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17 March 2023, Volume 44 Issue 2
Petroleum Geology
Predication methods of ultra-thin sandstone reservoirs and their application to blocks 14 and 17 in the Andes, Ecuador
Guangfu WANG, Hai XU, Fayou LI, Jianfang SUN, Wenlong DUAN
2023, 44(2):  247-263.  doi:10.11743/ogg20230201
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The minimum sand thickness for clastic reservoirs at medium burial depth between 2 500 and 3 500 meters to be predictable with current techniques is generally no more than 5 to 10 meters, while prediction of ultra-thin reservoirs with a thickness less than 5 meters remains a tough challenge. Based on the post-stack seismic data acquired and processed at different times from blocks 14 and 17 in the Andes of Ecuador, this study uses the post-stack consistent processing method driven by the structural trend surface to suppress and eliminate the interference of phase, energy, frequency and closure error on thin-bed reflection and reduce reservoir prediction uncertainty. The time-frequency attenuation, high-precision synthetic seismogram calibration method is employed to erase the accumulative time shift caused by formation absorption, accurately calibrate and analyse reflection characteristics of thin layers, and determine the minimum predominant frequency for resolving ultra-thin reservoirs. The weak reflection coefficient of thin layers is also effectively restored by using the high-resolution processing technology on post-stack broadband signals of “steady-state time-frequency-varying wavelet” without well data driving. The algorithm and workflow of facies-controlled waveform inversion are optimized based on broadband seismic waveform constraints. A series of technologies have then been developed and applied to the blocks, from which some tidal channel sand bodies of only 2 to 5 meters thick and 3 000 meters deep were successfully mapped. The drilling results of new appraisal wells and development wells verified that a prediction accuracy of at least 90 % with the methods has been reached.

Tectono-sedimentary evolution, paleo-geographic reconstruction and play fairway delineation of the Lower Paleozoic, Ordos Basin
Yan ZHOU, Siyi FU, Tao ZHANG, Hongde CHEN, Zhongtang SU, Juntao ZHANG, Chenggong ZHANG, Ziming LIU, Xiaoyu HAN
2023, 44(2):  264-275.  doi:10.11743/ogg20230202
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The Lower Paleozoic has been an important target for natural gas exploration in the Ordos Basin. Several suites of gas-bearing sequences, such as weathering crust reservoir on top of the Ordovician, middle assemblage dolomite and sub-salt dolomite, have been discovered successively, which proves that the Lower Paleozoic in the basin is of good exploration and development potentials, and thus paleo-geographic reconstruction there is in urgent need. The study proposes a new idea of play fairway delineation of the Lower Paleozoic in the Ordos Basin from the perspective of tectono-sedimentary evolution and paleo-geographic reconstruction therein. The research shows that Ordos Basin experienced five stages of evolution, namely, the Archean-Proterozoic fault depression-deposition stage, the Cambrian-Ordovician deposition-denudation stage, the Carboniferous-Triassic stable depression-deposition stage, the Jurassic-Cretaceous compressional hydrocarbon accumulation stage and the adjustment and finalization stage from the Late Cretaceous till present. The paleo-geographic pattern of the Lower Paleozoic Ordos Basin underwent the stages of paleo-land reduction, sediment filling, margin rifting and the alternated uplift-depression during the Early Ordovician, which controlled the distribution of quality play fairways. Besides, the latest exploration achievements help us identify four types of quality play fairways in terms of source-reservoir assemblage, spatial distribution and major controlling factors, that is, hydrocarbon accumulation within dolomite of marginal tidal-flat facies, hydrocarbon accumulation within the Ordovician pre-salt layer of intra-platform mound shoal facies, hydrocarbon accumulation within dolomite and shale of marginal tidal-flat and intra-platform mound shoal facies in the west, hydrocarbon accumulation within dolomite of karstification and shoal facies. The play fairways of different types feature separate hydrocarbon play elements and major controlling factors, and thereby targeted exploration strategies are in need.

Controlling mechanism of Wushenqi paleo-uplift on paleo-karst gas reservoirs in Ordos Basin
Faqi HE, Wei ZHANG, Xiaoqi DING, Zhuangzhuang QI, Chuntang LI, Hanjing SUN
2023, 44(2):  276-291.  doi:10.11743/ogg20230203
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It’s found that the Wushenqi paleo-uplift has a significant impact on controlling the hydrocarbon accumulation of the Ordovician Majiagou Formation in the northern Ordos Basin with a batch of deep wells successfully drilled in Da’niudi gas field there. The study makes full use of the drilling, seismic and laboratory data from the northern part of the basin to discuss the controlling effects of the paleo-uplift on reservoir forming and hydrocarbon accumulation in the Lower Paleozoic. The results are shown as follows. First, the Wushenqi paleo-uplift was formed in the Late Proterozoic and continued to be active until the end of the Ordovician. It underwent five stages of evolution featuring alternated development above and under water. Second, the continuous activities during the Early Paleozoic resulted in the development of fault system to the east of the paleo-uplift, which served as fluid channels for the confined aquifer karstification during the Caledon and Hercynian periods. Deep karst reservoirs were developed thereby. Third, the sedimentary water body on the paleo-uplift was shallow, favorable for the deposition of microbial rocks and enecontemporaneous dolomitization, and prone to generate porous dolomite reservoirs. Fourth, the faults to the east of the paleo-uplift acted as major passages for natural gas migration during accumulation. Multiple reservoirs get connected by these faults, featuring “hydrocarbon charging from three directions, fault-controlled reservoir formation and multi-layered hydrocarbon accumulation”. The study is of a new model to the exploration of the Lower Paleozoic and Ordovician in the Ordos Basin.

