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JOPC 2024 Vol.26 Number 5
2024, Vol.26 Num.5
Online: 2024-10-01
SPECIAL ISSUE ABOUT THICK SOURCE ROCKS
PALAEOGEOGRAPHY AND MINERAL RESOURCES
LITHOFACIES PALAEOGEOGRAPHY AND SEDIMENTOLOGY
REVIEW
 
SPECIAL ISSUE ABOUT THICK SOURCE ROCKS
1017 LIU Huimin, WEN Huaguo, ZHANG Kuihua, GUO Pei, ZHANG Guanlong, YU Hongzhou, XIONG Wei, ZHOU Jian, XU Wenli, SONG Meiyuan, LI Changzhi
Geological characteristics and exploration prospects of alkaline lake shale oil of the Fengcheng Formation, Hashan area,NW Junggar Basin
In recent years,the Permian Fengcheng Formation of the Mahu sag has become an important interval for shale oil exploration and newly discovered hydrocarbon reserves in the Junggar Basin. The Halaalate Mountain area(Hashan area)is located on the northern margin of the Mahu sag. Due to its proximity to the thrust belt and in the early stage of exploration,the sedimentary background,palaeogeographical location and shale oil exploration potential of the Fengcheng Formation of the Hashan area are still unclear. Based on the exploration and development practice of shale oil in the Hasan area during the past decade,the sedimentary environment,shale facies,source rock quality,oil content and reservoir quality of the Fengcheng Formation shale oil in this area were studied systematically. The results show that besides the main sag in the Mahu sag,there was another alkaline lake center of developed in the central part of the Hashan area,which is another hydrocarbon rich depression for the Fengcheng Formation. The volcanic eruption,sedimentary evolution and alkaline salts forming stage of the Hashan area bear a strong resemblance to that of the Wuxia area to the south. A four end-member classification scheme is proposed here for the alkaline lake shale in the Fengcheng Formation. The shale of the Fengcheng Formation can be divided into eight lithofacies,including four salty lithofacies and four non-salty lithofacies. On the whole,the clayey shale and calcareous shale of the Fengcheng Formation in the Hashan area have the highest organic matter abundance,the dolomitic shale and salty shale have relatively good oil content,and the dolomitic shale and siltstone have the best reservoir property. It is proposed that the dolomitic shale comprises the class-I sweet spot and the siltstone comprises the class-II sweet spot for shale exploration. For the Fengcheng Formation,shale in the Hashan area is similar to that of in the Wuxia area in terms of source rock and reservoir quality,both of which have high shale oil exploration potential.
2024 Vol. 26 (5): 1017-1036 [Abstract] ( 713 ) [HTML 1KB] [PDF 50968KB] ( 118 )
1037 WEI Shilin, JIAO Xin, LIU Yiqun, LI Hong, ZHOU Dingwu, Lü Yiran
Progress on volcanic-hydrothermal sedimentary dolostone in intracontinental lacustrine rift basin

Organic-rich black shales,which represent shale oil and gas resources,have become a research hotspot in the field of oil and gas exploration and development in China and around the world in recent years. These lithologies consist mainly of complexly sourced fine-grained sedimentary rocks in continental lacustrine basins in China,among which a special type of laminated microcrystalline dolostone is developed. The petrological,mineralogical micro-features,and geochemical characteristics of this type of dolostone are distinct from the commonly dolostones formed by dolomitization and are also different from microbial mediated dolostones. Hence,the dolostones have been defined as a new type of volcanic-hydrothermal sedimentary dolostone. Based on extensive review of domestic and foreign literature,this paper summarizes the global distribution and environmental characteristics of modern lacustrine dolomites and presents the research progress on the models of origins of dolostone. The focus is on the comparative study of volcanic-hydrothermal sedimentary dolostones found in the Lucaogou Formation in Santanghu and Junggar Basins, the Xiagou Formation in Jiuxi Basin,and the Tengger Formation in Erlian Basin,all of which are formed in an extensional tectonic setting. Four characteristics are summarized as follows: (1)These dolostones are well-preserved with μm-scale laminated structure and microcrystallized,indicating latterly weak diagenesis. (2)They contain a variety of volcanic-hydrothermal minerals,and are interbedded with semi-deep to deep lacustrine black shale. Evaporite characteristics are lacking. (3)Strontium isotopes indicate that the diagenetic fluid has a deep source(crust-mantle mixed)characteristic. (4)They are rich in organic matters and show a strong coupling relationship with hydrocarbon distribution. Finally,it is recommended to enhance research on this type of dolostone,utilize its unique geological information for tracing tectonic backgrounds,and explore its relationship with hydrocarbon generation as future research directions.
