three-dimensional evaluation technology based on available data of drilling, logging and well performance. The results show that the faults have the characteristics of "vertical layered deformation and plane piecewise evolution" in different structural layers. Their structural responses are also very different, revealing a vertical structural sequence from bottom up of vertical slipping, gypsum-salt detachment structure, flower structure, right-lateral echelon normal fault, left-lateral echelon normal fault and inherited left-lateral echelon normal fault. The structural styles of the faults in different structural layers exert different effect on oil and gas accumulations. This study provides a basis for better understanding the process of oil and gas accumulation and wiser selections of exploration targets, thus can be used as a guiding reference for oil and gas exploration and development in the future.

Evaluation of sealing effectiveness of gypsolyte during burial: A case study of the gypsolyte caprock in deep Cambrian, Tarim Basin

Tong Lin, Tongshan Wang, Wenqing Pan, Wenfang Yuan, Qiufen Li, Wei Ma

2021, 42(6):  1354-1364.  doi:10.11743/ogg20210610

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Gypsolyte has been widely accepted as a superior caprock for oil and gas reservoirs. However, not all gypsolytes are of effective sealing capacity, to which certain geological conditions are in need. The core and outcrop gypsolyte samples taken from the Tarim Basin are applied to carry out scanning electron microscopy (SEM), physical property analysis, breakthrough pressure test and triaxial stress test. The results suggest that (1) both gypsolyte and saline rock initially formed are of strong sealing capacity, which tends to grow weaker as the gypsum under dehydration transforms into anhydrite during evolution under burial, and the spaces formed thereby undermine its sealing performance; (2) The maximum burial depth at which gypsum-to-anhydrite transformation occurs is about 2000 m in the Tarim Basin, and the anhydrite developed under dehydration is significantly different from the initial gypsum in terms of shear resistance; (3) During the evolution process under burial, the sealing properties of gypsolyte are constantly changing under different formation temperatures and confining pressures, and it all depends on the sealing capacity and shear resistance of the gypsolyte under burial whether the gypsolyte can be eventually developed into effective caprock.

Application of laser in-situ U-Pb dating of calcite to determination of the absolute time of hydrocarbon accumulation in polycyclic superimposed basins: A case study on Tahe oilfield, Tarim Basin

Jiaxu Chen, Bin Wang, Xiaowen Guo, Zicheng Cao, Yongli Liu, Feng Geng, Xuyou Zhang, Hao Xu, Jianxin Zhao

2021, 42(6):  1365-1375.  doi:10.11743/ogg20210611

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The absolute time of hydrocarbon accumulation in polycyclic superimposed basins has always been a major problem to be solved in the research of hydrocarbon accumulation. Based on the observation of oil inclusions and fluorescence spectrum analysis, we classify the generation of calcite crystallization into four stages by cathodoluminescence and trace elements analysis carried out in the Tahe oilfield, Tarim Basin; then determine the absolute time of hydrocarbon accumulation by laser in-situ U-Pb dating of calcite generated in these stages. The results show that two types of oil inclusions with yellow and blue-white fluorescent respectively are developed in calcite veins of the Ordovician in the study area. The differences of cathodoluminescence and micro-area trace elements indicate two stages of calcite veins contain primary oil inclusions with yellow fluorescent, the ages of which just correspond to the two charging episodes of yellow fluorescent crude oil. The laser in-situ U-Pb isotope dating results of calcite veins show that the first episode of yellow fluorescent crude oil charging occurred during 329.7 Ma±1.6 Ma, the second ran from 249.3 Ma±2.6 Ma to 220.5 Ma±7.3 Ma. An integration of the isotope dating technique and hydrocarbon fluid inclusion analysis can effectively determine the absolute time of hydrocarbon accumulation in polycyclic superimposed basins, and is of great significance to reconstructing the dynamic process of hydrocarbon accumulation.