Quantitative characterization and dynamic evolution of pore structure in shale reservoirs of Chang 7 oil layer group in Yanchang area, Ordos Basin
Jing XU, Yunjin GE, Yonghong HE, Renhai PU, Linyu LIU, Liang DUAN, Kefeng DU
2023, 44(2):  292-307.  doi:10.11743/ogg20230204
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Quantitative characterization and dynamic evolution study of full-scale structure provides an important basis for understanding the forming mechanism of shale reservoirs and clarifying the relationship between pores and hydrocarbon-rock interactions. Using field emission scanning electron microscopy integrated with high-pressure mercury intrusion as well as N2 and CO2 adsorption experiments, representative core plugs of different maturity and corresponding samples subjected to hydrocarbon generation and expulsion simulation experiments from Chang 7 oil layer group in the Yanchang area, Ordos Basin, were quantitively characterized for their full-scale structure. Furthermore, combined with hydrocarbon generation and expulsion simulation and X-ray diffraction test results of whole rock/clay mineral content, the interaction of organic hydrocarbon generation and expulsion, mineral diagenesis and pore structure evolution was studied, and the dynamic evolution of reservoir pore structure and fractal characteristics was quantitatively analyzed. The results show that the micropores, small pores and mesopores in shale reservoirs of the Chang 7 oil layer group contribute more to the pore volume, while the micropores and small pores provide most pore specific surface area. With hydrocarbon generation and expulsion of organic matter, dissolution of inorganic minerals and transformation between minerals, the total pore volume first decreases and then increases, and the proportion of meso-macropores decreases first and then increases, and the heterogeneity of micro-small pores is generally enhanced, while the heterogeneity of meso-macropores has a trend of increasing first and then decreasing. The correlation analysis reveals that the pore size and mineral composition are coupled with the heterogeneity of the reservoir, in which the fractal dimension D1 value of micro-small pores is positively correlated with the proportion of micropores to micro-small pores and the content of clay minerals, while the fractal dimension D2 value of meso-macropores is negatively correlated with the proportion of macropores to meso-macropores, and positively correlated with the content of brittle minerals. This study provides an accurate determination of the full-scale structure and evolution of the shale in Chang 7 oil layer group, which is of great significance for identifying sweet spots in shale reservoirs of terrestrial facies.

Differential hydrocarbon accumulation model of the Ordovician in Tazhong block, Tarim Basin
Bin LI, Xingxing ZHAO, Guanghui WU, Jianfa HAN, Baozhu GUAN, Chunguang SHEN
2023, 44(2):  308-320.  doi:10.11743/ogg20230205
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The study aims to analyze the nature, distribution patterns and origin of fluids in the Ordovician by means of reservoir geochemistry, in view of existing problems concerned with coexistence of multiple phases of oil and gas, large difference of productivity and limited knowledge of hydrocarbon enrichment modes of the Ordovician reservoirs in TazhongⅡblock, Tarim Basin. The pressure-volume-temperature (pVT) analysis of the Ordovician reservoir fluids shows that condensate gas reservoirs and light oil reservoirs coexist along strike-slip faults. The crude oil maturity parameters indicate the oil is product of mature stage, and the carbon isotope and hydrocarbon composition ratio of natural gases reveals that the gas is a mixture of the gas associated with the crude oil and oil-cracking gas. The analysis of diamondoid compounds and natural gas shows that the charging of gas from cracking of oil in reservoirs below the Middle Cambrian evaporite layer is an important factor leading to the formation of the Ordovician condensate gas reservoirs. The analysis in combination with 3D seismic interpretation data of high precision indicates that the hydrocarbon distribution is controlled by strike-slip faults, and that the crude oil density and wax content are relatively lower, and gas/oil ratio, drying coefficient and 4-MDBT/1-MDBT ratio are relatively higher in horsetail-type graben, wing tail-type graben and intersection of strike-slip fault and thrust fault which are of favorable channels for hydrocarbon charging. The coupling of the periodical opening of strike-slip faults with the sealing mechanism of evaporite intervals controls the hydrocarbon accumulation of the Ordovician in the study area. It is considered that the hydrocarbon reservoirs in Tazhong Ⅱ block are characterized by juxtaposition of source rock and reservoir, vertical migration and enrichment along strike-slip faults, and multi-stage scattered hydrocarbon charging, all of which are an important reason for the differential hydrocarbon distribution in the Ordovician reservoirs. Strike-slip faults control hydrocarbon accumulation in the Ordovician, and there is great exploration potential in the horsetail-type graben, wing-tail-type graben, and intersection of strike-slip faults and thrust faults. Large-scale gas reservoirs may exist in sequences below the Middle Cambrian evaporite layer, and this is in need of further attention.

