Table of Content

30 June 2025, Volume 46 Issue 3

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Petroleum Geology

Frontiers and future prospects of international sedimentological research： Review on the 37th International Meeting of Sedimentology

Xiaomin ZHU, Xiaolin WANG, Xin HU, Xiang WANG, Zijie GAO

2025, 46(3):  685-704.  doi:10.11743/ogg20250301

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The 37thInternational Meeting of Sedimentology （also referred to as the 37th IMS） was held in June 2024 in Aberdeen， UK， focusing on the frontiers of sedimentology. Specifically， this meeting highlighted cutting-edge academic issues including the interactions between the Earth， life， and climate， Precambrian environments， source-to-sink systems， fluvial and lacustrine sedimentation， tidal action and neritic sedimentation， deep-water sedimentation， and the diagenesis and reservoir characterization of siliceous clastic rocks. Additional focal areas included carbonate rocks and evaporites， ichnology， volcanoes and geochemistry， planetary sedimentology， geothermal resources and energy transition， seismic sedimentology， and AI-based new methodologies and technologies for sedimentological research. This meeting emphasized the importance of paleoclimate research in deciphering deep-time global changes， presenting a novel approach to reconstruct paleoclimate using high-resolution sedimentary records. It underscored the coupling relationships among tectonic activities， climate change， and sedimentary processes， positing that elucidating source-to-sink transport mechanisms necessitates multi-scale geomorphological reconstruction and sediment flux simulations. Furthermore， this meeting underlined the transport and deposition mechanisms of fine-grained sediments in deep-water environments， and introduced novel machine learning-based methods for automatic sedimentary facies identification， reservoir prediction， and sedimentary process simulation. A comprehensive analysis of the cutting edge obtained from the 37th IMS reveals that future sedimentological research in China should prioritize research on paleoclimate and deep-time global changes， paleogeomorphology and source-to-sink systems， deep-water sedimentation and fine-grained sedimentology， and new technologies such as big data and AI. The purpose is to achieve the national energy strategic objectives and establish a sedimentological theoretical system that reflects the regional geological characteristics of China.

Classification and characteristics of pores in marine shale gas reservoirs in the Lower Paleozoic Wufeng-Longmaxi formations， Sichuan Basin

Zongquan HU, Qianru WANG, Baojian SHEN, Dongjun FENG, Wei DU, Chuanxiang SUN

2025, 46(3):  705-718.  doi:10.11743/ogg20250302

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Shale gas reservoirs contain complex inorganic minerals and abundant organic matter， and have experienced a multistage evolutionary process including diagenesis and hydrocarbon generation. All these lead to the formation of diverse pore types with complex origins and formation processes. Internationally， systematic， in-depth studies on the classification of shale gas reservoirs have been conducted generally. However， in China， the genetic classification of these reservoirs largely relies on the criteria used for conventional reservoirs. Accordingly， there remains a lack of detailed studies on the interactions between inorganic minerals and organic matter， as well as on the evolution of inorganic and organic pores and their potential mutual transformation. In this study， we investigate typical marine shale gas reservoirs in the Wufeng-Longmaxi formations within the Sichuan Basin. By characterizing pores based on their props， we analyze the characteristics of pores propped by inorganic minerals， organic matter， and a combination of them， and accordingly propose a pore classification scheme in terms of prop types， pore development locations， and pore origins. Using this scheme， pores in marine shale gas reservoirs are categorized into inorganic， organic， and composite pores， each of which are further subdivided. To deepen the pore evolution and transformation， we integrate the diagenetic processes of inorganic minerals with the hydrocarbon generation of organic matter， and thereby the pore evolution in marine shale gas reservoirs can be divided into three distinct stages， namely， the sedimentary to early diagenetic stage， the oil generation stage， and the gas generation stage. The first stage is characterized by the development of abundant inorganic pores， most of which are saturated with water. The oil generation stage features the development of inorganic pores and a small quantity of pores within in situ organic matter （also referred to as in situ organic pores）， which are dominated by oil. The gas generation stage is marked by the formation of numerous pores in migrated organic matter and a small number of inorganic pores. Most of the pores formed in this stage contain gas. By analyzing the evolution of inorganic pores， in situ organic pores， and pores in migrated organic matter， along with the transformation among these pore types， we propose that the evolutionary sequence of high-quality reservoirs consists of the formation of radiolarian-rich siliceous shales during the sedimentary stage， the coupling of biogenic silicon and organic matter during the diagenetic stage， the infilling of liquid hydrocarbons into the intergranular and intercrystalline pores of siliceous minerals during the oil generation stage， and finally， the infilling of asphalt into the intercrystalline pores of siliceous minerals and the development of organic pores within the asphalt during the gas generation stage.

Formation mechanisms of composite continental-margin basins and their control of hydrocarbon accumulation： A case study of the Pearl River Mouth Basin， northern South China Sea

Changgui XU, Yangdong GAO, Jun LIU, Guangrong PENG, Hongbo Li, Xinwei Qiu, Lin Ding, Zulie Long, Jinyun Zheng, Dapeng Jiang