Accumulation mechanism of tight sandstone oil in Gaotaizi reservoir in Qijia area, Songliao Basin

Weitao Wu, Jingzhou Zhao, Qi'an Meng, Tiefeng Lin, Ge Zhang, Jinyou Zhang, Shanghua Si, Yubin Bai

2021, 42(6):  1376-1388.  doi:10.11743/ogg20210612

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Tight oil has become an important exploration field in China's petroliferous basins. The study focuses on the tight oil of Gaotaizi reservoir of Qijia area in the Songliao Basin. An array of data obtained from core, thin section, and fluid inclusion observation, geochemical analysis and production test, are applied to analyze the characteristics of the Gaotaizi tight sandstone reservoir, and clarify the accumulation mechanism of tight oil. The results show that the pore space of Gaotaizi reservoir is dominated by residual intergranular pores and feldspathic dissolved pores, with a mean porosity of 9.17% and a mean permeability of 0.45×10^-3 μm². On map view, the porosity of Gaotaizi sandstone reservoir in the northern part of Qijia area is mostly over 12%, while that in the sorth-central area is mostly less than 12%, thus belonging to tight reservoir. The tight reservoirs are mainly of faulted, lithologic-faulted and lithologic types in quasi-continuous distribution and are laterally superimposed and connected and vertically overlapped. They are controlled by sand body distribution, faults and reservoir physical properties, instead of structural highs. The source rocks of the 1^st and 2^nd members of Qingshankou Formation (Qing 1 and Qing 2 members) are of high quality at the peak of oil generation, featuring relatively large thickness and a TOC content of 2.47% and 1.6%, respectively. The tight oil accumulation occurred in 3 episodes of 2 stages, that is the late Nenjiang period and Mingshui period. The faults which do not cut through the Qingshankou Formation, fractures and sand bodies provide pathways for tight oil migration. The burial depth of the top surface of the Gaotaizi reservoir was 800-1 400 m at the end of the Nenjiang Formation deposition, corresponding to a palaeo-porosity of about 21%-15%, and the maximum residual pressure was about 10 MPa, resulting in only conventional reservoir in the center. While its burial depth was 1 600-2 200 m at the end of Mingshui Formation deposition, corresponding to a palaeo-porosity of about 13%-7%, and the maximum residual pressure was over 25 MPa, resulting in tight oil reservoir in wide distribution. The accumulation model of tight oil therein can be concluded as a quasi-continuous type of dual sourced hydrocarbon charging, featuring an lower source rock-upper reservoir assemblage and source-reservoir alternating configuration.

2D NMR studies of fluids in organic-rich shale from the Qingshankou Formation, Songliao Basin

Longhui Bai, Bo Liu, Yaao Chi, Shichao Li, Xun Wen

2021, 42(6):  1389-1400.  doi:10.11743/ogg20210613

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Shale may contain a large amount of oil and gas that can be commericially exploited. Clarifying the content and occurrence of different fluids in shale is crucial to the development of shale oil. High frequency (23 MHz) two-dimensional (2D) NMR technology that can be used to quickly, nondestructively and quantitatively measure protium-containing (¹H) fluids in shale is employed to test shale samples (from the Qingshankou Formation in Songliao Basin) before and after extraction as well as during spontaneous imbibition and constant temperature rising experiments. The content and occurrence of different fluids in the samples as well as the loss of fluid from the samples after a period of static settlement are quantified and analyzed. The results show that on the 2D NMR maps, the signals of analogous solid organic matter and movable hydrocarbons, roughly separated by T₂=0.1ms, are mostly distributed in the region above T₁>10 ms. Good linear correlations can be observed between the signals and TOCs or S₁. Oil is either free or absorbed in shale samples from the Qingshankou Formation. Higher clay content means more absorbed oil and less free oil in shale. The 2D NMR spectrum obtained during spontaneous imbibition experiment indicates water swelling of clay minerals. The vacuum heating process of the samples shows both volatilization of light hydrocarbons and significant loss of water. Samples from sealing core drilling combined with 2D NMR can be used effectively in the restoration of the original oil saturation and water saturation.