Development characteristics and evolution mechanism of the uplifted segment of the No. 5 strike-slip fault zone in Shunbei area, Tarim Basin
Hongbo ZHANG, Yushuang ZHOU, Xuguang SHA, Shang DENG, Xiangcun SHEN, Zhongzheng JIANG
2023, 44(2):  321-334.  doi:10.11743/ogg20230206
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The No. 5 fault zone in Shunbei area, a main strike-slip fault zone running through two paleo-uplifts and being devided into north, middle, south and uplifted segments, has always been the focus of research on the formation and evolution of the strike-slip fault system in Shunbei area and its surroundings. In this study, the latest high-precision 3D seismic data newly acquired and processed are applied to carry out the fine analysis of the uplifted segment of the No. 5 fault zone in Shunbei area, determining its geometric and kinematic characteristics and revealing its formation and evolution process. According to the change of strike angle and segmented deformation pattern, the uplifted segment of the study area can be divided into three sections: the north translational section (NE16°)(uplifted segementⅠ), the middle pull-apart section (NE19°)(uplifted segment Ⅱ), and the south compressional-uplifted section (NE25°)(uplifted segment Ⅲ). The uplifted segment in the study area develops a layered deformation pattern composed of high and steep strike-slip faults and multiple suites of en echelon normal faults from bottom to top, while undergoing multi-stage strike-slip activities during the Middle Caledonian, Late Caledonian, Middle-to-Late Hercynian and Indosinian-Early Himalayan periods. In the study area, the strike-slip activities feature “strong in the segments Ⅱ and Ⅲ, weak in the segment Ⅰ”. Under the influence of stress transmission from south to north, the south and uplifted segments present sinistral strike-slip from south to north with No. 1 fault zone in Tazhong area as the boundary at the initial stage of development, while the north and middle segments present dextral strike-slip from north to south, with the maximum principal stress deflecting from NW to NE. After the Late Caledonian period, the No. 5 fault zone in Shunbei area presented sinistral strike-slip as a whole, while during the Indosinian-Early Himalayan periods, the uplifted segment reversed to dextral strike-slip, with the maximum principal stress reversing from NW to NE.

Strike-slip faults and hydrocarbon accumulation in the eastern part of Fuman oilfield, Tarim Basin
Xingguo SONG, Shi CHEN, Zhou XIE, Pengfei KANG, Ting LI, Minghui YANG, Xinxin LIANG, Zijun PENG, Xukai SHI
2023, 44(2):  335-349.  doi:10.11743/ogg20230207
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The strike-slip faults in the eastern Fuman oilfield have been targeted for Ordovician ultra-deep carbonate reservoirs in the Tarim Basin. However, they are difficult to be identified and interpreted with available data due to their weak activity, thus it is essential to deeply understand their development, evolution and reservoir-controlling characteristics. Based on the newly acquired 3D seismic data of the oilfield, three typical trunk faults (FI10, FI12 and FI17) are selected for a fine interpretation to clarify the activity characteristics and main faulting stages of the strike slip faults. Combined with oil and gas properties and reservoir-forming stages, the study analyzed the relationship between fault activity characteristics and hydrocarbon accumulation to deepen the understanding of fault development characteristics and hydrocarbon distribution in the area as well as clarifying the coupling relationship between multi-stage evolution of faults and multi-stage accumulation of hydrocarbons. The results show that the strike-slip faults in the Fuman oilfield has the characteristics of vertical stratified differential deformation, which can be divided into four tectonic deformation layers from bottom up: the subsalt basement tectonic layer (below T?2), salt tectonic layer (T?2-T?3), carbonate rock tectonic layer (T3-TO3t) and clastic rock tectonic layer (above TO3t). The active faulting can be divided into three stages: the early Caledonian, the third episode of the middle Caledonian and the late Caledonian-Hercynian, among which the third episode of the middle Caledonian is the main active faulting stage. Combined with the coupling relationship between the difference of oil and gas properties, the production performance of oil wells, the characteristics of fault activity and the period of hydrocarbon accumulation, comprehensive analyses show that faulting affects the opening of vertical migration pathway and controls the vertical hydrocarbon migration. The faulting lasts for a long time, which keeps the hydrocarbon migration pathways open and is conducive to the continuous charging of late highly mature oil and gas, resulting in fault-controlled reservoirs characterized by high hydrocarbon charging intensity, high maturity and high gas-oil ratio.