2024 Vol. 26 (5): 1037-1057 [Abstract] ( 286 ) [HTML 1KB] [PDF 20642KB] ( 83 )
1058 LUO Fan, DUAN Ke, ZHANG Hao, XU Lulu, WANG Yi, CHEN Wei, YANG Jie
Development characteristics of volcanic tuff of the Wufeng-Longmaxi Formations in Xianfeng area of western Hubei and its influence on enrichment of organic matter in shale
The organic-rich shale of Wufeng Formation and Longmaxi Formation in Xianfeng area of western Hubei Province contains multilayer volcanic tuff,but the internal relationship between volcanic activity and organic matter enrichment in marine shels has not been systematically studied. Based on the core and outcrop data of Well XY 1 in Xianfeng area,the organic-rich mud shale and volcanic tuff of Wufeng-Longmaxi Formations were systematically sampled,rock and ore identification,trace element,rare earth element and organic carbon content(TOC)tests were carried out,and the distribution characteristics,source rock composition,tectonic setting and genetic mechanism of volcanic tuff of Wufeng-Longmaxi Formations were expounded. The volcanic activity in this sedimentary period and its influence on the enrichment of organic matter in shale are discussed. The results show that the Wufeng-Longmaxi Formations can be divided into dense,sub-dense and undeveloped tuff segments. The primary magmatic properties of the tuff were mainly neutral andesite,and the volcanic ash originates from the volcanic eruption event of the island arc formed by the collision between the northern margin of the Yangtze and the ancient Qinling Ocean. Volcanic ash was an important material source in the Middle Yangtze Retention Basin where terrigenous detritus supply was insufficient in Late Ordovician and Early Silurian. On one hand,volcanic ash provided abundant nutrients for the growth of marine microorganisms and improved the marine palaeoproductivity. On the other hand,volcanism promoted the formation of anoxic environment in the ocean,which was conducive to the burial and preservation of organic matter. The results can provide basic geological data for further exploration of shale gas in the Wufeng-Longmaxi Formations in western Hubei Province.
2024 Vol. 26 (5): 1058-1071 [Abstract] ( 276 ) [HTML 1KB] [PDF 3677KB] ( 61 )
1072 CHEN Hao, SHEN Junjun, TAN Guangchao, WANG Yuman, LI Shanshan, YAN Jiakai, ZHOU Lin, LIU Jiyong
Passive margin shale sedimentary palaeoenvironment and their controls over organic matter enrichment: a case study of the Lower Silurian Longmaxi Formation in northern Chongqing,Upper Yangtze
At present,there are relatively few studies on sedimentary palaeoenvironment evolution of passive margin. In particular,due to limited understanding of the changing patterns of upwelling current activity,the main controlling factors for organic matter enrichment remain unclear. On these grounds,we investigate passive margin shale sedimentary palaeoenvironments and their controls on organic matter enrichment,based on an integrated core/outcrop,mineral petrology,organic geochemistry and elemental analysis of the Lower Silurian Longmaxi Formation in northern Chongqing. The results show that organic enrichment in the study area is controlled by preservation conditions,terrigenous input and palaeoproductivity,but the main controlling factors vary in different periods. During the Rhuddanian(LM1-LM5 depositional period),the accumulation of organic matter was controlled jointly by preservation conditions,terrigenous input and palaeoproductivity,and the TOC content was the highest. In the early Aeronian(LM6 depositional period),the basin entered the flexural-migration period of enhanced tectonic activity,the barrier in the Yubei area opened,resulting in increased upwelling current activity,and the deteriorated preservation condition was the main cause of the decrease of TOC content. During the mid Aeronian(LM7 sedimentary period),the migration effect was enhanced,with the barrier further opened and the upwelling current reaching an activity peak,and the deteriorated preservation condition and sharply increased palaeoproductivity were the main reasons for little change in TOC content compared to the LM6 deposition period. During the late Aeronian(LM8 depositional period),the barrier opening migrated to the northwest,the TOC content decreased,and the main controlling factors of organic matter enrichment were the same as those of LM1-LM5. During the early Telychian(LM9 deposition period),the sea level dropped to the lowest,the oxygen-rich environment predominated,and the TOC content decreased to the lowest. The preservation conditions were the main factors controlling the enrichment of organic matter. Overall,the changes of controlling factors of organic matter enrichment in different periods in the study area are closely related to the varying sedimentary palaeoenvironments caused by tectonic activities and sea level changes.