2025, 46(3):  719-739.  doi:10.11743/ogg20250303

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In recent years， significant advances have been achieved in research on rift basins along the passive continental margin of the South China Sea. However， consensus on the nature of these basins is yet to be reached， and there is a lack of in-depth studies on the formation mechanisms of these basins and their control of hydrocarbon accumulation. Using 3D seismic data， gravity and magnetic data， and drilling data from over 200 wells encountering basement， we examine the Pearl River Mouth Basin （PRMB） along the passive continental margin of the South China Sea. Specifically， we examine the lithological differences of magmatic arcs in the Mesozoic basement of the basin， as well as the geometric and kinematic characteristics of pre-existing faults within the arcs. Accordingly， we propose for the first time that the PRMB represents a composite continental-margin basin characterized by the dynamic transition， spatial superposition， and temporal succession between the Mesozoic arc-basin system and the Cenozoic ocean-continent system. The Mesozoic magmatic arcs in the PRMB are identified as a ternary structure consisting of back-， inner-， and fore-arc zones. The nature of the PRMB as a composite continental-margin basin and the architectural characteristics of the basement magmatic arcs exert significant controlling effects on the deep hydrocarbon accumulation in the basin. Specifically， pre-existing reverse faults in the inner-arc zone of the Mesozoic magmatic arcs feature higher density and larger dip angles， facilitating the formation of inverted and deep-seated faults in the Cenozoic basins. This contributes to the formation of large-scale， wide， deep faulted lacustrine basins， with the largest measuring up to 3 590 km² in area， creating favorable conditions for the generation of extensive， high-quality source rocks. The east-west lithological differences in the basement of the PRMB govern the east-west differences in deep diagenetic environment of the basin， that is， potassium-poor and potassium-rich fluids occur in the eastern and western parts of the basin， respectively. The potassium-rich fluids inhibit the kaolinite precipitation， while the potassium-poor fluids restrain the illite transformation， jointly forming the synergetic permeability preservation mechanism. This finding can effectively guide the mapping of deep reservoirs. In the context of complex deep fluid systems， the PRMB shows an efficient hydrocarbon preservation and enrichment mechanism characterized by supercritical CO₂-faciliated transport and joint sealing by alternating calcareous sandstones and thin-bedded mudstones. The concept of composite continental-margin basins introduced in this study enriches the types of continental-margin basins， providing a significant reference for in-depth research on the formation mechanisms and deep hydrocarbon accumulation of similar basins worldwide.

Geneses of dolomites and dolomite reservoirs： Review， advances， and prospects

Anjiang SHEN, Anping HU, Xiucheng TAN, Zhanfeng QIAO, Jianfeng ZHENG, Liyin PAN, Min SHE

2025, 46(3):  740-758.  doi:10.11743/ogg20250304

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This study systematically reviews and summarizes the history and advances of research on the geneses of dolomites and dolomite reservoirs， and outlines the prospects for future research orientations. The study of dolomites has had a history of over 200 years， while research on dolomite reservoirs has been kicked off since the 1950s. Previous studies established dolomitization models under varying geological conditions represented by seepage reflux and burial-compaction， and explained the geneses of dolomites with varying characteristics and occurrences in nature. Accordingly， the dolomite pores may mainly result from dolomitization as concluded. Over the past decade， progress has been made by incorporating modern sedimentary observations and simulation experiments， and four genetic types of dolomites have been identified， that is， the low-temperature organic precipitation type， the low- and high-temperature metasomatism types， and the high-temperature inorganic precipitation type. Based on this， the plot for identifying the geological and geochemical characteristics of dolomites is developed. It has been revealed that the genesis of dolomites cannot be simply equated with that of dolomite reservoirs. The porosity in dolomite reservoirs is mainly inherited from original porosity followed by modification， featuring more significant facies control and inheritance rather than modification. Although some dolomite pores are the products of burial dissolution， early-stage dolomitization is favorable for pore preservation. Two dolomitization pathways， i.e.， dolomitization with protolith texture preserved and burial-based limestone dolomitization， have been identified， with the former exhibiting the optimal reservoir-forming effects. The advancements in experimental techniques have deepened the understanding of the geneses of dolomites， while simulation experiments provide deep insights into the geneses of dolomite reservoirs. In response to hydrocarbon exploration and production， future research should intensify the fine characterization and modeling of dolomite reservoirs， as well as logging-based reservoir identification and seismics-based reservoir prediction using the geological models of reservoirs.

Types and distributions of endogenetic components in fine-grained sediments and their implications for the evaluation of shale oil and gas sweet spots

Zhensheng SHI, Tianqi ZHOU, Pengfei WANG

2025, 46(3):  759-776.  doi:10.11743/ogg20250305

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Efficient exploration and exploitation of shale oil and gas depend heavily on the accurate prediction of hydrocarbon sweet spot distribution. Meanwhile， the spatiotemporal differentiation and coupling of endogenetic components in fine-grained sediments produce a key effect on sweet spot distribution. Through an in-depth analysis of typical oil and gas-bearing shales both at home and abroad， we systematically reveal the types and distribution patterns of endogenetic components in fine-grained sediment， as well as the mechanisms by which these components control shale oil and gas sweet spots. The results indicate that the major endogenetic components include biogenic silica， biogenic carbonate minerals， biogenic phosphate minerals， authigenic organic matter， and intraclasts. These components are primarily concentrated in zones that terrigenous clastics pass by， zones that are difficult to reach for terrigenous clastics， areas influenced by upwelling ocean currents， and nearshore areas experiencing rapid transgression. The dissolution-reprecipitation process of biogenic silica， among others， leads to the formation of authigenic quartz， which inhibits organic matter degradation through hydrogen bonding. While the biogenic carbonate and phosphate mineral cementation maintains reducing environments by decreasing pore water mobility. All these are conducive to organic matter enrichment. The biogenic silica in endogenetic component-rich shales， through its synergy with organic matter， contributes to the development of pores and fractures， which in turn leads to the formation of a distinctive organic pore-microfracture coupling structure. Furthermore， the rigid support and pore networks provided by the biogenic silica frameworks provide， along with swelling stress and dissolution induced by the thermal evolution of organic matter， collectively drive the formation of a three-dimensional microfracture system. Endogenetic components play a determinant role in the distribution of shale oil and gas sweet spots by influencing organic matter enrichment， pore structure， gas adsorption， seepage characteristics， and rock brittleness. Accordingly， shale oil and gas sweet spots generally feature high endogenetic component content， elevated nano-scale porosity， abundant microfractures， and high brittle mineral content. Presently， there is a need for further investigation into the genesis and enrichment mechanisms of endogenetic components in fine-grained sediments， as well as the quantitative relationships of these components with the physical properties and brittleness index of reservoirs. Moreover， the mechanisms underlying the dynamic controlling effects of the endogenetic components on organic matter preservation and pore evolution during diagenesis warrant further exploration. In the future， it is necessary to further identify and analyze the endogenetic components using methods such as multi-scale characterization， artificial intelligence （AI）， and interdisciplinary integration， with a view to supporting the efficient exploitation of shale oil and gas.