Development characteristics and formation mechanism of lacustrine dolomite reservoirs in the Paleogene inter-salt shale sequence, Qianjiang sag, Jianghan Basin

Junjun Shen, Guoliang Tao, Kongquan Chen, Junjun Li, Pengwan Wang, Zhiming Li, Qigui Jiang, Jianghui Meng

2021, 42(6):  1401-1413.  doi:10.11743/ogg20210614

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This study deals with the 10^th cyclotherm of the 4^th oil layer group in the lower submember 3^rd of the member of Qianjiang Formation (hereafter referred to as Qian 3^L(4-10) cyclotherm), an inter-salt shale sequence in the Qianjiang sag, Jianghan Basin, with respect to the development characteristics and formation mechanism of dolomite reservoirs in the cyclotherm. The drilling, core and logging data serve as the research basis, in combination with analytical data from thin section identification, scanning electron microscopy (SEM), high pressure mercury intrusion porosimetry (MIP), X-ray diffraction, rock freezing and Rock-Eval pyrolysis, and gas chromatography-mass spectrometry (GC-MS) of saturated hydrocarbon. The results show that (1) the target layer features complicated mineral compositions of 4 lithic facies, including carbon-rich argillaceous dolomite, carbon-rich argillaceous limestone, carbon-rich dolomitic mudstone and carbon-rich dolomitic mudstone filled with glauberite. Among others, the carbon-rich argillaceous dolomite is considered the best, owing to its high abundance of organic matters in a range of 3.3% to 6.3%, high level of brittle minerals of 50% to 76%, low level of clay minerals of 18% to 33%, and high porosity of 10.8% to 26.3%; (2) The reservoir space of the dolomite reservoirs is dominated by intercrystalline pores, together with few dissolved pores and fractures, and these pores and fractures are characterized by poor structure and poor connectivity; (3) The macroscopic distribution of argillaceous dolomite reservoirs are significantly determined by sedimentary setting. For instance, the carbon-rich argillaceous dolomite mainly occurs in areas of moderate salinity during the middle phase of eustatic lake level descending. Microscopically, its pore development is markedly associated to compaction, while slightly to cementation and dissolution; (4) The weak compaction during the early diagenesis, the penecontemporaneous dolomitization and the hydrocarbon charging to reservoirs, all play a positive role in the preservation of intercrystalline pores, and are key to reservoir formation in the study area.

Application of standardized sequence stratigraphy theory in the shelf break-to-slope area: A case study of the Middle Miocene sedimentary sequence in the PRMB

Weitao Chen, Shaohua Xu, Zhen Sun, Min He, Jiali Yao, Yingmin Wang, Haiteng Zhuo

2021, 42(6):  1414-1422.  doi:10.11743/ogg20210615

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The application of standardized sequence stratigraphy theory is yet to be improved in the shelf break-to-slope area in deep water. As of now, the researches on the sequence stratigraphic framework mostly focus on deep-water channels or interior basin floor fans (BFFs) based on the high-resolution seismic data of shallow layers and outcrop observation, while ignoring systematic correlation with the upper slope and shelf system. In this regard, the study sets up a uniform sequence stratigraphic framework running from continental shelf to slope on the basis of systematic interpretation, attribute extraction and visualization of 3D seismic data, as well as analysis of drilling and logging data, while applying the standardized sequence stratigraphy theory to the Middle Miocene (around 13.8 Ma) shelf break-to-slope area in deep water in the Pearl River Mouth Basin (PRMB). As shown by the newly established framework, deposit sedimentary sequences in both transgressive systems tract (TST) and highstand systems tract (HST) are extremely thin as a whole; foreset beds of the falling stage systems tract (FSST) and lowstand systems tract (LST), however, are much thicker, and bottomset beds (mainly of deep-water fan) are widely developed in the LST. In a nutshell, the establishment of deep-water sequence stratigraphic framework is supposed to have a holistic consideration from shelf break to deep-water areas at first; 3D systematic comparison of seismic events is to be followed while analyzing the evolutionary features of "Source-to-Sink" system, with a view to better dissecting the system tracts and understanding the coupling relationship of shelf, slope and deep-water sedimentary facies.