2024 Vol. 26 (5): 1072-1089 [Abstract] ( 282 ) [HTML 1KB] [PDF 26122KB] ( 63 )
1090 WANG Yang, ZHANG Hanyu, ZHU Yanming, QIN Yong, CHEN Shangbin, WANG Zhixuan, CAO Wan
Sedimentary environment and organic matter enrichment of marine-continental transitional shale in the Shanxi-Taiyuan Formations in western Linqing Depression, Bohai Bay Basin, China

The exploration and development of marine-continental transitional shale gas is in the primary stage. The Shanxi-Taiyuan Formations shale has good hydrocarbon generation potential and is also an important horizon for the development of marine-continental transitional shale gas in China. In order to explore the relationship between sedimentary environment and organic matter enrichment of the Shanxi-Taiyuan Formations shale in the western Linqing Depression,Bohai Bay Basin,Well XJ1 was investigated in detail by means of petrology,organic geochemistry,element geochemistry and numerical analysis. The results show that the Shanxi-Taiyuan Formations shale is characterized by a low content of brittle minerals,a high content of clay minerals and a high resistance to hydraulic fracturing. It is also found that the Shanxi-Taiyuan Formations shale comprises a high-quality source rock with an average TOC content of 4.03%,which has the potential for shale gas exploration and development. During the deposition of the Taiyuan Formation mudstone/shale,the study area featured an anoxic,moderately stagnant oceanic environment under a warm and humid climate,with frequent variaions in terrigenous debris influx,low deposition rate and low to medium paleoproductivity level. During the deposition of the Shanxi Formation mudstone/shale,the study area featured anoxic,moderately to strongly stagnant and brackish to brine seawater under complex and ever-changing paleoclimatic conditions,with a stable and high terrigenous debris influx,medium deposition rate and medium to high paleoproductivity level. Given the linear correlation,grey correlation and robust regression analysis between paleoenvironmental conditions and TOC content,it is concluded that the enrichment of organic matter in the Taiyuan shale is mainly controlled by paleoclimate,paleoredox and ancient water salinity conditions,while the enrichment of organic matter in the Shanxi shale is mainly controlled by paleoclimate and terrigenous debris input conditions.
2024 Vol. 26 (5): 1090-1107 [Abstract] ( 304 ) [HTML 1KB] [PDF 3008KB] ( 96 )
1108 GAO Mengtian, YOU Li, HU Gaowei, GONG Yu, LI Yanli, CHEN Yabing, FENG Qi, HAN Junwei, LIU Guolin
Formative mechanisms of high-quality source rocks in continental fault basin of northern South China Sea: a case study from the Eocene Wenchang Formation in Shunde sag,Pearl River Mouth Basin
Three sets of favorable source rocks developed in the Eocene Wenchang Formation of the Shunde sag,Pearl River Mouth Basin. However,their stratigraphic attribution,organic matter characteristics and development patterns are unclear. Based on high-resolution 3D seismic,drilling,and geochemical data,this study documents sequence stratigraphic divisions of the Wenchang Formation,and sorted out the characteristics of hydrocarbon source rocks. The current study not only identifies main controlling factors for organic matter enrichment,but also establishes development patterns of high-quality hydrocarbon source rocks of the Wenchang Formation in Shunde sag. Results and observations from the current study suggest that the Eocene Wenchang Formation can be divided into three third-order sequences,among which the sequence SQ2 corresponds to the Member 2 of the Wenchang Formation. The lithofacies of source rocks developed during the development of SQ2 are mainly composed of brown-black and gray-black laminated shale with high organic matter abundance,and are considered as high-quality source rocks. The SQ1 and SQ3 sequences correspond,respectively,to the Members 3 and 1 of the Wenchang Formation. The lithofacies of source rocks developed during the development of SQ1 and SQ3 are mainly composed of dark-gray massive mudstone with silty mudstone with relatively low organic matter abundance. Also,the parent-source types of them were dominated by terrestrial higher plants and aquatic plankton,and the contribution of algae in the SQ2 sequence was more developed. The development of high-quality source rocks in the Wenchang Formation was constrained by tectonism-palaeoclimate coupling,which suggests that palaeoproductivity and palaeoredox conditions are the main controlling factors of organic matter enrichment. In addition,during the development of SQ2 sequence,the strong fault depression intensified volcanic activity which drived higher palaeoproductivity by large amount of volcanic dust subsiding stably. Also,the strong tectonic subsidence and the warm-humid palaeoclimate promoted the rise of lake levels,and the stratification of the water body under temperature control led to the oxygen depletion of the lake bottom water,giving rise to the thick organic-rich shales in the semi-deep lake environment.
