Table of Content

01 February 2024, Volume 45 Issue 1

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Petroleum Geology

Natural fractures in deep to ultra-deep tight reservoirs： Distribution and development

Lianbo ZENG, Lei GONG, Xiaocen SU, Zhe MAO

2024, 45(1):  1-14.  doi:10.11743/ogg20240101

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Natural fractures serve as effective storage spaces and primary seepage pathways in deep to ultra-deep tight reservoirs， affecting the hydrocarbon migration and enrichment， single-well productivity， and exploitation methods and outcomes of the reservoirs. Based on the summary of latest research results and literature review on fractures in tight reservoirs， this study delves into the distribution characteristics and developmental patterns of natural fractures in deep to ultra-deep tight reservoirs. The results show that the natural fractures are of large， meso， small， and micro scales， following a power law distribution. In other words， a larger scale corresponds to a smaller number of fractures， and vice versa. Large- and meso-scale fractures primarily facilitate seepage； small-scale ones mainly enable seepage and storage； and micro-scale ones principally serve as storage spaces. The type， occurrence， and mechanical properties of the natural fractures formed across different periods are determined by the evolution of stress regime during stratigraphic burial. The formation， distribution， and developmental degree of multi-scale fractures are subjected to the magnitude of tectonic stress， the mechanical properties of rock mechanical stratigraphy， and the thickness differences in mechanical layers. Structural deformation results in varied local stress and strain distribution at different structural locations， increasing fracture heterogeneity. Thrust faults control the distribution of faulted fracture zones by controlling the deformation of strata on the hanging walls. The combination style and movement mode of strike-slip faults， along with rock mechanical stratigraphy， jointly dictate the three-dimensional spatial distribution of related fractures. Furthermore， the crack-seal patterns of the fractures during formation and evolution determine their storage spaces and record the evolutionary history of their effectiveness.

Development of submarine depositional systems under dynamic interplays between sediment gravity flows and seafloor topography： A case study of the Potiguar Basin on the equatorial Atlantic Ocean

Naxin TIAN, Chenglin GONG, Gaokui WU, Kun QI, Yijie ZHU, Jingjing LIU

2024, 45(1):  15-30.  doi:10.11743/ogg20240102

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Seafloor topography plays a significant role in the modulation of gravity flow deposition and in the meantime， the associated depositional processes can modify the pre-existing topography. Given a lack of studies on the development of the submarine depositional systems as a consequence of the dynamic interplays between sediment gravity flows and seafloor topography， we analyze the local depositional records of the Potiguar Basin on the equatorial Atlantic Ocean. Using 3D seismic data along with spectral decomposition and spectral decomposition with red， green and blue （RGB） color blending techniques， we investigate the development of a submarine depositional system and then reveal the dynamic interactions between sediment gravity flows and seafloor topography. The results indicate that the main topographic low and the minor topographic low within initial topography of the study area played a major role in the deposition of early submarine channels and lobes. With the gentling of the slope along the main topographic low toward the distal provenance end， the velocities and competences of sediment gravity flows gradually decreased； correspondingly， their tendences of vertically downcutting and laterally broadening respectively weakened and strengthened， resulting in the cross-sectional geometries of submarine channels varying from V-shaped through deep U-shaped to dish-shaped downstream. Because submarine-channel confinement gradually decreased downstream， lobes occur at the unconfined terminal area， which grew headward， overlay on the early channel fills， and finally spilled out into the minor topographic low at the northwestern corner of the study area. The deposition of submarine channels and lobes significantly reduced the slope gradients in distal parts of the main topographic low. When subsequent mass flows were captured by the main topographic low， the capacity of mass flows to carry sediments was prone to get even weaker， thus resulting in more extensive accumulation of mass-transport complexes （MTCs） in distal parts of the main topographic low.

Organic matter enrichment mechanisms of terrigenous marine source rocks in the Qiongdongnan Basin

Dujie HOU, Keqiang WU, Li YOU, Ziming ZHANG, Yajun LI, Xiaofeng XIONG, Min XU, Xiazhe YAN, Weihe CHEN, Xiong CHENG

2024, 45(1):  31-43.  doi:10.11743/ogg20240103

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The Oligocene terrigenous marine source rocks in the Qiongdongnan Basin serve as a significant suite of source rocks in deep-water areas. We analyze their formation mechanisms from the aspects of the basin’s tectonic framework， paleoclimate， sedimentary facies， and delta scale. The results indicate that the tectonic framework of the basin generally determines the source rock types. Specifically， the terrigenous organic matter of the Yacheng Formation， deposited during the rifting stage， predominantly accumulates offshore and forms coal-measure source rocks. In contrast， the terrigenous organic matter from the Lingshui Formation （E₃l） forms terrigenous marine source rocks via fluvial-deltaic transport. The 3^rd member of the Yacheng Formation （E₃y³） and the 2^nd member of the Sanya Formation （N₁s²） witnessed the most luxuriant terrigenous higher plants， followed by the 1^st to 2^nd members of both the Yacheng and Lingshui formations （E₃y¹ to E₃y²， and E₃l¹ to E₃l²）. These contributed significantly to an abundant supply of terrigenous higher plants. Variations in sedimentary facies zones determine the differences in terrigenous organic matter， whose differential deposition leads to the formation of significantly varying terrigenous marine source rocks. Favorable areas for the terrigenous marine source rocks include delta fronts to inner neritic zones， along with submarine fans. Large-scale deltas facilitate the transport of terrigenous organic matter with higher biotic fluxes， thus improving the organic matter abundance of source rocks. As a result， significant and characteristic gas source rocks are formed in the sea area.