Characteristics of abnormal pressure and its influence on deep and ultra-deep tight reservoirs in the Junggar Basin
Jing SUN, Xincai YOU, Jingjing XUE, Yuanting CAO, Qiusheng CHANG, Chao CHEN
2023, 44(2):  350-365.  doi:10.11743/ogg20230208
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In recent years, discoveries and breakthroughs have been made successively in exploration of the deep and ultra-deep sequences in the Junggar Basin. The reservoirs found in the sequences are mostly tight with abnormally high pressure. In order to clarify the development characteristics of abnormally high pressure and its impact on reservoir quality, statistical analysis is carried out by using the pressure data obtained through systematic production tests in the key wells to determine the characteristics and types of pressure distribution in deep and ultra-deep layers of the basin, and its impact on the quality of tight reservoirs in combination with diagenesis. The results show that abnormally high pressure is widely developed in the deep and ultra-deep layers where residual pressure and pressure coefficient are high, vertical and lateral distribution of the abnormal pressure are broad. Four abnormally high pressure zones are recognized, i.e., the fairly high, moderately high, very high and super high pressure zones. Blocks in the western and eastern parts are located in the fairly high or moderately high pressure zones, blocks in the central part are located in the fairly high or very high pressure zones, and blocks in the southern part have fairly high, moderately high, very high or super high pressure. The vertical evolution of pressure coefficients can be described as gradual, abrupt and changeable due to different circumstances. The influence of abnormal high pressure on deep and ultra-deep tight reservoirs is mainly reflected in enhanced pressure bearing capacity of pore fluid, which in turn would reduce the pressure of rock skeleton particles, inhibit compaction, and contribute to the preservation of a large number of primary intergranular pores in the reservoirs. The abnormal high pressure also inhibits the formation and precipitation of cement and thus decreases continuously the volume fraction of cement with increasing burial depth, restrains the evolution of organic matter to extend hydrocarbon generation and expulsion period as well as the time and scope of organic acid action, enhances dissolution to form secondary pores, and promotes the accumulation of pore fluid pressure to facilitate the formation of non-structural fractures when the pore fluid pressure exceeds 60 % of overburden pressure. Abnormally high pressure plays a key role in the improvement of the quality of deep and ultra-deep tight reservoirs in the basin, and serves as one of the control factors for reservoirs to become effective.

Brittleness features and controlling factors of continental shale from Da’anzhai Member of the Lower Jurassic Ziliujing Formation, northeastern Sichuan Basin
Ruyue WANG, Zongquan HU, Fuqiang LAI, Yuejiao LIU, Zhonghu WU, Jianhua HE, Guan’gui ZOU, Pengwei WANG, Zhihao LI
2023, 44(2):  366-378.  doi:10.11743/ogg20230209
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The brittleness features and their controlling factors of shale are clarified based on the systematic study of lithofacies and mineral composition, dynamic and static rock mechanical properties, and diagenesis of the Da’anzhai Member of the Lower Jurassic Ziliujing Formation in the northeastern Sichuan Basin. The following results are obtained. First, the continental shale sequence of the Da’anzhai Member is mainly composed of clayey shale, silty shale, shell limy clayey shale and shell limestone, featuring high content of clayey minerals and local enrichment of carbonate minerals. The source of felsic minerals is mainly terrigenous detritus, and the correlations between the TOC, clay, felsic and carbonate minerals are complex. Second, the brittleness of shale strata is under a joint effect of mineral content, mineral architecture and their interactions. The Da’?anzhai Member is at the middle diagenetic stage, with a low degree of concretion, and its felsic mineral content has a weaker effect on brittleness compared with carbonate minerals. The increasing content of calcareous shells leads to differences in dynamic and static rock mechanical parameters and reduces the energy consumed for rock fracturing and fracture propagation as required. Third, the interbeds in the Da’anzhai Member are diverse in type and complicated in mineral composition and architecture. The dynamic and static mechanical parameters of shale with interbeds of homogeneous architecture are in good positive correlation, while those of shale with interbeds of heterogeneous architecture (i.e. shell interbed) are in negative correlation in terms of Young’s modulus. In all, factors such as lithology, interbed type, differences in rock mechanical properties, and diagenesis should be fully considered to establish an evaluation system for the brittleness and fracability of continental shale in the Jurassic. The conclusions achieved above are of important referential value to deepening the understanding of the brittleness and the evaluation of geological-engineering sweet spots of continental shale reservoirs with interbeds of complex types.

Control of organic matter enrichment on organic pore development in the Permian marine organic-rich shale, eastern Sichuan Basin
Pengwei WANG, Zhongbao LIU, Dianwei ZHANG, Xiong LI, Wei DU, Haotian LIU, Peng LI, Ruyue WANG
2023, 44(2):  379-392.  doi:10.11743/ogg20230210
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Organic matter abundance, macerals and occurrence that have been suggested to significantly control upon pore development in marine shales have not been well studied. Guided by organic geochemistry, organic petrology and other theoretical methods, this study is focused on a systematic analysis and comparison of the abundance, macerals and occurrence of organic matter in the overmature marine shales in the Permian Maokou and Wujiaping Formations at the eastern Sichuan Basin through scanning electron microscope observation and energy spectrum testing among others, to discuss the control of organic matter enrichment on organic pore development in shales and establish an evolution model for the pores. The following results are obtained. Both the Maokou and Wujiaping marine shales are characterized by “high organic matter abundance (TOC) and predominance of algae in their primitive components”. However, they also have some differences in respect of organic matter occurrence. The Maokou Formation shale has relatively higher TOC, with organic matter enriching along bedding or at local position. The Wujiaping Formation shale has relatively lower TOC, with organic matter containing structural vitrinite and structureless vitrinite enriching dispersively or locally. The existence of favorable primitive organic components are a prerequisite for the pore development. Pores are mostly developed in sapropel compositions such as algae but are rarely found in vitrinite. TOC is the key to the pore development. Within a certain range, TOC is positively correlated with organic porosity. The organic matter occurrence influences the pore preservation. Pores are well preserved in dispersed organic matter within brittle mineral framework. The organic matter enriched locally or along bedding usually has less pores preserved under compaction due to a lack of effective framework support.