2024 Vol. 26 (5): 1108-1126 [Abstract] ( 272 ) [HTML 1KB] [PDF 5221KB] ( 55 )
1127 ZHENG Mingyu, MA Yiquan, WEN Huaguo
Depositional environments and mechanism of differential organic matter enrichment for shale of the Lower Jurassic Quse Formation in Qiangtang Basin,Tibet
The Lower Jurassic Quse Formation in the Qiangtang Basin is an important source rock series in the basin. However,a systematic study on the mechanism of organic matter enrichment in this source rock is still lacking,which limits the understanding of the material basis of hydrocarbon accumulation in this area. In this paper,the 100-meter thick organic matter-rich shale of the Lower Jurassic Quse Formation in the Biluoqiao area of the South Qiangtang Depression is used as a study subject. we systematically investigated the the palaeoenvironmental and palaeoclimatic characteristics of shale at the time of deposition,analyzed the main controlling factors of the organic matter enrichment in the shale,and established models of differential organic matter enrichment in the shale of Quse Formation based on the integrated core,TOC,major and trace elements data. The results show that the shale can be divided into two intervals by the organic matter content which increased upward at both intervals. The lower interval(59~110 m)had a generally low organic matter content(the TOC contents range from 0.3% to 8.1%,with an average of 2.7%),and contained three thin organic matter-rich intervals. However,the organic matter content in the upper interval(0~59 m)was relatively high(the TOC contents range from 0.8% to 16%,with an average of 4.9%). The deposition of Quse Formation generally occurred in a humid environment,and was influenced by hydrothermal processes. The organic matter enrichment in the lower interval was mainly related to moderate terrestrial inputs,redox conditions and palaeoproductivity,and the organic matter enrichment in the upper interval was mainly controlled by redox conditions and palaeoproductivity. The difference in the organic matter enrichment mechanism between the upper and lower intervals of the Quse Formation may be closely related to the driving forces of palaeoclimate and sea level changes.
2024 Vol. 26 (5): 1127-1139 [Abstract] ( 275 ) [HTML 1KB] [PDF 2547KB] ( 55 )
1140 HAI Qing, LIANG Chao, YANG Bo, HAN Yu, HAN Wanlu
Paleosalinity control on development of lacustrine shale: an example of shale from the lower submember of Member 3 of Paleogene Shahejie Formation,Dongying sag
The development of lacustrine high-quality hydrocarbon source rocks is often associated with lake salinization,but the impact of salinization on shale development remains unclear. In this paper,taking the Paleocene shale of the Dongying sag as an example,we analyze the changes in shale composition,laminae structure,and organic matter abundance across different salinity evolution stages. This is done by means of thin-section observation,XRD,pyrolysis test of rocks,2D elemental scanning of XRF,and trace elemental test,combined with organic geochemistry data and variations in elemental content. The results show that the shale within the depth range of 3120-3160 m in the lower submember of Member 3 of the Shahejie Formation of Well F42 can be divided into five lithofacies. The environmental evolution of the target section can be divided into five stages,which are controlled by factors such as climate and terrestrial input. During the deposition of the lower submember of Member 3 of the Shahejie Formation,the paleoclimate transitioned from warm and humid to relatively wet and cold,then to warm and humid,relatively dry and cold,and finally to relatively warm and humid. Terrestrial input showed multi-period cyclic changes. Characteristics of sedimentary environments varied among different lithofacies. High-salinity lakes had high nutrient concentration,high primary productivity,strong water stratification,and weak water circulation. The position of oxycline/halocline was relatively high,and the oxygen-rich zone was shallow,facilitating the rapid deposition of organic matter into reducing environments,which favored organic matter preservation. The carbonate mineral content was high and developed as laminae in stagnant water layers.
2024 Vol. 26 (5): 1140-1151 [Abstract] ( 266 ) [HTML 1KB] [PDF 9175KB] ( 54 )
 
PALAEOGEOGRAPHY AND MINERAL RESOURCES
1152 DU Yuansheng, YU Wenchao, WENG Shenfu, LEI Zhiyuan, LI Peigang, QIN Ying, DENG Keyong, LU Shufan, LUO Xiangjian, FU Hongbin, ZHANG Jiawei, WU Bo, DENG Xusheng, CHEN Qun, GUO Shangyu, ZHANG Qilian, QIN Feng, ZOU Yingzhong, PANG Dawei, ZHOU Jintao, CHENG Long
Lithologic(ore)types, formation sequence and metallogenic mechanism of bauxite and associated critical metal deposits
Continental weathering represents a pivotal mechanism in the Earth’s surface system,encompassing a diverse range of energy forms,including solar,wind,chemical,and gravitational potential energy,alongside a series of intricate physical and chemical processes. This multifaceted process facilitates migration and redistribution of geochemical elements across different Earth subsystems(e.g.,lithosphere,hydrosphere,biosphere,and atmosphere). During this transformation,anomalous elemental enrichment can lead to the formation of industrial-scale weathering-leaching deposits,which can be categorized into two types: weathering deposits and sedimentary-leaching deposits. However,traditional approaches in mineral deposit studies and sedimentology face limitations in understanding the stages and dynamics of mineralization,leading to unresolved issues in both fundamental research and exploration. To address these exigent issues,this study delineates the fundamentals of continental weathering,as well as the concepts of weathering crust(including paleoweathering crust)and soil(extending to paleosol),providing a comprehensive analysis of modern and ancient weathering-leaching profiles. We conducts a meticulous sedimentological analysis and comparison of profiles examing modern weathering and leaching profile found in the gibbsite deposit in Guigang,Guangxi,in contrast with ancient deposits l from the Early Carboniferous-Late Permian bauxite deposits and sedimentary leaching Rare Earth Element(REE)deposits in Southwest China. Our findings underscore that weathering-leaching deposits primarily consist of structural components such as clastic,aggregate and coating grains,massive clay. These correlate to the genesis of clastic,pelletizing,and massive ores within weathering-leaching deposits. Diagenetic processes are largely driven by the cementation and infilling activities of oxide minerals and clays. Drawing upon these conclusions,the study proposes a classification framework for weathering-leaching deposits,where the vadose and phreatic zones within the weathering profile serve as foundational criteria for division. This framework further discriminates between the weathering ore sequence and sedimentary-leaching ore sequence,primarily based on the presence or absence of sedimentary units. By integrating paleoclimatic analysis and recent advances in mass balance calculations of element migration during the leaching process,this study elucidates the mechanisms of weathering-leaching deposit formation,summarizing the process into three phases: (1)the formation of ore-forming parent material,(2)subsequent weathering of these metallogenic substances,and(3)later stages transformations. This in-depth exploration promotes a nuanced understanding of the intricate processes underlining continental weathering,paving the way for future research avenues in this pivotal facet of Earth’s surface system.