Diagenesis and low-permeability tightening mechanisms of the deep Paleogene reservoirs under high temperature and highly variable geothermal gradients in the Baiyun Sag， Pearl River Mouth Basin

Guanghui YUAN, Guangrong PENG, Lili ZHANG, Hui SUN, Shuhui CHEN, Hao LIU, Xiaoyang ZHAO

2024, 45(1):  44-64.  doi:10.11743/ogg20240104

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In the Baiyun Sag of the Pearl River Mouth Basin （PRMB）， the oil and gas exploration targets have gradually shifted from the conventional reservoirs in the shallow to moderately deep Hanjiang-Zhujiang formations to the low-permeability， tight reservoirs in the deep to ultra-deep Zhuhai-Enping formations. Due to their distinct geological setting of highly variable geothermal gradients， the low-permeability， tight reservoirs exhibit significantly different diagenesis and tightening mechanisms from the conventional reservoirs. Using techniques such as casting thin section observation， scanning electron microscopy （SEM）， physical property tests， diagenetic reconstruction， and physical property restoration， we systematically analyze the diagenetic processes of the Paleogene sandstone reservoirs from the Zhuhai Formation’s lower member to the Enping Formation in the area from the northwestern low uplift to the central sub-sag zone in the Baiyun Sag and their disparities. Considering the tectonic evolution setting， stratigraphic burial history， and current physical property data， we investigate the major factors influencing the relationships among the reservoirs’ physical properties and explore their tightening processes and mechanisms. The results suggest that the reservoirs from the Zhuhai Formation’s lower member to the Enping Formation have experienced intense compaction， two-stage carbonate cementation， three-stage siliceous cementation， and three-stage feldspar dissolution. During their diagenetic processes， the reservoirs exhibited varying compaction rates due to changes in geothermal gradients and underwent water-rock interactions in different open-closed systems. These are major reasons for the different physical properties of the reservoirs across various tectonic zones in the Baiyun Sag. Compaction emerged as the primary factor leading to the reservoir tightness， which was further enhanced by siliceous and carbonate cementation. In contrast， dissolution improved the physical properties of the reservoirs. From the low uplift to the sub-sag zone， strata from the Zhuhai Formation’s lower member to the Enping Formation exhibited increasing geothermal gradients and burial depths. Accordingly， their reservoirs in the low uplift， slope zone， and sub-sag zone are in the middle diagenetic stage A2， middle diagenetic stage B， and late diagenetic stage， respectively， with diagenetic intensity gradually increasing. The diagenetic variations significantly impacted the evolution of the reservoirs’ physical properties. Specifically， the reservoirs in the sub-sag zone had become tight prior to the late-stage hydrocarbon charging， while those in the slope zone underwent a gradually tightening process during this period. In contrast， the reservoirs in the low uplift still feature low permeability currently. The slope zone， with burial depths of generally less than 5 km and overlying strata exhibiting excellent sealing capacity， emerges as a focus of deep oil and gas exploration in the Baiyun Sag.

Sedimentary evolution and genetic mechanisms of typical marine nearshore sandbars

Junwei ZHAO, Haihang SUN, Dongwei ZHANG, Heng WANG

2024, 45(1):  65-80.  doi:10.11743/ogg20240105

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Beach-bar sandbodies are important hydrocarbon reservoirs formed under complex and varied hydrodynamic mechanisms. However， due to the limitations of data on outcrops and modern analogs， the understanding of their developmental processes， geometric morphologies， dynamic evolutionary patterns， and superposition within internal architectures remains unclear. Given this， we analyze the sedimentary evolution and hydrodynamic mechanisms of typical shore-normal， linearly distributed nearshore bars using sedimentary numerical simulations combined with modern analogs. Furthermore， we delineate the developmental and evolutionary patterns of these typical nearshore bars. The results show that a typical marine nearshore bar undergoes five stages of development： the formation of tapered bars， the development of crescentic bars， and the formation， expansion， and developmental termination of a banded compound bar. Modern analogs reveal bars at various developmental stages， which were formed under diverse coastal hydrodynamic conditions. Tapered bars and crescentic bars are a series of small-scale bars that are roughly equally spaced. The tapered bars evolve into crescentic bars due to the continuous erosion from uprush and longshore currents. In between the tapered and crescentic bars， there exhibit inter-bar rip channels and backflow trenches on the top. Both bar types represent typical topset deposits. Over time， the crescentic bars and their inter-bar rip channels receive deposits， forming a relatively uniform banded compound bar， demonstrated as typical lateral depositon This bar， after widening and thickening under the action of onshore and bottom currents， finally emerges above water and constitutes barrier shorelines. The internal architecture of a compound bar， which may comprise several tapered bars， crescentic bars and inter-bar channels， displays the tapered bar， interbar channel and tapered bar deposits superimposition or the crescentic bar， interbar channel and crescentic bar deposits superimposition that， corresponding to thick-thin-thick sequences on profiles.