Characteristics of natural fractures in carbonate reservoirs and their impacts on well productivity in the Sinian Dengying Formation, central Sichuan Basin
Jinxiong SHI, Xiangyuan ZHAO, Renfang PAN, Lianbo ZENG, Zhengping ZHU
2023, 44(2):  393-405.  doi:10.11743/ogg20230211
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Carbonate reservoirs in the Sinian Dengying Formation, central Sichuan Basin, are a hot spot for hydrocarbon exploration and development in deep and ultra-deep sequences in China. Multi-type and multi-scale natural fractures are widely seen in the Dengying reservoirs, and of significant impacts on the seepage flow pattern and well productivity. In this study, the genetic types and developmental characteristics of natural fractures in the fourth member of Dengying Formation (Deng 4 Member), Gaoshiti-Moxi area, are clarified utilizing the cores, thin sections, image logs, experimental tests and production testing data. Major geological factors controlling the development of natural fractures are used to analyze the impact of natural fractures on well productivity. Results show that natural fractures in the Deng 4 Member reservoirs in Gaoshiti-Moxi area are mainly of tectonic and diagenetic types. The tectonic fractures can be divided into shear fractures and tension fractures, and the diagenetic fractures are mainly bedding fractures and stylolites, among others. The shear fractures dominate the Deng 4 Member reservoirs, and are mainly of high-angle ones and striking in the NNW-SSE, nearly EW, NE-SW and nearly SN directions. Factors controlling the shear fracture effectiveness include the timing of fracture formation, cementation, dissolution, fracture occurrence and current in-situ stress. The shear fractures in NNW-SSE and nearly EW orientation are more effective than others. The development degree of shear fractures is closely related to the lithology, mechanical layer thickness and faults. The shear fractures are more highly developed in micritic dolomites, and their density decreases and scale enlarges along with the increasing thickness of mechanical layers. The shear fractures near the main strike-slip faults, in particular within the tip and superimposed part of faults, are better developed. The development degree and effectiveness of fractures jointly determine gas well productivity. The effective fractures in different occurrences and scales can form fracture network, and get relatively isolated pores interconnected, which greatly improves the seepage capacity of reservoirs and elevate well productivity. In the Deng 4 Member reservoirs of Gaoshiti-Moxi area, the shear fractures in NNW-SSE and nearly EW orientation are of stronger ability to enhance reservoir permeability, and make greater contribution to the improvement of well productivity.

The effects of paleogeomorphology on the development of fractures in reservoirs of weathering metamorphic zone in an exposed Archean burial hill, Block B, Bohai Bay Basin
Xinwu LIAO, Runcheng XIE, Wen ZHOU, Yue WANG, Wenchao LIU, Weilin LIU, Qi CHENG, Xiaojun XIONG, Ziwei Luo
2023, 44(2):  406-417.  doi:10.11743/ogg20230212
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Burial hill is one of the important targets for oil and gas exploration and development in the Bohai Bay Basin where Block B in an exposed Archean burial hill is recently found to be hosting the largest gas condensate field in eastern China. The paleo-geomorphology is suggested to have played an important role in the formation and development of reservoirs in the field that caused differences in duration and intensity of weathering upon the burial hill of metamorphic rocks and thus shaped the development of reservoirs and even the formation of fractures in the weathering zone of the burial hill. Based on core description and image logging interpretation of fractures as well as the restoration and division of the ancient landform of the burial hill, the control of paleogeomorphology on the formation of fractures in the reservoirs of burial hill is discussed in detail. The following results are obtained: ① The weathering and leaching effect is relatively stronger at the higher parts of the paleogeomorphology, resulting in denser fractures. The paleogeomorphology directly affects the development of fractures in the weathering zone, weakens the rock properties of the reservoir in the inner zone, and indirectly controls the development of fractures in the inner zone to a certain extent. ② The depth ranges of effects of paleogeomorphology on weathering zone of buried hill are different in different areas, which controlled the formation of fractures in different depth of the burial hill. The paleogeomorphology in the east part of Block B is very different from that in the west part, with the thickness of the weathering fracture zone in the west part (277 m) significantly larger than that in the eastern part (193 m). The depth of the weathering fracture zone affected by the paleogeomorphology in the west part is also greater than that in the east part. ③ There is a clear correlation between paleogeomorphology and the fracture density of each fracture group/system (stage), i.e., the fracture development in the weathering zone is not only controlled by paleogeomorphology, but also related to the tectonic activities of each period, especially the early tectonic activities (Indosinian period) which controls jointly with paleogeomorphology the formation of early fractures in the weathering zone. ④ Block B contains 3 large high-pitched ridge remnant mound areas with well-developed fractures. Fractures, especially the NW-SE-trending fractures of Indosinian period in the weathering zone are controlled by geomorphologic amplitude. More complete water-rocks reaction occurs in the gentle flanks of the remnant mounds, where the weathering and leaching effect is strong enough to reduce the mechanical properties of the rocks and facilitate the formation of fractures under various stresses and large-scale dissolution, all providing favorable conditions for the formation of high-quality weathering zone reservoirs.