Progress and prospects of helium exploration and production in China

Chenglin LIU, Sijie HONG, Xinpei WANG, Liyong FAN, Haidong WANG, Jianfa CHEN, Yuxin ZHU, Zhengang DING, Xue ZHANG, Haoran HU

2025, 46(3):  777-789.  doi:10.11743/ogg20250306

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Helium is recognized as an important strategic resource， and analyzing the progress and prospects of helium exploration and development in China holds great significance. In this study， we analyze the current situation of helium exploration and development in China， organize the geological conditions of its typical helium-bearing gas fields （reservoirs）， and discuss the prospects for helium development therein. The results indicate that associated with hydrocarbons or non-hydrocarbons， helium are usually found in both conventional and unconventional gas reservoirs. Helium-rich gas reservoirs in China span multiple strata and types， with the Ordos， Sichuan， and Tarim basins identified as three major helium-rich basins. In the compressional basins of western China， crust-derived helium predominates， followed by a small quantity of mantle-derived helium， both associated with hydrocarbons and nonhydrocarbons. In central China， the basins are dominated by crust-derived helium associated with hydrocarbons. In the extensional basins of eastern China， mixed crust-mantle-derived helium is predominant and is primarily associated with nonhydrocarbons. Furthermore， seven distinct helium generation-migration-accumulation patterns are identified in line with tectonic settings. Compared to other countries and regions worldwide， China exhibits unique geological conditions， resource potential， and distribution patterns for helium due to its special geological background and tectonic evolutionary history. The helium enrichment theories tailored to China’s geological conditions， an accurate understanding of helium resource potential， and innovative technologies for helium exploration and development are critical to the rapid increase in production and stable development of helium in China.

Current status and future trends of global research on natural hydrogen

Shuangbiao HAN, Jin WANG, Jie HUANG, Chengshan WANG

2025, 46(3):  790-808.  doi:10.11743/ogg20250307

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Low-carbon or even zero-carbon energy will become an integral part in future energy landscape， with natural hydrogen emerging as a particularly promising resource. In the context of the global surge in natural hydrogen exploration， reviewing the history and advances in research on natural hydrogen， as well as assessing current challenges and predicting future trends in the research， will provide a reference for determining the subsequent exploration directions and research schemes. In this study， we systematically retrieve global academic articles on natural hydrogen from databases such as CNKI and Web of Science. By analyzing the distribution of over 200 articles， we identify the research institutions， scientific communities， and funding sources involved. Furthermore， we assess the status quo of global research on natural hydrogen and explore the challenges faced at the present stage and future directions. The results of this study indicate that compared to international publications on natural hydrogen， domestic articles are fewer in number and exhibit theoretical systems poorly-established. However， the proportion of state-funded research projects involved in the domestic articles has significantly increased， indicating that the domestic research is typically entering a rapid development stage. For a prolonged period in the future， constructing a comprehensive natural hydrogen system and developing theoretical methods for scientific resource evaluation will remain a central focus of natural hydrogen research. Fundamental geological research on natural hydrogen centered around dedicated natural hydrogen exploration wells， will be undoubtedly valued. Furthermore， research on key topics， such as hydrogen source potential， the occurrence state of natural hydrogen， and the sealing performance of natural hydrogen reservoirs， will be undertaken systematically. Interdisciplinary integration will be essential for advancing the natural hydrogen field. Furthermore， the rapid advancement in natural hydrogen research and exploration in China will require national-level policies and financial support， technological breakthroughs， and international cooperation.

Genesis and impact of dissolution in deep dolomite reservoirs： A case of the 2nd member of the Sinian Dengying Formation， Penglai area， central Sichuan Basin

Qian TAN, Haifeng YUAN, Tao WANG, Zili MA, Bojun TANG, Qiu PENG, Wenjie LI

2025, 46(3):  809-826.  doi:10.11743/ogg20250308

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To further determine the genesis of deep dolomite reservoirs in the Sinian Dengying Formation within the Sichuan Basin， we analyze the developmental and filling characteristics of various dissolution pores and vugs in dolomite reservoirs in the 2nd member of the Dengying Formation （also referred to as the Deng 2 Member） in the Penglai area， central Sichuan Basin， through core observation， thin section identification and cathodoluminescence （CL） imaging. In addition， by integrating the analyses and tests of inclusions， carbon-oxygen-strontium isotopes， major and trace elements， and rare earth elements （REEs）， as well as U-Pb isotopic dating， we investigate the stages and genetic mechanisms of dissolution in the dolomite reservoirs. The results indicate that the dolomite reservoirs in the Deng 2 Member primarily underwent the superimposed modification of three stages of dissolution， namely the syngenetic to penecontemporaneous dissolution， weathered-crust karstification， and hydrothermal dissolution. Such superimposed modification produced a spectrum of storage spaces and strong reservoir heterogeneity. The fabric-selective dissolution pores and reticulate dissolution vugs in the reservoirs occur in the middle to upper part of sedimentary cycles， exhibiting multi-set superimposition and vertical rhythmicity. These pores and vugs are predominately filled by fascicular fast dolomite （FFD） cements， exhibiting unique botryoidal-lace and prehnite textures. They are the product of syngenetic to penecontemporaneous dissolution driven by frequent sea-level fluctuations. Brecciated karst caves and dissolution vugs in honeycomb pattern are principally found beneath the unconformities of the Deng 2 Member. The former is filled with collapsed karst- breccia and argillaceous materials， while the latter is half filled with radial slow dolomite （RSD） cements， both resulting from the superimposed modification of weathered-crust karstification during the Tongwan Movement. High-angle fractures and vugs in the reservoirs are distributed near E-W-trending strike-slip faults. They are associated with CO₂-rich hydrothermal dissolution caused by the Emeishan volcanism during the Late Hercynian and are largely half filled with hydrothermal mineral assemblages. The comprehensive analysis reveals that the mound-shoal deposits modified by syngenetic to penecontemporaneous dissolution provide a basis for the formation of high-quality reservoirs， while the superimposed modification of weathered-crust karstification is critical to their formation. Additionally， hydrothermal dissolution leads to the redistribution of reservoir spaces， thus governing the distribution of high-quality reservoirs.