Characteristics and origin of high-quality gravity-flow sandstone reservoirs in the 3^rd member of the Dongying Formation， southern Liaozhong Sag， Bohai Sea

Guanmin WANG, Xiaojun PANG, Xiaobo HUANG, Xuefang ZHANG, Kai PAN

2024, 45(1):  81-95.  doi:10.11743/ogg20240106

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Gravity-flow sandstone reservoirs in the medium to deep parts of the Bohai Sea exhibit significantly varying physical properties， making it difficult to predict high-quality reservoirs. This study aims to address this oil/gas exploration challenge. Based on the analyses and tests of conventional physical properties， casting thin sections， clay mineralogy， and geochemistry， as well as data from well logging， core observations， and seismic survey， we investigate the sedimentary characteristics of the deep-water gravity-flow sandstone reservoirs in the 3^rd member of the Dongying Formation （also referred to as the Dong 3 Member） in the Liaozhong Sag， as well as primary factors influencing the development of high-quality reservoirs. The results show that the gravity-flow deposits in the Dong 3 Member in the study area are of the sandy debris flow type， with reservoirs primarily characterized by medium-to-low porosity and permeability. The storage spaces in the reservoirs are dominated by intergranular dissolution pores， followed by primary pores. Among the reservoirs， medium- and coarse-grained sandstones exhibit the optimal physical properties， followed by fine-grained sandstones， while siltstones show poor physical properties. The key factors influencing the development of high-quality reservoirs include： （1） Good sorting， low clay content， and feldspar enrichment act as the material basis for the formation of high-quality reservoirs； （2） In the early diagenesis stage of the Dongying Formation， the rapid subsidence led to undercompaction-induced overpressure， preserving numerous primary pores. During the hydrocarbon generation in the late stage， overpressure prolonged the generation period of organic acids， producing sufficient acidic fluids for the dissolution of minerals like potassium feldspar and calcite in sandstones； （3） Faults connected the reservoirs and source rocks， leading to enhanced dissolution in medium-deep reservoirs， which facilitated the formation of dissolution pores in quantity； （4） Tectonic uplift and lateral compression caused reservoir fracturing， as well as the formation of extensive fractures and dissolution zones， improving the physical properties of the reservoirs.

Distribution and origin of overpressure in the Paleogene Dongying Formation in the southwestern sub-sag， Bozhong Sag， Bohai Bay Basin

Xiaoyi YANG, Chenglin LIU, Feilong WANG, Guoxiong LI, Dehao FENG, Taozheng YANG, Zhibin HE, Jiajia SU

2024, 45(1):  96-112.  doi:10.11743/ogg20240107

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Overpressure is extensively distributed in the Paleogene Dongying Formation in the southwestern sub-sag of the Bozhong Sag， Bohai Bay Basin. However， there is a lack of a clear understanding of the pressure distribution， the formation mechanisms of overpressure， and the effects of overpressure on hydrocarbon accumulation in this formation. In this study， we explore the present-day pressure in the formation and its spatial distribution using measured formation pressure and logs. Based on a comprehensive analysis of log curves and the cross plots of sonic velocity vs. density， we analyze the genetic types of overpressure in the formation. Furthermore， through basin modeling， we reconstruct the paleopressure evolution during hydrocarbon accumulation to assess the effects of the formation paleopressure on hydrocarbon accumulation. Key findings are as follows： （1） The Dongying Formation exhibits a complete overpressure system vertically. Horizontally， the overpressure is distributed around hydrocarbon-generating sub-sags， intensifying closer to the sag center； （2） The genetic type of overpressure shifts from predominant undercompaction in shallow strata to a combination of undercompaction and hydrocarbon generation-induced expansion in deep layers； （3） The overpressure within the source rocks of the Dongying Formation， including the 3^rd member and the lower sub-member of the 2^nd member， offers a sufficient dynamic for hydrocarbon migration. Furthermore， it forms seals for the underlying overpressured fluids， preventing the upward escape of hydrocarbons. It is proposed that ancient buried hills represent favorable targets for future deep and ultra-deep hydrocarbon exploration in the southeastern sub-sag of the Bozhong Sag.

Accumulation mechanism and model of multi-type deep coarse-grained siliciclastic reservoirs in the eastern Jiyang Depression， Bohai Bay Basin

Wenzhi LEI, Dongxia CHEN, Yongshi WANG, Jianqiang GONG, Yibo QIU, Qiaochu WANG, Ming CHENG, Chenyang CAI

2024, 45(1):  113-129.  doi:10.11743/ogg20240108

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Deep coarse-grained siliciclastic rocks in the eastern Jiyang Depression， Bohai Bay Basin hold great potential for oil and gas exploration. Given the presence of various types of deep coarse-grained siliciclastic reservoirs and the significant geological differences in hydrocarbon accumulation between the deep and the medium-to-shallow reservoirs in the depression， there is an urgent need to understand the formation mechanisms behind these differentiated geological features in order to advance the exploration and exploitation of deep oil and gas reserves. Employing techniques such as thin-section microscopy， reservoir fluid inclusion analysis， and basin simulation， we investigate pore types in the deep reservoirs and the origin of differentiated formation pressure in the Jiyang Depression. The accumulation models of various types of deep coarse-grained siliciclastic reservoirs in the depression are established thereby. The key findings are as follows： （1） The diagenetic evolution reveals the formation mechanisms of the various pore types of reservoirs. The preservation of primary intergranular pores in the deep coarse-grained siliciclastic reservoirs is attributed to the combined effects of early hydrocarbon charging and overpressured fluids. In contrast， the secondary dissolved pores in the reservoirs are developed largely due to dissolution reactions induced by acidic fluid charging during the hydrocarbon generation of source rocks. In addition， the overpressure in the reservoirs facilitates the development of microfractures. All these lead to the formation of pore-fracture reservoir spaces； （2） The evolution of fluid overpressure in the deep reservoirs is governed by hydrocarbon generation and depositional process. The formation of overpressure environment hinges on preservation conditions， and the pressure distribution in hydrocarbon reservoirs within structural-lithologic traps is regulated by the fault-sand body transport system. In deep normal pressured-weakly overpressured reservoirs， the residual pressure difference between source rocks and reservoirs serves as the primary driving force to hydrocarbon accumulation. While in overpressured reservoirs， this residual pressure difference gradually decreases over time. Furthermore， buoyancy provides the main driving force for the secondary migration and adjustment of hydrocarbons in areas with well-developed faults and in sand bodies with good vertical connectivity； （3） Based on the joint control of hydrocarbon generation-reservoir-pressure on hydrocarbon accumulation， we establish three hydrocarbon accumulation models for deep coarse-grained siliciclastic reservoirs in the eastern Jiyang Depression， including the model of vertically differentiated hydrocarbon accumulation in multi-phase superimposed nearshore subaqueous fans， the model of top-lateral joint hydrocarbon sealing by mudstone within nearshore subaqueous fans， and the model of near-source hydrocarbon accumulation within nearshore subaqueous fans-tubidite fans. The elucidation of these hydrocarbon accumulation mechanisms and models of multi-type deep coarse-grained siliciclastic reservoirs in the Jiyang Depression lays a solid foundation for subsequent in-depth exploration.