Genesis of high-quality source rocks in volcano-related alkaline lakes and implications for the exploration and development of shale oil and gas

Changzhi Li, Pei Guo, Xianqi Ke, Yan Ma

2021, 42(6):  1423-1434.  doi:10.11743/ogg20210616

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In order to clarify the controlling mechanisms of volcanic activities on the development of high-quality source rocks in non-marine petroliferous basins, this study summarizes the relationships between volcanic activity, alkaline lakes and high-quality source rocks through an extensive review of previous studies and proposes that alkaline lakes act as a vital link between volcanic activities and source rocks. It is suggested that large amount of CO₂ emitted by volcanic activities would enter hydrothermal fluids, underground waters or rivers and then produce a large amount of HCO₃^- through accelerated silicate hydrolysis process, leading to the formation of alkaline lakes. The high pH in alkaline lakes would activate a variety of nutrient elements and compounds such as Mo, phosphate and silicate, thus improving the primary productivity of water body. In addition, the high pH also would lead to an exponential increase of silica solubility in alkaline waters. During the process of pH decrease by initial degradation of organic matter, the dissolved silica would precipitate and form a siliceous layer, which could effectively prevent further degradation of organic matter. Based on these assumptions, this study proposes a genetic model for volcanism-alkaline lake-high-quality source rocks chain, which is the main reason of the occurrence of brittle and porous shale reservoirs for oil due to a high content of microcrystalline dolomite and tuff materials that are easily converted from montmorillonite to zeolite, potassium feldspar, and sodium feldspar.

Deformation and evolution of mud diapirs in Rio Del Rey Basin, Niger Delta

Yixin Yu, Tao Wang, Zhankun Chen, Yushan Su, Jixin Zhang, Shixiang Wu

2021, 42(6):  1435-1439, 1474.  doi:10.11743/ogg20210617

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Several mud diapir belts developed in the Akata Formation mudstone are closely linked to the accumulation of hydrocarbon in the Rio Del Rey Basin, northeast Niger delta. According to their heights and contact relationships to peripheral strata, the mud diapirs can be divided into complicated pierced and simple buried styles, with S-N trending elongations or scattering dots in a plane view. Influenced by mud diapirs, three kinds of normal faults have developed in the overburden. The evolution of mud diapirs experienced three stages: the deposition and weak activity stage before the middle Oligocene, intense rising stage during the late Oligocene and Miocene, and weak activity and buried stage after the Miocene. In addition to the influence exerted by gravitational sliding and differential loading induced from the overburden, the formation of mud diapirs are also subjected to some Atlantic transform faults in the southeast part of the Rio Del Rey Basin and basement faulting activities. The study is helpful to the analysis of the impact of these diapirs upon the development of traps and reservoirs as well as hydrocarbon migration in the basin.

Depositional characteristics of locally restricted channel complex in deep water and its influence on reservoir properties: A case study of the Eocene series, Rovuma Basin

Hui Sun, Guozhang Fan, Dali Shao, Guoping Zuo, Shaozhi Liu, Hongping Wang, Hongxia Ma, Xiaoyong Xu, Yintao Lu, Chun Yan

2021, 42(6):  1440-1450.  doi:10.11743/ogg20210618

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The Eocene channel sandstone reservoirs in the Rovuma Basin, East Africa, are of deep sea gravity flow deposits. The Eocene channel complex is characterized by unilateral migration and multi-level erosion and filling. Research methods and technologies including core observation, well logging analysis, and 3D seismic attribute analysis, etc., are applied to study the channel complex. The relationship between the sedimentary facies of channel complex and its grade is clarified. The Eocene channel complex of locally restricted type can be divided into 3 subfacies, 3 grades and 4 sedimentary periods; moreover, the evolutionary process of the 4 periods is described, and the impact of sedimentary unit type, sedimentary period of the channel complex and the distance from the channel mouth on the reservoir properties is analyzed. The Eocene channel complex in the Rovuma Basin is composed of 3 sedimentary subfacies, i.e. complex substrate, main deposits of confined filling and overflow wedge. Channel filling deposition includes 3 grades of channel complex, composite channel and channel. Under the influence of bottom current, a north-to-south migration occurs within a single composite channel and between composite channels of different stages from old to young; and thereby the reservoir is mainly of the superimposing channel facies in laterally continuous distribution. On the other hand, the reservoir properties of the Eocene channel complex of local restriction type are closely associated with the sedimentary period, the type of sedimentary units and the distance from the channel mouth. Sandstone reservoirs are mainly developed in superimposing and meandering channels, and the superimposing channel facies is the most favorable to reservoir development. The earlier the channel deposition occurs, the thicker a single sandstone layer will be; however, the later the channel deposition begins, the better the physical properties of a reservoir will be, and the closer to the channel mouth, the greater the reservoir thickness and the lower the porosity will be. The variation of shale content and permeability with extension distance is complicated. In a nutshell, the research results serve for deepening the study of sedimentary reservoir of deep-water channel complex facies, as well as are of practical value to establishing geological models in the early stage of oil/gas field development.