2024 Vol. 26 (5): 1152-1166 [Abstract] ( 451 ) [HTML 1KB] [PDF 6718KB] ( 58 )
1167 CHENG Long, YU Wenchao, DU Yuansheng, ZHOU Jintao, XIONG Guolin, WENG Shenfu, GUO Shangyu
Genesis of colloidal pellets of Danping bauxite deposit in northern Guizhou Province and estimation of their mineralization duration
In most bauxite deposits around the world,concentric colloidal pellet structures are commonly observed. The growth process of these structures records valuable information about the mineralization of bauxite,offering the potential to reconstruct the bauxite-forming environment. Unlike the more extensively studied oolitic structures in carbonate deposits,the research on similar multilayered pellet structures in bauxite deposits is still limited. The basic definition,formation mechanism,and time constraints of these structures remain unclear,and thus require further investigation. In view of this issue and the character of pelleted structure in the deposition-cementation layers of modern tropical laterite profiles,this research was carried out on the Lower Permian bauxite deposits in the Danping mining area of Wuchuan-Zheng’an-Daozhen region in northern Guizhou,China. The concentric layered structure of bauxite deposits is proposed here to be named as “colloidal pelleted structure”. On the basis of the geological characteristics of ore-bearing lithostratigraphic profiles in the Danping bauxite deposit,this study focuses on investigating the colloidal pellet structures. The colloidal pellets in the Danping bauxite deposit consist of alternating layers of different types of core and surrounding alternating Al-hematite and boehmite minerals. The sphericity of the colloidal pellets was statistically analyzed using the dichotomy,which showed that they are predominantly circular or nearly circular. The normalized pellet layers range from 11.2 to 319.5 μm in thickness. According to the statistics of the number of pellet layers,the majority are around 1-3 layers. Based on the above statistical data,Fick’s first law and its extended form were used to estimate the growth time of the colloidal pellets,and construction of a diffusion growth model was attempted. The results show that the formation time of the Al-hematite layers within the pellets ranges from 0.1 to 96.9 years,while the formation time of the boehmite layers ranges from 0.2 to 143.6 years. The formation time of the microscopic-scale colloidal pellets(0.1~0.4 mm)and hand-sized colloidal pallets(2.0~5.0 mm)is estimated to be around 0.6 to 481.2 years and 1178 to 7364 years,respectively. Furthermore,the estimated formation time of colloidal pellets of various sizes(0.1~5.0 mm)in bauxite deposits is 0.6 to 7364 years. These findings indicate that there is a complex relationship between colloidal pellet structures and leaching processes during bauxite formation. The formation of interlayered concentric structures indicates the periodic agglomeration of colloidal pellets within the aluminum-bearing strata in the Danping area,providing a microscopic depiction of the periodic ore-forming rhythm. The geochemical behavior of aluminum during the formation of colloidal pellets may be the dominant factor,and it is influenced by key control conditions such as the concentration of organic acids in weathering profiles,Redox Conditions and the surface charge properties of layered minerals affecting the adsorption and enrichment processes of hydroxyaluminum and exchangeable aluminum.