Overpressure in moderately deep to deep strata in the Lengjia-Leijia area， western Liaohe Depression， Bohai Bay Basin： Origin and effects on hydrocarbon accumulation

Chang CHEN, Nansheng QIU, Rongjin GAO, Xiaolong ZHOU, Yonghe SUN, Linlin YANG, Jian FU

2024, 45(1):  130-141.  doi:10.11743/ogg20240109

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The moderately deep to deep strata in the Lengjia-Leijia area， western Liaohe Depression exhibit significant overpressure. It’s of great significance to understanding its genetic mechanism and its effects on hydrocarbon migration and accumulation in terms of hydrocarbon exploration therein. Based on measured formation pressure and temperature， log data， and structural analysis， we comprehensively investigate the characteristics and genetic mechanisms of overpressure in these strata and explore its role in hydrocarbon accumulation therein. The results indicate that the Lengjia-Leijia area is characterized by composite thermo-pressure system of the overpressure， normal-pressure， and underpressure types， with the overpressure interval predominantly found in the 3^rd and 4^th members of the Shahejie Formation （also referred to as the Sha 3 Member and the Sha 4 Member， respectively） within the sub-sag zone， at depths ranging from 2 500 to 4 000 m approximately. The overpressure in the area primarily results from undercompaction， followed by a combination of hydrocarbon generation-induced pressurization， diagenesis， and tectonic compression. There exist two major hydrocarbon migration and accumulation modes in the study area： the self-sourced indigenous mode within overpressure zones and the cross-pressure mode from an overpressure zone to a normal-pressure/underpressure zone. The former is predominantly found within the overpressure zones of the Sha 3 and 4 members in the sub-sag zone. In contrast， the latter occurred during the period of faulting， including overpressure-driven hydrocarbon charging into the Leijia and Gaosheng structural zones from source rocks in the Sha 4 Member in the Chenjia sub-sag， and hydrocarbon accumulation in the eastern steep slope area and Xinglongtai buried hill in the Lengjia area after transporting through faults and sand bodies from the overpressure zones in the lower to middle sub-members of the Sha 3 Member.

Discovery of suspected intrusive rock bodies in Gufengzhuang area， Ordos Basin and its geological significance

LiHua YANG, ChiYang LIU, Lei HUANG, Yijun ZHOU, Yongtao LIU, Yang QIN

2024, 45(1):  142-156.  doi:10.11743/ogg20240110

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Magmatic rocks within the Ordos Basin are crucial for reconstructing tectono-thermal events， paleogeothermal fields， and petroleum geological processes. Based on the processing and interpretation of high-resolution 3D seismic data， we identify suspected intrusive rock bodies in shallow strata in the Gufengzhuang area， Ordos Basin， marking the first discovery of such geological structures in this area. Furthermore， we characterize their spatial distributions and explore their geological significance. The results reveal six rock bodies interpreted as intrusive rocks in the Gufengzhuang area， most of which are ring- or cloud-shaped in a planar view. As shown on seismic sections， strata closer to the cores exhibit more pronounced chaotic reflections and more significant characteristics of mounded structure. Based on their distribution patterns and seismic reflection characteristics， these intrusive rock bodies as interpreted can be classified into two types： the deep scattered and shallow concentrated type with magma upwelling and the shallow concentrated type with magma upwelling. The geological structures of the former type exhibit chaotic seismic reflections as caused by magma intrusion from deep to shallow， significantly distinct from the seismic reflections of their surrounding rocks. Furthermore， they are characterized by nearly vertical occurrences and associated faults. In contrast， only the shallow strata of the latter type display chaotic mounded seismic reflections （corresponding time slices at 1 000 ms and less）， while the deeper strata show normal layered reflections with almost no disturbance. Analyses suggest that the differential magma upwelling is primarily caused by the presence of pre-existing faults and their quantity， scale， degree of vertical superimposition， and throws at cores and ends， as well as formation pressure. By combining the developmental characteristics and formation eras of other magmatic rocks within the Ordos Basin， this study proposes that the intrusive rock bodies as interpreted in the Gufengzhuang area were formed during the late Early Cretaceous， which is consistent with the scale hydrocarbon generation and accumulation period of Paleozoic-Mesozoic oil and gas， and the hydrothermal activity in this stage is of great importance to the generation， preservation and accumulation of hydrocarbons in this area.