Influence of mantle-derived CO2 on hydrocarbon accumulation in Qinhuangdao 29 tectonic zone, Bohai Sea
Wei LI, Jianyong XU, Zhifeng LIU, Youchuan LI, Wenqi ZHU
2023, 44(2):  418-428.  doi:10.11743/ogg20230213
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Reservoirs of high CO2 content are rarely found in Bohai Sea, offshore of the Bohai Bay Basin, and Qinhuangdao 29 tectonic zone (QHD29) is one of the few typical reservoirs of the type. To study the correlation of hydrocarbon accumulation with the migration and accumulation of high-CO2-content gas is of great significance to oil and gas exploration and the discovery of CO2 gas reservoirs. Based on geochemical analysis of saturated and aromatic hydrocarbons, petrologic characteristics of fluid inclusions, we systematically discuss the interrelationship between CO2 charging and hydrocarbon accumulation in QHD29 tectonic zone and the genetic mechanism of hig-CO2-content reservoirs in the tectonic zone. The results are shown as follows. First, the hydrocarbons sourced from the source rocks of the first and third members of Shahejie Formation (E2s1 and E2s3) mainly accumulate in the E2s1, followed by Dongying Formation (E3d). Second, QHD29 reservoirs underwent two stages of hydrocarbon charging. The early stage features crude oil charging dominated by north-south lateral migration from the Qinnan Sag to the uplift along the sand bodies; and the late stage features instantaneous gas accumulation caused by magmatic-hydrothermal fluid as dominated by vertical migration. The supercritical CO2, fluid rich in hydrocarbons, moved upward along the deep-rooted faults, entered the major oil reservoirs, and then moved laterally from south to north. Third, QHD29 reservoirs obviously experienced gas washing under both early oil and late gas accumulation events, forming a hydrocarbon distribution pattern with heavy oil and bitumen enriched at the bottom, light oil and gas condensate in the middle, and CO2 gas dominating on top of the major reservoirs.

Genetic mechanism and reservoir distribution of shallow-marine multi-stepped submarine fans in the Miocene Huangliu Formation of Dongfang 1-1 gas field, Yinggehai Basin
Hua LI, Zhaoqiang YANG, Wei ZHOU, Youbin HE, Yu WANG, Xuan PENG, Yaru LI, Xinbo LYU
2023, 44(2):  429-440.  doi:10.11743/ogg20230214
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Deep sea submarine fan is one of the key sedimentological subjects for oil and gas exploration, while shallow sea submarine fan is comparatively less documented. To fill up the gap, the shallow sea multi-stepped submarine fans in the Miocene Huangliu Formation of the Dongfang 1-1 gas field in the Yinggehai Basin are thoroughly investigated to determine their genetic mechanisms and distribution pattern of reservoirs by using core, drilling, seismic, lab test data, etc. The following results are obtained. ① The multi-stepped submarine fans host channels, sheet sands and distal silts, of which the channels are medium- to fine-grained sandstone with lenticular-shaped high-amplitude seismic reflections and low seismic impedance values, the sheet sands are dominantly fine-grained sandstone and siltstone with parallel beddings, showing high-amplitude seismic reflections and lower seismic impedance values (compared to channel deposits), and the distal silts are commonly mudstone and siltstone, showing low-amplitude seismic reflections and high impedance. ② The channels, forming the main parts of the fans in the northwest, middle, and southeast of the study area, are NW-SE oriented and can be divided into three steps. The sheet sands are developed at flanks and terminals of the channels. The distal silts are generally formed at the margin of the fan. ③ Both the channels and sheet sands are considered potential reservoirs with the latter being slightly higher in reservoir quality than the former. They are usually disconnected, quite ideal for the formation of lithological and stratigraphic traps. ④ The fans are suggested to have experienced an early supercritical to subcritical flow transform stage and a late filling-overbanking stage. During the early deposition of the Huangliu Formation, high-energy gravity flow led to the formation of multi-stepped channels via transformation from supercritical to subcritical flow; and during the late deposition of the Huangliu Formation, flow transformation was replaced by filling-overbanking as the gravity flow got weakened.