2024 Vol. 26 (5): 1167-1184 [Abstract] ( 262 ) [HTML 1KB] [PDF 8423KB] ( 46 )
1185 WANG Dongdong, HU Hongchang, MAO Qiang, Lü Dawei
Genesis and palaeoenvironmental significance of siderite nodules in the Taiyuan Formation,Zibo area of Shandong Province,China
Siderite nodules are widely developed in the Late Paleozoic marine-continental transitional facies strata in North China. The siderite nodules formed in the syndepositional stage contain rich sedimentary paleoenvironmental information. This study focuses on the siderite nodule layer within the Taiyuan Formation in the Zibo area of eastern North China. By conducting petrological,sedimentological,and geochemical analyses,we explored the formation conditions,diagenetic stage,and paleoenvironmental context of the siderite nodules. The main conclusions are as follows: 1)The siderite nodules in Zibo area were formed in the syndepositional stage. The chemical composition and characteristics of siderite nodules can effectively reflect the palaeoenvironmental characteristics of the sedimentary period;2)Siderite nodules were formed in brackish water,weak reduction-reduction,weak alkaline-alkaline environment of marine-continental transitional facies;3)The siderite nodules were formed in the Early Permian Asselian-Sakmarian transition period(295.6±1.1~296.7±1.1 Ma),correlating with the terminal phase of the Early Permian’s discontinuous glacial P1;4)During their formation,the region experienced significant terrestrial weathering,with a generally warm and humid climate. The bottom seawater temperature of the epicontinental sea was between 9.3 ℃ and 19.6 ℃,with an average of 13 ℃.
2024 Vol. 26 (5): 1185-1200 [Abstract] ( 272 ) [HTML 1KB] [PDF 9976KB] ( 38 )
 
LITHOFACIES PALAEOGEOGRAPHY AND SEDIMENTOLOGY
1201 KUANG Mingzhi, ZHANG Xiaobing, YUAN Haifeng, CHEN Cong, ZHANG Xihua, PENG Hanlin, XU Ting, XIAO Qinren, LI Tianjun, SHAN Shujiao
Sequence stratigraphy and sedimentary facies characteristics of the Maokou Formation carbonate rocks in central Sichuan Basin
The carbonate rocks of the Permian Maokou Formation in the Sichuan Basin are crucial for oil and gas exploration. Significant breakthroughs have been achieved in the exploration of the Maokou Formation in the Bajiaochang-Moxi-Longnüsi area of central Sichuan Basin. However,the unclear stratigraphic structure impedes the study of sedimentary evolution and reservoir prediction of the Maokou Formation. This research is based on high-precision lithofacies characteristics of the Maokou Formation at the Er’ya section in the Huaying Mountain of Guang’an in central Sichuan Basin. It integrates actual drilling data to analyze the sedimentary facies and sequence stratigraphic framework of the Maokou Formation,explaining the influence of the sequence framework on the development characteristics and distribution patterns of the reservoirs. The research findings indicate that: (1)The carbonate rocks of the Maokou Formation in the central Sichuan Basin can be divided into eight lithofacies. During the late stage of deposition of the Maokou Formation,tectonic activity facilitated the transition from a gentle slope sedimentary system to a “platform-platform interior sea trough” sedimentary system. (2)The upper sequence interface SB1 of the Qixia Formation and the upper sequence interface SB2 of the lower submember of Member 2 of the Maokou Formation are both classified as type II lithofacies transition interfaces. The top interface SB3 of the upper submember of Member 2 of the Maokou Formation is identified as a type I drowning unconformity interface,while the top interface SB4 of Member 3 of the Maokou Formation is classified as a type I erosion unconformity interface. (3)The Maokou Formation in the Er’ya section can be divided into three third-order sequences and five fourth-order sequences. Member 1 of the Maokou Formation constitutes the transgressive system tract of third-order sequence SQ1. The middle to lower parts of the lower submember of Member 2 correspond to the high-position system tract of third-order sequence SQ1,while the upper part develops the transgressive system tract of third-order sequence SQ2 at the margin of the continental shelf. The upper submember of Member 2 and Member 3 of the Maokou Formation correspond to third-order sequences SQ2 and SQ3,respectively. (4)In the Moxi-Longnüsi area,the dolomitized reservoirs of the lower submember of Member 2 of the Maokou Formation are distributed in the high-position system tract of third-order sequence SQ1 and the continental shelf margin system tract of third-order sequence SQ2. These reservoirs are primarily dominated by late diagenetic dolomitization and early diagenetic dolomitization,controlled by type II sequence boundary SB2. In the Guang’an-Bajiaochang area,the dolomite reservoirs of the upper submember of Member 2 of the Maokou Formation in the platform margin shoal facies are found in the high-position system tract of third-order sequence SQ2 and are controlled by type I sequence boundary SB3. These reservoirs undergo stages of early diagenetic matrix dolomitization,late diagenetic grain dissolution,and burial dolomitization. This study provides constraints on sequence and sedimentation for predicting the distribution patterns and investigating the genesis of dolomite reservoirs in the Maokou Formation in the Sichuan Basin,which is significant for oil and gas exploration in the Maokou Formation.