Migration differentiation of hydrocarbon components in the 7^th member of the Triassic Yanchang Formation， central Ordos Basin

Liang SHI, Bojiang FAN, Zhonghou LI, Ziwei YU, Zijin LIN, Xinyang DAI

2024, 45(1):  157-168.  doi:10.11743/ogg20240111

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The shale sequence of the 7^th member of the Yanchang Formation （also referred to as the Chang 7 Member） in the Ordos Basin consists of mudstones， shales， and sandstones. To delve into the migration differentiation of shale oil components in different lithologies， we investigate the migration differentiation of typical hydrocarbon components in the central Ordos Basin. The findings show that the rocks exhibit different contents of the four group components： saturated hydrocarbons （SAT）， aromatic hydrocarbons （ARO）， polar nitrogen-， sulfur-， and oxygen-containing compounds （NSO）， and asphaltenes （Asph）. Specifically， the sandstones manifest high SAT content and low NSO+Asph content. Conversely， the shales and mudstones exhibit low SAT content and high NSO+Asph content. Regarding n-alkanes， the sandstones demonstrate high nC_11-15 content and low nC_23-39 content， while the shales and mudstones display low nC_11-15 content and high nC_23-39 content. Compared to the sandstones， the shales and mudstones show more significant variations in the Pr/Ph， Pr/nC₁₇， and Ph/nC₁₈ ratios. These rocks manifest low C₂₇/C₂₉ regular sterane ratios and low Ts/Tm and Ts/（Ts+Tm） ratios， while the sandstones show opposite trends. They demonstrate substantial variability in 22S/（22S+22R） ratios for C₃₂ homohopane， in contrast to the relatively concentrated ratios for the sandstones. Besides， the sandstones are characterized by high gammacerane index values varying in a broad range， whereas the shales and mudstones exhibit the opposite trend. Notably， the shales and mudstones generally have 8β（H）-drimane/8β（H）-homodrimane ratios exceeding 1， which， however， are typically less than 1 for the sandstones. A comprehensive analysis indicates that the variations in the distributions of the various hydrocarbon group components within the source rocks can be attributed to both differential adsorption and migration differentiation.

Effects of diagenetic evolution on gas-bearing properties of deep tight sandstone reservoirs： A case study of reservoirs in the 1^st member of the Permian Shanxi Formation in the Qingyang gas field， southwestern Ordos Basin

Jiangjun CAO, Jiping WANG, Daofeng ZHANG, Long WANG, Xiaotian LI, Ya LI, Yuanyuan ZHANG, Hui XIA, Zhanhai YU

2024, 45(1):  169-184.  doi:10.11743/ogg20240112

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This study aims to analyze the effects of diagenetic evolution on the gas-bearing properties of reservoirs. Using data from casting thin section observations， scanning electron microscopy （SEM） images， physical property tests， and comprehensive log interpretation， as well as previous research results， we investigate the diagenetic characteristics of deep tight sandstone reservoirs in the 1^st member of the Permian Shanxi Formation （hereafter referred to as the Shan 1 Member） in the Qingyang gas field. Key findings are as follows： （1） The multi-stage thick-bedded sandstones in the middle tributary channels of the Shan 1 Member in the Qingyang gas field exhibit two types of superimposition relationships： continuous and intermittent deposition. The pore assemblages in the sandstone reservoirs of the continuous deposition type are dominated by dissolution pores， intergranular pores and intercrystalline micropores， while those in the sandstone reservoirs of the intermittent deposition type predominantly include dissolution pores and intercrystalline micropores. Compared to the intermittent deposition type， the sandstone reservoirs of the continuous deposition type exhibit lower content of fillings， more favorable physical properties， and enhanced gas-bearing properties； （2） Compaction is identified as the most significant destructive diagenesis that undermines the gas-bearing properties of reservoirs in the Shan 1 Member. The sandstone reservoirs of the continuous and intermittent deposition types exhibit compaction rates of 55.4 % and 61.2 %， respectively. Dissolution acts as the most significant constructive diagenesis that enhances the gas-bearing properties of these reservoirs. The sandstone reservoirs of the continuous and intermittent deposition types present dissolution rates of 27.9 % and 19.7 %， respectively. The sandstone reservoirs of the continuous deposition type underwent a diagenetic evolution characterized by moderate compaction， followed by moderate cementation， and then moderate dissolution， indicating weak destructive diagenesis. In contrast， the reservoirs of the intermittent deposition type experienced a diagenetic evolution from moderate to high compaction， to weak to moderate cementation， and finally weak to moderate dissolution， suggesting strong destructive diagenesis； （3） The sandstone reservoirs in the Shan 1 Member experienced reservoir tightening first and then hydrocarbon charging. Due to the influence of diagenetic evolution， the sandstone reservoirs of the continuous deposition type exhibit lower-level tightness， larger-scale natural gas charging， and more favorable gas-bearing properties compared to the intermittent deposition type； （4） The diagenetic intensity of reservoirs in the Shan 1 Member was quantitatively determined using the comprehensive diagenetic index. Regarding the planar distribution， the trunk channels to the west of the Qingyang area primarily consist of sandstones of the intermittent deposition type， with reservoirs exhibiting low comprehensive diagenetic index and poor gas-bearing properties. In contrast， in the trunk channels to the east of the Qingyang area， sandstones of the continuous deposition type are predominant， with reservoirs featuring high comprehensive diagenetic index and favorable gas-bearing properties. Therefore， gas-bearing reservoirs primarily occur to the east of the Qingyang area.