Methods and Technologies

Discussion on evaluation method of total oil and movable oil in-place

Qiulin Guo, Jian Wang, Xiaoming Chen, Ningsheng Chen, Xiaozhi Wu, Zhuangxiaoxue Liu

2021, 42(6):  1451-1463.  doi:10.11743/ogg20210619

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The calculation of total oil yield (TOY) and movable oil yield (MOY) of shale is a core technology in assessing shale oil resource potential. A method of evaluating the heterogeneity of TOC content in shale samples and a calculation method to correct the content of adsorbed oil are proposed based on data of two separate pyrolysis experiments. The calculation method is subsequently applied to evaluate 29 shale samples from the Qianjiang Formation in the Jianghan Basin and 32 shale samples from the Shahejie Formation in the Bohai Bay Basin, and the results show that: (1) The mean deviations of TOC are 0.16% and 0.34%, respectively, indicating heterogeneity of the two sample groups; (2) The adsorbed oil content deviations of pre-and post-correction are 0.85 mg/g and 0.84 mg/g respectively, showing that the equivalent TOC correction can lead to more accurate results in calculating adsorbed oil, TOY and MOY. Besides, a method to calculate evaporative hydrocarbon loss is proposed based on shale oil density and formation oil volume factor. These newly proposed methods are used to evaluate the 7^th shale oil layer group of Yanchang Formation (Chang 7 shale oil layer group) in the Ordos Basin. The results are as follows: (1) Adsorbed oil accounts for 63% of the TOY, the MOY accounts for 37% of the TOY, the TOY is 3.5 times of S₁, and the evaporative hydrocarbon loss accounts for 9% of the TOY and 29% of S₁; (2) The shale oil in-place is 111.2×10⁸t and the movable oil in-place is 40.1×10⁸t in the study area, indicating that the Chang 7 shale oil layer group is of great exploration potential.

Application of big data analytics to hydrocarbon exploration for favorable basin selection in Central Asia

Ke Zhang, Yina Zhang

2021, 42(6):  1464-1474.  doi:10.11743/ogg20210620

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With the advent of the digital era, oil companies have invested more in obtaining and integrating basic data, and constantly improved the utilization of big data analytics, as an emerging trend, in oil and gas industries, with a view to discovering "big oil and gas". With the use of big data, companies can capture large data in real time; in contrast, traditional statistical analysis is characterized by poor timeliness in capturing large volumes of data, as well as a lack of efficient methods for analysis and critical evaluation parameters, seriously restricting the in-depth application of the big data analytics in hydrocarbon exploration. Central Asian is a region rich in natural resources, including oil and gas, and also a key area and ideal choice for China's oil companies to implement the Belt and Road Energy Cooperation Strategy. However, it is difficult to carry out effective petroleum geological analysis at the stage of study area selection, given the large scope of researches in short time, and the lack of data in seismic interpretation and from wells, and a macro-understanding to guide decision-making cannot be reached as a result. In this regard, we firstly carry out big data analysis following deep mining into and secondary development of purchased databases, integrating massive multi-sourced heterogeneous data, creating a knowledge base for strategic area selection in Central Asia, serving to lay a data foundation for big data analysis in petroleum exploration. Secondly, methods of data mining and big data analytics are innovated for hydrocarbon exploration, and a comprehensive key parameter indicator (KPI) scoring model based on a Trinity of petroleum geological conditions, exploration maturity and commercial value, is established to select multiple petroliferous basins of great exploration potentials and effectively guide the strategic area selection in Central Asia. The study provides new ideas and solutions, and expounds the necessity and feasibility of big data analytics for petroleum exploration from the oil companies' point of view. In all, it is of great significance to application and promotion.