Controlling effect of slope-break zone and paleovalley on sedimentation in rifted lake basins: A case study of the Paleogene Wenchang Formation in Haifeng 33 Subsag, Pearl River Mouth Basin
Zichen HE, Hao LIU, Heming LIN, Xinwei QIU, Xudong WANG, Yongtao JU, Xiaoming QUE
2023, 44(2):  441-451.  doi:10.11743/ogg20230215
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Rifted lake basins usually have their sedimentary filling controlled by the intensity of tectonic activities, sea level fluctuation and sediment flux. The distribution of sand bodies in these basins are also controlled by slope-break zones and paleovalley systems. It is of great significance to explore the configuration relationship between the slope break zones and paleovalley systems for oil and gas exploration in rifted lake basins. The Pearl River Mouth Basin is the most important petroliferous basin in the South China Sea. The less explored Haifeng 33 Subsag in the basin is selected to study the factors controlling the development of sedimentary systems so as to support the mapping of reservoirs and deployment of oil and gas exploration. Based on 3-D seismic data and logging data of the subsag as well as on the basis of regional structural interpretation, this paper analyzes the types and development characteristics of the slope-break zones and the paleovalleys of the Paleogene Wenchang Formation in the subsag, and clarifies the control of the slope-break zones and paleovalleys on sediment filling. The results show that there are three types of slope-break zones in the study area: fault slope-break zone, flexure slope-break zone and sedimentary slope-break zone. The paleovalleys are mainly V-shaped, U-shaped, W-shaped and single fault groove types, among which the V-shaped and U-shaped valleys are more in number and more concentrated in-plane distribution. The slope-break zones control the boundary of denudation and provenance, thus mobilizing the sediments in a certain direction for a certain distance within certain space. Different sedimentary systems are believed to be developed in different periods under the influence of these zones. Serving as transport channels, paleovalleys control the direction of sediment transport, location of sediment discharge as well as scale of depositional system. It is concluded that slope-break zones together with paleovalleys determine the types, scales, development and evolution of depositional systems.

Micro-architecture, deformation and source-reservoir significance of organic-clay composites in shale
Xiaoxia LI, Yuantao GU, Quan WAN, Shuguang YANG
2023, 44(2):  452-467.  doi:10.11743/ogg20230216
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The organic matter and clay minerals in shale can combine with each other to form organic-clay composites during sedimentation and evolution, which are an important source of parent materials of hydrocarbons. An integration of field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) is applied to study the deformation and micro-architecture of organic-clay composites, and discuss their source-reservoir significance in depth. The object of study is the four suites of typical shale reservoirs, that is, the Triassic Yanchang Formation of the Ordos Basin, the Ordovician Wufeng-Silurian Longmaxi formations of northern Guizhou, the Cambrian Niutitang Formation of central Guizhou, as well as the Permian Shanxi Formation of the Southern North China Basin. The organic-clay composites in shale feature complex components, diverse geometries and being prone to deform, and the main mechanisms driving the deformation mainly include tectonic stress, mineral particles, the occurrence of organic matter and the transformation of clay minerals. The deformation caused by tectonic stress outside the composites and mineral particles (imposed on the surrounding clay layer and organic layer) inside the composites could change the local stress environment of the composites, resulting in extensional environment, which could in turn drive the large-scale development of nano-pores in the composites. Under the protection of clay layer, hydrocarbons stored in the nano-pores are less likely to loss, which could effectively improve the storage capacity of shale reservoir. The conclusions achieved in the study are of beneficial value to understanding the diagenetic evolution of shale, as well as the generation, migration and storage of shale oil and gas, thus guiding the exploration and development of shale oil and gas.

Methods and Technologies
Characterizing the heterogeneity of tight sandstone in outcropped Permian Shanxi Formation, Liujiang Basin
Hongqi YUAN, Xinyu DENG, Huiyao DU, Yinghua YU, Fengming XU
2023, 44(2):  468-479.  doi:10.11743/ogg20230217
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Tight sandstone in outcropped Permian Shanxi Formation of the Liujiang Basin is highly heterogeneous as a result of various affecting factors, making quantitatively characterization of the feature quite a headache to parties of interest. A systematic and intensive sampling of the tight sandstone were performed and reservoir properties and relevant quantitative data were obtained through geological statistical analysis, testing and microscopic observation of thin sections. Hendrik Lorentz curves were drawn based on the analysis and the analytic hierarchy process (AHP) was also applied to determine the weights of influence factors such as permeability, porosity, surface porosity, matrix content and particle size, with a comprehensive and quantitative characterization method of the heterogeneity proposed. The study shows that the tight sandstones are highly heterogeneous with nearly identical porosity-permeability characteristics at both the vertical and lateral directions, indicating the invalidity of traditional assessment parameters such as breakthrough coefficient, variation coefficient and gradient in heterogeneity characterization due to the poor correlation between porosity and permeability. Meanwhile, the proposed method yields better results that fit well with the field observation. This method is also applicable to the description of heterogeneity of other types of unconventional reservoirs.