The Jinqiu gas field in the Sichuan Basin： A typical helium-bearing to helium-rich gas field with the Mesozoic sedimentary rocks as helium source rocks

Baoshou ZHANG, Benjian ZHANG, Hua WANG, Jianfa CHEN, Kaixuan LIU, Shuang DOU, Xin DAI, Shuangling CHEN

2024, 45(1):  185-199.  doi:10.11743/ogg20240113

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The lack of studies over an extended period leads to an unclear understanding of helium accumulation conditions and mechanisms， along with fierce controversy over the types and formation conditions of helium source rocks. Focusing on the helium-bearing to helium-rich Jinqiu gas field， this study presents the following key findings through detailed analysis： （1） The Jinqiu gas field exhibits a helium content ranging from 0.05 % to 0.10 % primarily， with an average of 0.07 %. In some wells， the helium content exceeds 0.10 %， reaching a maximum of 0.20 %. Isotope analysis reveals that helium in the gas field originates from the crust， without any contribution from the mantle. （2） The Jinqiu gas field is identified as of a typical helium-bearing to helium-rich type with the Mesozoic sedimentary rocks serving as helium source rocks. Both the Upper Triassic Xujiahe Formation and the Jurassic strata demonstrate high uranium and thorium contents， along with considerable thickness. They exhibit a high helium-generating intensity in the Jinqiu area， serving as helium source rock basis for the formation of helium-bearing to helium-rich gas reservoirs. Helium is presumed to primarily originate from the Jurassic reservoirs rather than the hydrocarbon source rock sequence of the Upper Triassic Xujiahe Formation. （3） The formation of the Jinqiu gas field is primarily under three factors. First， helium source rocks with a high helium-generating intensity act as a sound material basis. Second， the presence of hydrocarbon gases with a moderate charging intensity contributes to helium enrichment. Last， the uplift and erosion of strata， accompanied by a decrease in temperature and pressure， result in the exsolution of in-situ dissolved helium， conducive to helium gas enrichment.

Karst paleogeomorphology and reservoir control model of the 2^nd member of Dengying Formation in Penglai gas area， Sichuan Basin， China

Zhili ZHANG, Yanping QIAO, Shuang DOU, Kunyu LI, Yuan ZHONG, Luya WU, Baoshou ZHANG, Xin Dai, Xin JIN, Bin WANG, Jinmin SONG

2024, 45(1):  200-214.  doi:10.11743/ogg20240114

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An integration of core and thin section observation， log and 3D seismic data is applied to systematically analyze the epigenetic karst paleogeomorphology and reservoir development pattern of the 2^nd member of the Dengying Formation （hereafter referred to as Deng 2 Member） in the Penglai gas area， Sichuan Basin， China. Key findings are summarized as follows. First， the Deng 2 Member in the Penglai gas area can be vertically divided into the epigenetic karst zone， vertical vadose zone， superimposed zone， horizontal phreatic zone， and deep tranquil flow zone. Notably， the horizontal phreatic zone exhibits the thickest reservoirs； second， the karst paleogeomorphology of the Deng 2 Member was restored using the dual interface method. The results indicate that it can be categorized into four types of first-order paleogeomorphic units， namely karst highland， karst slope， karst depression， and karst basin， exhibiting one karst highland， one karst slope， and two karst depressions overall； third， the karst highland is situated in the Gaoshiti-Moxi area in the east， the karst slope is present in the central part of the study area， and the karst depressions are found on the northwest and southwest sides of the study area. The karst highland and karst slope primarily contain vertical vadose zones and horizontal phreatic zones， respectively. It is proposed that reservoirs in the Deng 2 Member of the Penglai gas area are jointly influenced by karst paleogeomorphology and sedimentary facies； fourth， epigenetic karst reservoirs emerge as the predominant reservoir type in the study area， with high-quality reservoirs occurring in the karst slope with superimposed microfacies of mound-shoal complexes.

Recognition of the Carboniferous strata in the Hongche fault zone， Junggar Basin and its hydrocarbon accumulation characteristics

Hong PAN, Qingsen YU, Xiaoshan LI, Junqiang SONG, Zhibin JIANG, Li WANG, Guanxing LUO, Wenxiu XU, Haoyu YOU

2024, 45(1):  215-230.  doi:10.11743/ogg20240115

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This study focuses on the Carboniferous strata in the Hongche fault zone， Junggar Basin， which are characterized by complex lithology， interior architecture， and stratigraphic contact. To establish the Carboniferous stratigraphic sequence in the fault zone， we conduct a comprehensive analysis under the guidance of the Carboniferous stratigraphic system based on outcrops. This analysis involves examining the lithologic assemblage features， seismic wave group characteristics， and stratigraphic contacts through the fine calibration of lithological properties， electrical properties， and seismic attributes. By comparing the Carboniferous strata in wells with the Carboniferous stratigraphic system based on outcrops in the mountainous area on the northwestern margin of the Junggar Basin， we establish the stratigraphic sequence profiles of the Carboniferous in the fault zone and conduct fine division and correlation of the strata. Additionally， we analyze dominant lithofacies in the Carboniferous strata based on the established Carboniferous stratigraphic framework. Key findings are as follows： （1） Drilling data from the Junggar Basin reveals that the Carboniferous strata in the Hongche fault zone can date back to the Late Carboniferous and consist of the Aladeyikesai （C₂a）， Hala’alate （C₂h）， and Chengjisihanshan （C₂c） formations top to bottom. These strata demonstrate a westward overlapping and uplifting trend overall， resulting in a tectonic framework characterized by west-east zonation and north-south partition； （2） The C₂c is composed of volcanic breccias of explosive facies. In contrast， the C₂a and C₂h exhibit overlapping and pinchout due to napping. Both formations exhibit decreasing denudation under explosion in the southern part， with the main body primarily presenting volcanic and clastic sedimentary facies； （3） In the Hongche fault zone， the primary oil-bearing pay intervals include the C₂c and C₂h， with volcanic breccias in the southern part and localized andesitic basalts serving as favorable targets for hydrocarbon exploration.