2024 Vol. 26 (5): 1201-1220 [Abstract] ( 453 ) [HTML 1KB] [PDF 17847KB] ( 47 )
1221 ZENG Jianjun, TAN Xiucheng, ZHAO Dongfang, HE Ruyi, LUO Wenjun, LIU Yun, XU Wei, LI Shaorui, DENG Yu, WU Hao
Characteristics of eogenetic karst and its influence on reservoir of the Member 2 of Dengying Formation from Wellblock GS1, central Sichuan Basin
The genesis of reservoirs in Member 2 of the Sinian Dengying Formation in the Sichuan Basin has long been a subject of debate,including weathered crust karst,pene-sedimentary karst and buried hydrothermal karst,which significantly impedes the establishment of reservoir prediction model and the exploration and development benefit. This paper uses Member 2 of the Dengying Formation from Wellblock Gaoshi 1 as a case study. By analyzing drilling core samples,thin sections,well logging data,etc.,the karst characteristics of Member 2 of the Dengying Formation is identified and categorized. Furthermore,the variations in karst features and their impact on reservoir development and maintenance have been discussed. The results indicate that: (1)Member 2 of the Dengying Formation exhibits features such as high-frequency corrosion surface,fabric-selective corrosion cavity,geopetal structure,semi-dissociation zone,graniphyric textures,spongy corrosion structures,and karst breccias;(2)The dissolution characteristics are closely associated with the high-frequency shallower-upward sedimentary sequence,predominantly developing in the middle and upper parts of this sequence;(3)Based on the karst strength,three types of karst sequences can be identified: fabric selective corrosion,karst zonation,and integral brecciation;(4)Additionally,a negative bias in δ13C and δ18O as well as a positive bias in87Sr/86Sr,are observed at the high-frequency cycle interface. Further analysis indicates that the thin reservoir of Member 2 of the Dengying Formation is also developed in the middle and upper parts of the high-frequency shallower-upward sedimentary sequence. A significant number of residual karst caverns are filled with micritic envelopes and fibrous isopachous cements. Additionally,some of these caverns exhibit geopetal structures,indicating that they re-entered the submarine diagenetic environment after the formation of karst caverns. Considering the characteristics of negative carbon and oxygen isotope biases and positive strontium isotope biases at high-frequency cycle interfaces,it is proposed that the development of this kind of porous thin reservoir is primarily influenced by eogenetic karst processes driven by the high-frequency sea level changes. The vertical superposition exhibits a distribution pattern characterized by “multiple thin layers”,with distinct facies-controlling features. The karstification with different intensities change the connectivity of pores. Relatively isolated small dissolved pores are primarily formed during the fabric selective solution stage,where they remain more isolated and preserved for extended periods due to seabed cements. In contrast,during the karst zonation and integral brecciation stage,these dissolved pores become increasingly interconnected. In the later stages,they are more prone to mechanical filling and chemical bonding,which enhances reservoir heterogeneity and can significantly deteriorate reservoir performance. Furthermore,it has been revealed that the connectivity of the karst system controls the maintenance pathway of dissolution pores during the early diagenetic stage,under different karst strength transformation. This finding offers a new perspective for understanding the formation and preservation of ancient deep carbonate reservoirs.
2024 Vol. 26 (5): 1221-1234 [Abstract] ( 269 ) [HTML 1KB] [PDF 18460KB] ( 52 )
1235 LI Ying, LI Fei, LU Ziye, LI Yangfan, WANG Zengjun, LI Yalan, XIE Hui, ZHANG Tianshu
Characteristics and formation mechanism of patchy dolostone of the Cambrian Xiannüdong Formation in eastern Hannan-Micangshan area
Patchy dolostone,a common type of dolostone,is distinguished by its heterogeneity and complex internal structure,requiring a comprehensive investigation of its origins and formation mechanisms. In this study,the Yangjiagou section in the eastern Hannan-Micangshan area is selected as a representative example. Through comprehensive field observations and various methods including petrography,sedimentology and geochemistry,13 types of lithofacies and five dolomite types were identified in the Xiannüdong Formation. Geochemical methods were employed to analyze the carbon and oxygen isotopes as well as trace element compositions of these dolostone.Our findings indicate that the occurrence of patchy dolostone is strongly associated with the location of vertical tectonic fractures and transverse extension zones. The fractures within the patchy dolostone and surrounding limestones are filled by coarse-grained saddle dolomite. Notably,the δ18O values of the patchy dolomites (δ18O: -8.17‰ to-6.58‰) are lower than those of the contemporaneous seawater-derived dolomites. Additionally,in-situ trace element measurements reveal that dolomite crystals exhibit high Mn/Sr ratios(mean of 14.8),a positive Eu anomalies(mean of 1.3),and a middle rare earth elements-enriched pattern when normalized to shale. After excluding other possibilities,it is inferred that the formation of patchy dolostone in the study area can be attributed to the influence of regional fault-controlled hydrothermal fluids. The significance of this study lies in its ability to identify the distribution and development of Cambrian dolomites within the Xiannüdong Formation in the surrounding area,as well as to clarify the identification markers and formation processes of hydrothermal dolomites. This study may provide valuable insights for oil and gas exploration in the northern Sichuan Basin.