Impact of bottom currents in the northern South China Sea on deep-water depositional systems and their petroleum geological implications

Xingxing WANG, Xing WAN, Yunlong HE, Qiling SUN

2025, 46(3):  827-845.  doi:10.11743/ogg20250309

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Bottom currents are identified as a significant driving force in the formation and evolution of deep-water depositional systems， playing an important role in governing the formation， distribution， and physical properties of deep-water hydrocarbon and natural gas hydrate （NGH） reservoirs. In this study， we conduct a comprehensive analysis of the sedimentary geology in the northern South China Sea using 2D and 3D seismic data， as well as drilling and log data. The results indicate that the northern continental margin of the South China Sea exhibits active and diverse effects of deep-water bottom currents. Bottom-current deposits formed since the Miocene are extensively distributed along this margin. Influenced by the tectonic framework evolution of marine basins and local topography， these deposits show significant spatiotemporal differences in their distribution， with the strongest bottom currents identified along the northeastern continental margin. Bottom currents have played a key role in the formation of critical sedimentary landforms in the northern South China Sea， including large-scale linear erosional troughs formed during the Early Miocene， the Central Canyon and migration canyon group that have developed since the Middle Miocene， and the dendritic canyon systems formed since the Pliocene. The formation and evolution of gravity-flow depositional landforms， such as submarine canyons and channels， in the northern South China Sea， have significantly modified the continental slope morphology， thereby controlling the gravity-flow transport pathways and the formation and distribution of turbidite sand bodies. Areas with strong bottom currents exhibit sandy bottom-current deposits. Furthermore， bottom currents in these areas modify gravity-flow deposits， with resulting bottom-current reworked sandstones serving as significant deep-water hydrocarbon and NGH reservoirs. Investigating bottom-current deposits holds great significance for understanding the deep-water sedimentary area in the northern South China Sea， and more research efforts should be directed toward the relationships between paleo-oceanic conditions and tectonic subsidence in the northern South China Sea， as well as the bottom-current depositional processes under complex topographic conditions and the mechanisms underlying interactions between bottom and turbidity currents.

Migration of the Late Eocene knickpoints and their source-to-sink system along the northern margin of the Dongsha Uplift， South China Sea

Qianghu LIU, Guangrong PENG, Pei LIU, Xiongbiao YU, Wanlin XIONG, Ziqiang ZHOU, Hongtao ZHU

2025, 46(3):  846-859.  doi:10.11743/ogg20250310

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This study aims to verify whether remnant landscapes preserved by stratigraphic overlapping can provide useful insights into characterizing sediment production. Focusing on the Dongsha Uplift， the largest uplift unit in the Pearl River Mouth Basin （PRMB）， and using full-coverage seismic data and well constraints， we extract the longitudinal channel profiles of key catchments from the remnant landscapes of the Late Eocene Enping Formation， characterize the boundary fault activity and quantify the spatiotemporal variations in sand-body grain size and sediment flux. Furthermore， the correlation between the characteristics of remnant landscapes and variations in the grain size and sediment flux in the sedimentary areas is assessed. The results indicate that the observation of tectonic knickpoints in the channel profiles of the two key catchments is consistent with the activities of boundary faults during the Late Eocene. These knickpoints may have facilitated interconnections between closed drainage systems during their headward migration， leading to overall finer grain size and greater sediment flux as archived in the sink area. Importantly， our results illustrate that geomorphological features of the source area and sedimentary records of the sink area can be successfully reconciled in terms of landscape evolution； this implies that remnant landscapes with weak post-burial deformation still preserve deep-time source-to-sink information. The fine analysis of remnant landscapes， and detailed characterization of source area slope， bedrock lithology， and knickpoint migration contribute to the reconstruction of deep-time source-to-sink systems across different stages， as well as improvement of the predictions of sand-body properties and scales in the systems.

AI-driven seismic sedimentological characterization of shelf sand bodies： A case study on the Neogene Zhujiang Formation， Huizhou Sag， Pearl River Mouth Basin

Jiawang GE, Cong CHEN, Pei LIU, Xiaoming ZHAO, Zhen YI, Yan ZHEN, An ZHANG, Xiaolong TANG

2025, 46(3):  860-875.  doi:10.11743/ogg20250311

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Neritic shelf sand bodies generally hold favorable geologic conditions for the formation of lithologic traps. However their small reservoir thickness （< 1/8 wavelength）， rapid lateral variation， and complex lithologies， lead to the sand body prediction results highly ambiguous. In this study， we investigate shelf sand bodies ZJ3A and ZJ3B， the major pay zones within the 3rd member of the Neogene Zhujiang Formation （also referred to as the Zhu 3 Member； confined by third-order sequence boundaries SB1 and SB2） in the Huizhou Sag， Pearl River Mouth Basin （PRMB）. Using well logs， as well as data from core and thin section observations， we establish quantitative lithological interpretation standards and select the optimal seismic attributes sensitive to sand body morphology. Accordingly， several artificial intelligence （AI） algorithms are employed for fitting and training based on multiple seismic attributes. As a result， an AI-driven seismic sedimentological characterization system is developed. The results indicate that the fitting of multiple seismic attributes using the random forest （RF） algorithm yielded the most accurate prediction for ZJ3A. The characterization results reveal that this sand body comprises predominantly asymmetric geomorphological units characterized by wide southern parts and narrow northern parts. These units exhibit a maximal length of up to 17.15 km and an average area of 11.23 km². In contrast， the boosted regression tree （BRT） model yielded the optimal prediction for ZJ3B. The characterization results show that this sand body consists primarily of symmetric geomorphological units characterized by roughly the same widths in the northern and southern parts. These units are small in scale， with an average area of 6.21 km². The seismic geomorphologies of ZJ3A and ZJ3B indicate the dynamic modification effects of unidirectional coastal currents and bidirectional tides， respectively. Therefore， their seismic geomorphologies jointly reflect the differential hydrodynamic response mechanisms of coastal currents and tidal dynamics during sea-level fluctuations. Seismic sedimentological research combined with fitting and selecting the optimal seismic attributes using multiple AI algorithms can enhance the seismic prediction accuracy of sand bodies. Furthermore， the quantitative characterization of seismic geomorphological units holds implications for the depositional hydrodynamic research of the study area. The results of this study will provide a robust geological basis for selecting the optimal lithologic trap targets subsequently.