Igneous rock intrusions in the western Shunbei area， Tarim Basin： Characteristics and coupling relationships with faults

Yuemeng NIU, Jun HAN, Yixin YU, Cheng Huang, Bo Lin, Fan YANG, Lang YU, Junyu CHEN

2024, 45(1):  231-242.  doi:10.11743/ogg20240116

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Igneous rock intrusions are extensively distributed in the western Shunbei area， Tarim Basin， playing a significant role in transforming stratigraphic architecture and petroliferous systems. However， there is a lack of clear understanding of their characteristics and coupling relationships with deep strike-slip fault zones. Using the latest 2D and 3D seismic data of the area， we present a fine-scale characterization of their morphological features， categorizing their types and combinations. Furthermore， we identify their developmental stages and define their coupling relationships with deep strike-slip faults. The results show that the igneous rock intrusions in the western Shunbei area exhibit multiple morphotypes， including stratum-concordant， tongue-， saucer-， and half-saucer-shaped intrusions. Individual intrusive sheets can combine to form sill complexes through processes of conjunction， connection， and fault-induced dislocation. The igneous intrusions in the Shunbei area predominantly occur between the tops of the Middle-Lower Ordovician and the Middle-Lower Devonian， with the activity types of intrusive sheets being categorized into single-stage and single-layer， single-stage and multi-layer， and multi-stage and multiple-layer types. Unconformities and faults， acting as magma conduits， dictate the quantity and scale of the igneous rock intrusions， leading to notable disparities in igneous rock intrusions between the southern and northern parts of the study area. Specifically， the northern part of the study area featuring the deep strike-slip faults serving as primary magma conduits， exhibits intense vertical magma intrusion. In contrast， the southern part featuring unconformities acting as magma conduits， shows stronger horizontal magma intrusion.

Methods and Technologies

Efficient progressive exploration and exploitation technology for subtle reservoirs in the Oriente Basin and its applications to blocks 14 and 17 in Ecuador

Guangfu WANG, Fayou LI, Jianfang SUN, Hai XU, Yaxiong ZHANG, Yuliang FENG, Feng DING, Shuangjiang YE, Shiwang CHEN, Jie WU, Yu SUN

2024, 45(1):  243-255.  doi:10.11743/ogg20240117

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In blocks 14 and 17 of Ecuador， both situated in the foredeep zone of the Oriente Basin， the main oil-bearing strata comprise the M1， U， and T members of the Napo Formation. The developed oilfields in these blocks have entered a high water-cut stage， posing challenges to resource replacement. The trend surface-driven， post-stack consistent processing of seismic data， the time-frequency attenuation-based calibration and interpretation of high-precision synthetic seismograms， and anisotropic-variable-velocity mapping， are jointly applied to finely characterize low-amplitude structures， with many low-amplitude structural reservoirs discovered. Thin layers’ weak-reflection coefficients， determined through frequency-division iterative denoising， contribute to the reconstruction of effective post-stack broadband signals. With these signals as constraints， we quantitatively predict 2- to 5-m-thick tidal channel sandstones at a burial depth of 3 000 m through a seismic phase-controlled nonlinear inversion of waveforms， identifying several ultra-thin lithologic reservoirs in the M1 member. Based on regional hydrodynamic conditions， the oil-water contact trends of low-amplitude structural reservoirs， and the energy characteristics of reservoirs， we determine hydrodynamic oil reservoirs in the LU member and perform step-out drilling. The observation of numerous thin sections from cores reveals the presence of glauconites as cements and particles in the quartzose sandstone reservoirs. Accordingly， a dual-component volume model of glauconites for log interpretation is developed， allowing for reservoir evaluation and identification of low-resistivity reservoirs in the UT member. Based on the characteristics of tropical rainforest surfaces and subtle reservoirs in the subsurface， we achieve progressive exploration， evaluation， and rapid production onstream following the strategy of “overall deployment， batch-wise implementation， tracking and evaluation， and timely adjustment”， with a success rate of over 90 % for both exploration and appraisal wells.

Status quo and development trends of research on shale gas adsorption and seepage in shale gas reservoirs

Yi ZHANG, Bin ZHANG, Banghua LIU, Jie LIU, Qiansheng WEI, Qi ZHANG, Hongjun LU, Pengyu ZHU, Rui WANG

2024, 45(1):  256-280.  doi:10.11743/ogg20240118

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Shale gas reservoirs are characterized by large amounts of adsorbed gas within nano- to micron-scale organic pores and the mosaic form of organic matter in the inorganic matter. To understand these unique features， we initially review current methodologies used to characterize shale gas adsorption and seepage. Afterward， we summarize the mathematical representations of critical factors influencing these processes， including changes in the gas adsorption layer thickness， changes in the thickness of water film in inorganic pores， and changes in gas desorption-induced pores. Subsequently， we point out major problems in the current models of calculating the apparent permeability of shale gas reservoirs： （1） inaccurate characterization of reservoir pore structures； （2） a lack of methods to characterize the effects of the desorption of gas molecules dissolved into the solid organic matter on the adsorption of seepage flux； （3） inadequate characterization of the discrete distribution of organic matter in shale gas reservoirs； and （4） the unreasonability in direct application of parameters obtained from laboratory isothermal adsorption experiments， which neglects the differences between adsorbed shale gas and that dissolved into the solid organic matter in adsorption and desorption. Then， we analyze the advantages of the molecular simulation technique over physical experiments in examining shale gas adsorption and seepage. Accordingly， we summarize molecular simulation technique-based methods for modeling and simulating shale gas adsorption and seepage， as well as the simulation results. Finally， suggestions for further advancement in the molecular simulation of shale gas adsorption and seepage are put forward， including the improvement of multi-medium and multi-scale modeling methods based on conventional molecular models of inorganic and organic pores to be in line with the actual situation of shale gas reservoirs， as well as the necessity to develop methods for shale gas adsorption and seepage simulation that are more suitable for actual conditions.