2024 Vol. 26 (5): 1235-1255 [Abstract] ( 363 ) [HTML 1KB] [PDF 15843KB] ( 57 )
1256 FAN Chengmeng, YANG Hailin, YANG Yang, WEN Hanjie, FAN Haifeng, ZHANG Hongjie, LI Fanghui, XIAO Jiaolong, ZHOU Yu
Calcimicrolith types and paleoenvironment evolution of the Sinian Dengying Formation in northern margin of Huangling Uplift,Three Gorges of Hubei Province
The Sinian Dengying Formation at the northern margin of the Huangling Uplift not only records the co-evolution of the environment and life during the late Sinian,but also contains a large number of calcimicroliths,which serve as important indicators for studying the paleoenvironment of the Neoproterozoic-Paleozoic transition. In this paper,based on the detailed field investigation and indoor lithofacies analysis and interpretation of the Sinian Dengying Formation along the northern margin of Huangling Uplift,nine types of sedimentary microfacies are identified,including dolomitic microbialite,dolomitic microlithite and dolomitic microallolite,with dolomitic microbialite being the most dominant type. Based on the analysis of calcimicrolith lithology types and their combinatory relationships,it is proposed that a variety of subtidal-intertidal-supertidal stratigraphic sequences have developed,characterized by upward stacking and meter-scale stratigraphic cycles composed of multiple grain beaches and microbial mounds. On this basis,four transgression-regression secondary cycles are described in detail. A shallow water carbonate platform model,exhibiting typical lithofacies characteristics of tidal flat facies,mound beach bodies,and grain beaches during the late Sinian period at the northern margin of the Huangling Uplift,has been established. This carbonate sedimentary model is similar to that of modern microbial carbonate,with a notable characteristic being that the thrombolites are formed in a low-energy subtidal environment,situated below the normal wave base and deeper than dolomitic laminated stone and dolomitic stromatolites. Based on the lithofacies characteristics,lithofacies assemblage,and the evolution of sedimentary facies of calcimicrolith,it is inferred that the palaeoenvironmental evolution in this area is primarily influenced by secondary fluctuations in sea level. The development of calcimicrolith in the Sinian Dengying Formation,located at the northern margin of the Huangling uplift,may respond to the increased oxygen content and the formation of marine rocks abundant in microorganisms following the Neoproterozoic oxidation event. Additionally,it may be closely associated with the emergence of the Shibantan biota and the decrease of microbial content due to the evolution of metazoans. Analyzing the types of calcimicrolith development in the Sinian Dengying Formation at the northern margin of Huangling uplift not only constrains global sea-level changes and the evolution of paleo-ocean environment during the late Sinian,but also holds significant implications for studying the early life evolution. The Dengying Formation is characterized by a high abundance of calcimicrolith,which may provide a substantial basis for future exploration of oil and gas reservoirs.
2024 Vol. 26 (5): 1256-1270 [Abstract] ( 270 ) [HTML 1KB] [PDF 11621KB] ( 54 )
 
REVIEW
1271 LI Pan, XIAO Dongsheng, SHI Xiaofei, MA Qiang, LIN Tong, YANG Runze
A review of megabeds
Megabed is a widely used term in the English literature,but has not been reported in official Chinese publications. Megabeds refer to giant event beds formed by sediment gravity flows. They can reach tens of meters to hundreds of meters in thickness and hundreds of cubic kilometers in volume,that is,approximately 1-2orders of magnitude larger than their surrounding normal deepwater intervals. Megabeds tend to be distinct from enclosing strata in terms of their thickness,grain size,composition,and commonly show multi-layer vertical structures that may record gravity flow transformation,differentiation and other deepwater transport and depositional processes. They are of great significance to geological events and basin analysis,provenance/paleogeography restoration,geohazard mitigation,and oil and gas exploration. In this paper,the conceptual connotation of megabed and related terms is firstly reviewed,and then the sedimentary characteristics of megabeds are analyzed and summarized,including their scale,composition,vertical structure and sequence. On this basis,the transport and depositional processes,as well as the initiation mechanisms of megabeds are discussed. Finally,existing problems and future research directions are discussed. The aim is to arouse extensive attention and in-depth research on megabeds across China.
2024 Vol. 26 (5): 1271-1286 [Abstract] ( 445 ) [HTML 1KB] [PDF 6636KB] ( 61 )
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