Technologies and prospect of full-cycle development of low-permeability tight gas reservoirs with horizontal wells, Ordos Basin
Jinbu LI, Yuehua CUI, Yougen HUANG, Shixiang FEI
2023, 44(2):  480-494.  doi:10.11743/ogg20230218
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The geological characteristics of low-permeability tight gas reservoirs (developed mostly with horizontal wells) in the Ordos Basin are systematically described based on previous gas development experiences. A successful development of these reservoirs with horizontal wells usually comprises four stages based on a study of the targets, technical parameters, and performance indicators of the wells: exploring all possibilities, tackling technical problems, scaling up of technology implementation as well as optimization and improvement. The technical connotation of horizontal well development in Ordos Basin is elaborated and represented by a series of development technologies including multidisciplinary sweet spot optimization technology, overall deployment technology of multi-layer large well cluster, multidisciplinary integrated geosteering technology, and fine fracturing technology with a full life cycle production management. A comprehensive evaluation of gas development through horizontal wells from different types of gas reservoirs indicates that horizontal wells perform better in tight gas reservoirs, that strengthening the joint efforts on multi-disciplinary research for the whole process is the main direction of horizontal well technology development as well as the key to a large-scale development of low-grade natural gas resources with profit. For developing reservoirs with more complex conditions such as those buried deep in thin layers with low abundance of gas, horizontal wells will be further improved to deal with the challenges in the brown Sulige gas field where targeted optimization of the technology will be carried out, the green gas fields such as Qingyang and Yichuan where supporting technology will be developed, and the new forms of gas resources such as shale gas, coal-bed methane, etc, that require effective development technologies, thus ensuring a steady increasing gas production in the Ordos Basin.

Quantitative analysis of nitrogen adsorption hysteresis loop and its indicative significance to pore structure characterization:A case study on the Upper Triassic Chang 7 Member, Ordos Basin
Zezhang SONG, Abula ABIDE, Mingyang LYU, Yueqiao ZHANG, Fujie JIANG, Zheyu LIU, Wei ZHENG, Xiayang WANG
2023, 44(2):  495-509.  doi:10.11743/ogg20230219
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Low-temperature nitrogen adsorption can form a “hysteresis loop”, whose geometry and area can effectively reflect the pore structure of porous media and its retention effect on adsorbed gas. However, the role of the “hysteresis loop” in quantitative characterization of shale pore structure has often been ignored. The study aims to clarify whether shale can form “hysteresis loop” in low-temperature nitrogen adsorption-desorption experiment and the determinants on hysteresis loop’s area with experiments on the 7th member of the Upper Triassic Yanchang Formation shale (Chang 7 shale) in the Ordos Basin. Various measures are applied in the study, including qualitative observation of pore structure under field-emission scanning electron microscopy (FE-SEM) and quantitative characterization of pore structure by low-temperature nitrogen adsorption test, hysteresis-loop quantitative analysis, total organic carbon (TOC) content analysis, pyrolysis and X-ray diffraction (XRD) experiments. The following results are obtained. First, whether shale can form a hysteresis loop in the low-temperature nitrogen adsorption-desorption experiment has an apparently positive correlation with the specific surface area, specific pore volume, clay mineral content, and pore structure fractal dimension, and an evidently negative correlation with the TOC content, while no apparent correlation with the average pore size, pore surface fractal dimension, highest pyrolysis peak temperature, and content of brittle minerals. Second, the hysteresis loop area depends on the development degree of the cylindrical pores with both ends open, ink-bottle pores, or parallel plate pores, the proportion of which to the total pores can be quantitatively evaluated by the hysteresis loop area. Third, the open-ended cylindrical pores, ink-bottle pores, or parallel plate pores in the samples from Chang 7 shale are mainly of the intergranular pores in clay minerals. Therefore, there is an apparently positive correlation between the hysteresis loop area and the content of clay minerals.

Tracer analysis of alkyl dibenzothiophenes migration based on displacement experiment and molecular simulation
Gang YAN, Yaohui XU, Baolei LIU, Yanze SUN, Shanshan LI, Shouyu ZHAO, Ming ZHONG
2023, 44(2):  510-520.  doi:10.11743/ogg20230220
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The distribution of alkyl dibenzothiophenes in crude oil along the migration pathway are under the effect of various factors such as geochromatographic effect, source, maturity, and depositional setting of the organic matter. To investigate the migration traces of alkyl dibenzothiophenes merely under the geochromatographic effect, we discuss the distribution characteristics and migration fractionation patterns of these compounds along the displacement pathway by gas chromatography-mass spectrometry (GC-MS), following the simulation experiments on displacement. Furthermore, the migration fractionation mechanism is verified by molecular simulation. The results indicate that with migration, the absolute concentration of dibenzothiophene (DBT), methyl dibenzothiophene (MDBT) and dimethyl dibenzothiophene (DMDBT) in the adsorbed oil shows a decreasing trend, parameters 4-/1-MDBT, 4,6-/(1,4+1,6)-DMDBT and 4,6-/2,4-DMDBT tend to significantly decline, parameters 2,4-/(1,4+1,6)-DMDBT and (2,6+3,6)/(1,4+1,6)-DMDBT present a significantly rise trend on the other hand. The molecular simulation results show that the adsorption capacity of alkyl dibenzothiophenes in crude oil and aqueous medium is closely related to the surface area of Connolly molecule, a compound, and dipole moment. In general, the larger the surface area and dipole moment, the stronger its adsorption capacity, and the molecular dipole moment is dominant among these two influential factors.

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