Differential development characteristics and main controlling factors of high-quality clastic reservoirs in the middle-deep strata： A case study on the Paleogene Yacheng Formation and Neogene Sanya Formation around Ya’nan area， Qiongdongnan Basin

Xuecheng ZHANG, Quansheng CAI, Wei WANG, Qianwei HU, Lijuan REN, Ao SU, Mingyi HU, Zhonggui HU, Qingjie DENG

2025, 46(3):  876-893.  doi:10.11743/ogg20250312

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The study aims to clarify the reservoir development characteristics and genetic mechanisms of different zones. A batch of analytical data is applied to comprehensively investigate the medium-to-deep clastic reservoirs in the Yacheng uplift， Yacheng 13-1 low uplift， and Ya’nan low uplift， focusing on the reservoir characteristics and diagenetic environment， as well as the main factors controlling the high-quality reservoir development. The results indicate that the clastic reservoirs in Yacheng uplift mainly consist of the deltaic sandstone of the Neogene Sanya Formation， characterized by fine grain sizes. These reservoirs exhibit burial depths ranging from 2 800 to 3 100 m， suggesting a normal temperature and pressure （NTP） environment， and with grains predominantly in point-line contacts. Additionally， their primary pores are well-developed， with an average porosity of 17.1 % and an average permeability of 494.317 × 10^-3 μm². The Yacheng 13-1 low uplift primarily exhibits deltaic sandstone reservoirs of the Paleogene Yacheng Formation and the Lingshui Formation. These reservoirs are dominated by medium-to-coarse-grained sandstones， with burial depths ranging from 3 800 m to 4 100 m. They occur in a high-temperature， normal-pressure environment， with grain contacts dominated by linear and concave-convex types， and dissolution pores widely seen within. The Lingshui Formation has an average porosity of 13.5 % and an average permeability of 285.661 × 10^-3 μm²， while the Yacheng Formation shows an average porosity of 9.2 % and an average permeability of 49.158 × 10^-3 μm². The Ya’nan low uplift predominantly exhibits fan-delta reservoirs of the Paleogene Yacheng Formation， characterized by coarse-grained sediment and burial depths ranging from 4 350 m to 4 800 m. They occur in a normal-temperature， high-pressure environment， with grain contacts dominated by point and linear types. What’s more， these reservoirs contain well-developed dissolution pores， with ferruginous cements observed， exhibiting an average porosity of 15.8 % and an average permeability of 107.435 × 10^-3 μm². The discrepancies in sedimentary and diagenetic environments are identified as the primary reason for the differential physical properties among medium-to-deep clastic reservoirs across varying structural zones in the circum-Ya’nan area. Specifically， weak compaction under shallow burial conditions is the major factor contributing to the development of high-quality reservoirs in the Yacheng uplift. In contrast， in the Yacheng 13-1 low uplift， early-stage dissolution and strong compaction act as destructive factors that restrict the formation of high-quality reservoirs. Meanwhile， the overpressure environment resulted from rapid burial during the late stage is the key to the growth of deep， high-quality reservoirs in the low uplift.

Developmental characteristics and transport mechanisms of the Upper Triassic submarine fans controlled by narrow shelf in the Taohe Basin， West Qinling Orogenic Belt

Mingxuan TAN, Saisai YUAN, Mianpei CHEN, Haonan SUN, Hongxia MA, Chengcheng ZHANG

2025, 46(3):  894-909.  doi:10.11743/ogg20250313

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The Taohe Basin， located in the West Qinling Orogenic Belt （WQOB）， exhibits a series of exposed Upper Triassic submarine fan systems controlled by narrow shelf， which records the last-stage deep-marine deposition along the northeastern margin of the Paleo-Tethys Ocean. Using field and laboratory sedimentological analyses of typical outcrops， we determine the sedimentary assemblages， sedimentary units， and genetic mechanisms of submarine fans in the Upper Triassic Nalu Formation in the Lintan-Zhuoni areas. The results indicate that the Upper Triassic submarine fans in the study area contain various gravity flow deposits， which show four sedimentary assemblages： low- and high-density turbidites， hybrid event beds， hyperpycnites， and reflected turbidites. Presently， most studied outcrops of deep-water deposits exhibit the mid-to-distal sedimentary features of submarine fans， with the dominance of various sedimentary units in the lobes （e.g.， lobe channels， axial and off-axis units of lobes， and lobe fringes）. The major feeder channels and natural levees， however， are largely absent in the submarine fans. The narrow shelf-controlled submarine fans of the Upper Triassic in the Taohe Basin are developed by multiple transport mechanisms. Specifically， slope instability and sediment reworking via littoral currents on the shelf are subject to indirect sediment supply， whereas hyperpycnal flows act as a direct sediment source. The alternation between both mechanisms is intimately associated with the shelf characteristics and high-frequency sea-level functuations. The narrow-shelf setting is closely related to the semi-closed tectonic framework of the Taohe Basin， while frequent submarine landslides have enhanced the ruggedness of the submarine topography. Influenced by the continuous uplift of the Bailongjiang Uplift， the distal deposits of the Upper Triassic submarine fans in the Taohe Basin are locally transformed by reflected currents， leading to the formation of the partially restricted submarine fan deposits with dual genetic characteristics.