Key stages in hydrocarbon migration and accumulation in layers outside source rocks and the evaluation methods related： A case study of the lower member of the Minghuazhen Formation， Bozhong Sag， Bohai Bay Basin

Hongguo ZHANG, Haifeng YANG, Wen SU, Chunqiang XU, Zhi HUANG, Yanjun CHENG

2024, 45(1):  281-292.  doi:10.11743/ogg20240119

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Hydrocarbon migration and accumulation are critical for large-scale reservoir development in layers far from the source rocks. Using abundant 3D seismic and drilling data， we delve into the primary driving forces for hydrocarbon migration and accumulation in three major stages in the Bozhong Sag， Bohai Bay Basin， namely the deep vertical transport along fault zones， secondary convergence in the Guantao Formation， and lateral diversion along sand traps in the lower member of the Minghuazhen Formation. Furthermore， we predict the hydrocarbon column heights within sand bodies in the lower member of the Minghuazhen Formation. Key findings are as follows. First， the coupling between overpressure in source rocks and fault development is crucial to the vertical hydrocarbon transport along fault zones， reflecting a high vertical transport efficiency with a fractal dimension exceeding 0.7 on the condition of abundant hydrocarbon sources. Second， the buoyancy pressure of old oil reservoirs in sand-rich sequences serves as the main driving force for the secondary hydrocarbon convergence. Convergence coefficient calculated using the size and effective range of traps， is in line with the scale of reserves. The lower member of the Minghuazhen Formation has the potential for the formation of large- to medium-scale oilfields when the convergence coefficient of the Guantao Formation exceeds 700 km³. Third， fault depressurization determines the lateral hydrocarbon diversion in shallow sand bodies. The depressurization coefficient is calculated based on the fault-sand contact area in both the high and low parts of the sand bodies. The results show that the hydrocarbon column heights within sand bodies in the lower member may exceed 30 m when the depressurization coefficient is greater than 2.0. These findings underscore that the three major stages are interlocking， which is a necessary condition for the formation of high-abundance hydrocarbon reservoirs in strata outside source rocks in a hydrocarbon-rich sag.

Seismic sedimentological characterization of an offshore area with sparse well control under the constraint of a high-resolution stratigraphic framework： A case study of the Paleogene Huagang Formation in block X of the central anticlinal zone in the Xihu Sag， East China Sea Basin

Xinxu DONG, Xinghai ZHOU, Kun LI, Renhai PU, Aiguo WANG, Yunwen GUAN, Peng ZHANG

2024, 45(1):  293-308.  doi:10.11743/ogg20240120

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Exploration in vast offshore survey areas faces the challenge of limited single-well data. To reveal the lateral sedimentary microfacies variations still in controversy and vertical sedimentary evolution pattern of the Huagang Formation， we focus on the Huagang Formation in block X of the central anticlinal zone in the Xihu Sag based on a comprehensive analysis of available logs and seismic data. Using the seismic sedimentological interpretation technology， which allows for analyzing the evolutionary pattern of sedimentary facies with the assistance of paleogeomorphic reconstruction， we establish a high-resolution isochronous stratigraphic framework through the wavelet transform of logs and seismic data. Furthermore， we calibrate the seismic facies types based on single-well sedimentary facies and delineate the distribution of sedimentary facies using frequency-division spectral decomposition red， green， and blue （RGB） attributes. Key findings are as follows： （1） The wavelet transform results from logs and seismic data， demonstrating high consistency， allow for the division of the Huagang Formation of the study area into eight fourth-order cycles： three in the upper member （H1—H3） and five in the lower member （H4—H8）. Consequently， a high-resolution isochronous stratigraphic framework is formed； （2） A comprehensive analysis of core facies， logging facies， and seismic facies suggests that the Huagang Formation in the study area had a sedimentary environment of braided deltas. Frequency-division seismic amplitude attributes reveal that sandstone layers of varying thicknesses exhibit the most significant seismic amplitude responses in single-frequency volumes of 15 Hz， 30 Hz， and 45 Hz. Therefore， the frequency-division RGB blends can be used to effectively delineate the boundaries of different sedimentary bodies in a planar view； （3） In the study area， the H1 and H2 cycles generally consist of delta-front sediments， the H3 to H6 cycles feature a combination of delta-plain and delta-front sediments in a shallow-water setting， and the H7 and H8 cycles comprise braided delta plain sediments. The vertical sedimentary facies evolution of these sand beds is governed by sediment supply， relative lake level， and paleogeomorphology. Based on the isochronous stratigraphic framework established through time-frequency analyses of logs and seismic data， the distributions of sandstones with different thicknesses can be characterized through RGB blends of seismic attributes selected via correlation analysis. This approach enhances the identification accuracy of sedimentary body boundaries while diminishing the influence of researchers’ subjectivity， thus providing a reference for sedimentary facies characterization of other zones with sparse well patterns.