Sedimentary filling characteristics of the Paleogene Baoshi Formation in the western slope of Xihu Sag， East China Sea Shelf Basin

Hui WANG, Lanzhi QIN, Jingqi XU, Yongjun CHEN, Wei ZHANG, Yong DAI, Hehe CHEN

2025, 46(3):  910-925.  doi:10.11743/ogg20250314

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Hydrocarbon exploration in the Xihu Sag of the East China Sea Shelf Basin is concentrated primarily in the Eocene Pinghu Formation and the Oligocene Huagang Formation. In recent years， it has shifted to the Baoshi Formation in the western slope of the sag. However， fundamental research on the Baoshi Formation remains limited， leading to an unclear understanding of its sedimentary environment and characteristics. In this study， we investigate the sedimentary filling characteristics of the Baoshi Formation in the Xihu Sag using seismic， borehole， log， and paleontological data， as well as data from elemental geochemical analysis. The results indicate that the Baoshi Formation occurs between the T{}_{\mathrm{4}}^{\mathrm{0}} and T{}_{\mathrm{8}}^{\mathrm{0}} seismic reflection interfaces. This formation consists of a series of clastic layers， with its top and bottom boundaries characterized by pronounced onlapping seismic reflection and truncation. It has a thickness of up to 2 000 m at the sag center， providing a material basis for the formation of source rocks. The Baoshi Formation is deposited under the control of segmented fault activities， resulting in the formation of multiple independent depocenters. The western slope exhibits varying structural styles from north to south. Specifically， the northern Hangzhou slope zone occurs as a reversely harmonic tectonic ramp， the central Pinghu slope zone consists of a steep slope structure shaped by multi-stage fault superimposition， while the southern Tiantai slope zone represents a normally harmonic tectonic ramp. The Baoshi Formation is formed in a brackish， weakly oxidizing to weakly reducing sedimentary environment， with paleosalinity increasing from north to south. This formation consists of the deposit of the marine-continental transitional facies in shallow water with frequent lake/sea level fluctuations. It exhibits distinct sedimentary systems in its northern and southern portions. Specifically， the northern Hangzhou slope zone is characterized by a braided river delta-bay sedimentary system， whereas the southern Tiantai and Pinghu slope zones （excluding the Pingbei area） are dominated by a tidal flat-tidal delta-littoral neritic sedimentary system in a restricted marine sedimentary environment. The northern and southern sedimentary systems along the western slope are dissected by a low salient in the central Pinghu slope zone， which serves to shelter lagoon and bay sedimentary systems in a restricted setting. The thickly laminated mudstones in these sedimentary systems act as the major source rocks of the Baoshi Formation in the Xihu Sag.

Differential fluid overpressure evolution and hydrocarbon accumulation mechanisms of deep Ordovician reservoirs in areas with and without strike-slip faults， Tarim Basin

Yifan XUE, Haitao ZHAO, Yahao HUANG, Zhigang WEN, Yukun LIU, Yintao ZHANG, Zhanfeng QIAO, Tao LUO

2025, 46(3):  926-943.  doi:10.11743/ogg20250315

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The deep Ordovician reservoirs in areas with and without strike-slip faults in the Tarim Basin exhibit significant difference in fluid pressure evolution characteristics. Existing exploration practices have shown areas without strike-slip faults hold significant potential for deep hydrocarbon exploration and exploitation. However， there is a lack of systematic studies on the hydrocarbon accumulation and overpressure evolution processes in these areas. In this study， we examine these processes in areas with and without strike-slip faults within the Fuman oilfield based on log data， reservoir characteristics， paleo-pressure reconstruction from fluid inclusions， and the calcite U-Pb isotopic dating. The results are as follows. First， the areas without strike-slip faults in the Tarim Basin underwent three critical pressure evolution stages， namely normal pressure， weak overpressure， and strong overpressure in sequence. On the other hand， the areas with strike-slip faults in the basin also experienced three critical pressure evolution stages： normal pressure， weak overpressure， and normal pressure in sequence. Second， during the Middle-to-Late Caledonian， crude oil charging occurred for reservoirs in these areas， resulting in weak overpressure. In the late stage， numerous pores and fractures are formed in carbonate reservoirs in these areas under tectonic fracturing combined with the dissolution of fluids migrating along faults. The interconnected reservoirs facilitate the redistribution of overpressure within the connected fault systems. As a result， the formation pressure in these areas was gradually reduced to be normal. Third， in the areas without strike-slip faults， the formation pressure increased rapidly to strong overpressure during the Early Yanshanian due to two factors： the lateral migration of deep gas from oil cracking to the Ordovician reservoirs along faults and unconformities and the tectonic compression. The overpressure has been maintained up to now. Fourth， hydrocarbon accumulation in the areas without strike-slip faults followed a sequence characterized by early-stage oil generation， late-stage gas invasion， and lateral adjustment.

Characteristics and formation mechanisms of the clastic reservoirs in the Paleozoic cratonic basins of North Africa

Xueyan LU, Zhe CAO, Guangqing YANG, Yuyin ZHANG, Jingjing LIU, Jin LI, Zhongmin ZHANG

2025, 46(3):  944-958.  doi:10.11743/ogg20250316

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The widely distributed Paleozoic clastic rocks are identified as the primary target for hydrocarbon exploration and production in the cratonic basins of North Africa. The diagenetic evolution and late-stage modification vary greatly in different regions across North Africa， resulting in significantly different reservoir quality. Based on the data from 492 hydrocarbon reservoirs across eight tectonic units in North Africa， sourced from IHS Markit， as well as the burial， geothermal， and diagenetic evolution histories of these reservoirs， we investigate the differences in the characteristics of the Paleozoic clastic reservoirs across different epochs and regions in North Africa. Furthermore， the major factors controlling the development of high-quality reservoirs are clarified and the favorable areas for reservoir development are predicted. The results indicate that the Cambrian， Ordovician， and Devonian sandstone reservoirs of the Paleozoic are major pay zones in the cratonic basins of North Africa. These reservoirs consist primarily of fluvial， littoral-nearshore， and neritic clastic rock deposits with extensive distribution and high continuity. Two types of burial history occurred in these reservoirs， leading to varying diagenetic processes of reservoirs across different regions and epochs with differential tectonic evolution. The compaction and secondary overgrowth and cementation of quartz during diagenesis are detrimental to the physical properties of reservoirs， while feldspar dissolution， and modifications by multi-stage tectonic fractures are of constructive effects on their physical properties. The favorable Cambrian-Lower Ordovician reservoirs generally occur in the uplifted areas that have been significantly influenced by the Hercynian movement. The high-quality reservoirs of the Upper Ordovician are principally distributed in the Murzuq， Illizi， and Ahnet basins near the Hoggar Shield. The high-quality reservoirs of the Lower Devonian exhibit more extensive distribution， primarily found in the Illizi， Ahnet， Timimoun， and Reggane basins， as well as the northern uplift and slope areas of the Ghadames Basin.

Methods and Technologies

Threshold pressure gradient characteristics and drainage radius determination of low-permeability reservoirs

Wei TIAN, Zhongchao LI, Chuanmou YU, Hongyan HAN, Kaicheng WANG, Liqiang GUO, Kun WANG, Xiao YANG

2025, 46(3):  959-966.  doi:10.11743/ogg20250317

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In view of the limited insights into the threshold pressure gradients （TPGs） of low-permeability reservoirs and inadequate methods for determining their drainage radii， we present a detailed analysis of the variation characteristics and mechanisms of the TPG in combining experiments with numerical simulations， and propose a method for calculating the drainage radii of low-permeability reservoirs which considering the stress conditions of actual reservoirs. The results indicate that the TPG of a reservoir is not a fixed value. Instead， it changes with the net confining pressure， exhibiting a strong positive correlation. The curve showing the relationship between the TPG and the net confining pressure follows a three-stage pattern. In the first stage， plastic deformations occur， primarily involving the closure of microfractures and the sharp contraction of large pore throats. This stage has a significant impact on the seepage capacity of reservoirs， with the TPG increasing by more than five times. The second stage is characterized by pseudoplastic deformations， principally reflected in the contraction of large pore throats and microfractures， with the TPG increasing by over 40 %. The third stage shows elastic deformations， characterized primarily by the contraction of small pore throats. This stage has a minimal impact on the seepage capacity， with the TPG increasing by only 17 %. Notably， the stress state of rocks in most actual low-permeability reservoirs corresponds to this elastic deformation stage. Based on the dynamic changes in the TPG and the presence of pressure drop funnels in actual reservoirs， we determine the changes in the net overburden pressure and then calculate the threshold pressure along a certain seepage distance in reservoirs through integration. Assuming that the threshold pressure equals the driving pressure difference， we derive the seepage distance （i.e.， the driving drainage radius） through inversion. Finally， the feasibility of the proposed method for determining the drainage radius is verified through practical application. This study enhances the understanding of the characteristics of low-permeability reservoirs while also providing a significant theoretical basis and technical support for the assessment and exploitation scheme optimization of low-permeability reservoirs.

Mechanisms behind CO₂-brine-Sandy Conglomerate interactions and resulting variations in mineral components， fluid occurrence， and pores： A case study of the Mahu Sag， Junggar Basin

Liu YANG, Fei GONG, Xiaoyu JIANG, Zhaoyang LIU, Guangtao DONG, Jiawei CAI

2025, 46(3):  967-982.  doi:10.11743/ogg20250318

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Tight Sandy Conglomerate reservoirs in the Mahu Sag， Junggar Basin exhibit extreme heterogeneity， posing significant challenges to the field tests and prediction of the CO₂ sequestration performance following CO₂ flooding. Understanding the mechanisms underlying CO₂-brine-Sandy Conglomerate interactions is crucial to evaluating the CO₂ sequestration performance following reservoir development. Based on CO₂-brine saturation experiments and data from integrated quantitative evaluation of minerals by scanning electron microscopy （SEM）， SEM-based quantitative evaluation of minerals （QEMSCAN）， micro-CT scanning， and nuclear magnetic resonance （NMR） spectroscopy， we investigate mechanisms underlying CO₂-brine-Sandy Conglomerate interactions and resulting variations in the mineral components， fluid occurrence， and pores in tight Sandy Conglomerate reservoirs of the Mahu Sag. The results indicate that CO₂-brine-Sandy Conglomerate interactions led to the dissolution of rock minerals. This mineral dissolution and subsequent migration resulted in elevated average pore-throat sizes and enhanced pore connectivity in cores. The X-ray diffraction （XRD） analysis reveals that the interactions increased albite and illite contents by 3.20 % and 2.32 %， respectively， while producing minimal impact on quartz content. SEM analysis results demonstrate that CO₂ soaking led to an increase in both the quantity and width of grain-edge fractures， as well as the formation of new lateral fractures. The carbonate and feldspar dissolution generates substantial intragranular micropores and microfractures， with these microfractures propagating along cleavages. NMR experiment results reveal that saturated water primarily occurs as clay-bound water， capillary-bound water， and movable water in the cores. CO₂ soaking contributes to the enhanced content and expanded distribution range of saturated water in the pores， with movable water in macropores changing the most significantly. The research results highlight the complexity of CO₂ interactions in Sandy Conglomerate reservoirs， which is of great significance for guiding the field test effectiveness of CO₂ storage after flooding in the Mahu Sag Sandy Conglomerate reservoirs.

Degradation of the mechanical properties of medium-rank coals under supercritical CO₂ soaking and its mechanisms

Huanpeng CHI, Caiqin BI, Dongping WAN, Chengxiang WEI, Chengzheng CAI, Shouceng TIAN, Fanshi ZHENG, Tianyu WANG

2025, 46(3):  983-994.  doi:10.11743/ogg20250319

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Exploring the mechanical properties of coals subjected to supercritical carbon dioxide （SC-CO₂） soaking holds great significance for enhanced hydraulic fracturing performance of deep coalbed methane （CBM） reservoirs and CO₂ geological sequestration. Using tri-axial compression tests， mineralogical analysis， and porosity and permeability measurements， we systematically reveal the SC-CO₂ soaking-induced degradation mechanisms of the macroscopic mechanical properties of medium-rank coals under varying confining pressures. The results indicate that SC-CO₂ soaking leads to more pronounced initial compaction and residual failure characteristics in the stress-strain curves of coals， accompanied by a significant reduction in peak stress. This can effectively diminish the strength enhancement of coals caused by confining pressure. After reaching peak strength， coals experience rapid instability and failure， with a higher confining pressure associated with more rapid failure. SC-CO₂ soaking induces more significant degradation of the compressive strength of coals compared to their elastic modulus. With an increase in the confining pressure， both the compressive strength and elastic modulus of coals trend upward before and after SC-CO₂ soaking； however， the degrees of their degradation gradually decrease. Under low confining pressure， SC-CO₂ soaking induces particularly prominent degradation of the mechanical properties of coals. Regarding the degradation mechanisms， SC-CO₂ undergoes physicochemical reactions with mineral components in coals， leading to the dissolution of carbonate minerals. This process alters the pore structure and promotes the formation and propagation of micropores. As a result， both the porosity and permeability of coals increase significantly， accompanied by the breakdown of the coals’ surface structures. Compared to active water soaking， SC-CO₂ soaking causes more severe degradation of coals. Under the same confining pressure， the degrees of degradation of coals’ compressive strength and elastic modulus caused by SC-CO₂ soaking increase by 7.7 % to 36.7 % and 1.7 % to 19.4 %， respectively， compared to those induced by active water soaking. SC-CO₂ soaking degrades the mechanical properties of coals primarily through chemical dissolution， while active water soaking induces degradation mainly via physical dispersion. This difference is due to the greater permeability and chemical reactivity of SC-CO₂. It has been demonstrated that the chemical and physical synergistic degradation mechanism of SC-CO₂ can significantly weaken the strength of coals while enhancing their permeability. The application of SC-CO₂ soaking to the well involved in deep CBM tests in the Jixi Basin， Heilongjiang Province， has successfully reduced the fracturing pressure by 6 MPa and enabled a stable CBM production exceeding 1000 m³/d. Therefore， the degradation of coal properties induced by SC-CO₂ soaking plays a significant role in promoting deep CBM production via volume fracturing.

Molecular simulation of methane adsorption/desorption characteristics of deformed kerogen in shales

Liang HUANG, Zhenyao XU, Qiujie CHEN, Baohua TIAN, Qin YANG, Xinni FENG, Zishuo QU, Zhe YANG, Sirun AN

2025, 46(3):  995-1005.  doi:10.11743/ogg20250320

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Current studies on the methane adsorption/desorption behavior in kerogen are primarily conducted based on fixed pore structures. Consequently， the influence patterns and mechanisms of the pore deformation of kerogen on methane adsorption/desorption remain poorly understood. Using molecular simulation， we explore both the methane adsorption/desorption behavior in shale kerogen and the coupled pore deformation characteristics. By constructing and characterizing a nanopore model of kerogen， we investigate the influence of pressure on kerogen deformation. Using a coupling method combining Grand Canonical Monte Carlo （GCMC） and molecular dynamics （also referred to as the GCMC-MD method）， we analyze the coupling characteristics of methane adsorption and kerogen deformation. Additionally， through methane desorption simulation using a kerogen model under high-pressure adsorption equilibrium， we investigate the characteristics and causes of methane desorption hysteresis. The results indicate that ultramicropores in Type ⅡD kerogen serve as the primary migration pathways， while isolated ultramicropores also have the potential to act as migration pathways due to the influence of pore deformation. The pore structures of kerogen generally are reversible during the pressure increase and decrease cycles. The methane adsorption-induced kerogen deformation can be divided into two stages： volume expansion and pore reorganization. The structural deformation of kerogen enhances the methane adsorption capacity， while the selective adsorption of methane onto high-affinity heterogeneous sites in kerogen leads to pronounced changes in pore space. Small desorption hysteresis loops are observed within the deformed structures of kerogen. The methane desorption hysteresis is primarily caused by pore deformation rather than the thermodynamic differences between adsorption and desorption and the capillary condensation effect.

Proxy of paleo-seawater pH reconstruction based on boron isotopes of marine carbonates and its applications

Rui WANG, Chuntong LIU, Songye WU, Hui HUA

2025, 46(3):  1006-1018.  doi:10.11743/ogg20250321

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Boron （B） is widely distributed in nature. The fractionation of its stable isotopes ¹⁰B and ¹¹B establishes boron as a significant tool for reconstructing paleoceanographic and paleoclimate conditions. The sensitivity of the δ¹¹B values to seawater pH makes it possible to establish a δ¹¹B-paleo-seawater pH proxy. This proxy enables the inversion of paleo-seawater pH using the boron isotope values of marine carbonate minerals， which in turn facilitates the analysis of the atmospheric CO₂ concentration changes. Although the δ¹¹B-paleo-seawater pH proxy has great theoretical potential， its accuracy remains a challenge. By analyzing the boron cycling in the ocean and methods for boron isotope determination， we investigate the underlying mechanisms and application potential of the δ¹¹B-paleo-seawater pH proxy. The results indicate that major factors affecting the proxy accuracy include boron deviation caused by biological effects and inaccuracies in the fractionation coefficient between boron isotopes， insufficient understanding of the exchange processes between B（OH）₃ and \mathrm{B}{(\mathrm{O}\mathrm{H})}_{\mathrm{4}}^{-} in the seawater， and the inability of lab simulations to fully replicate the fractionation of boron isotopes during carbonate precipitation in the natural environment. To enhance the accuracy of the proxy， it is recommended to incorporate additional proxies such as the B/Ca ratio and to select carbonate samples less affected by biological effects. Furthermore， it is necessary to enhance the precision of boron isotope fractionation coefficient calculations， establishing a more scientific empirical method for correcting the biological effects. These advancements will contribute to more scientific and broader applications of boron isotopes in reconstructing paleoceanographic and paleoclimate conditions.