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《Bulletin of Mineralogy, Petrology and Geochemistry》
JOPC 2024 Vol.26 Number 4
2024, Vol.26 Num.4
Online: 2024-08-01
SEOUENCE STRATIGRAPHY AND PALAEOGEOGRAPHY
LITHOFACIES PALAEOGEOGRAPHY AND SEDIMENTOLOGY
SPECIAL ISSUE ABOUT DEEP.WATER SEDIMENTATION
SEOUENCE STRATIGRAPHY AND PALAEOGEOGRAPHY
763
JIA Jinhua
Sequence characteristics and sedimentary palaeogeography evolution of the Triassic in Awati sag,Tarim Basin
In order to clarify the sedimentary-palaeogeography characteristics and sequence framework of the Triassic in Awati sag,Tarim Basin,based on the research ideas of terrigenous sequence stratigraphy and lake-basin sedimentology,and using basic data consisting of drilling core and seismic data,this paper puts forward new understanding on sequence division,sediment provenance,and sedimentary-palaeogeography distribution and evolution. In this study,the Ohobluk Formation and Karamay Formation of the Triassic in Awati sag are divided into seven third-order sequences,among which the Lower Triassic Ohobluk Formation is divided into five(SQ1-SQ5)and the Middle Triassic Karamay Formation is divided into two sequences(SQ6-SQ7). Each sequence is composed of the lowstand system tract(LST),transgressive system tract(TST),and highstand system tract(HST). It is suggested that the Triassic in Awati sag is characterized by fan deltas,braided river deltas,and shore-shallow lacustrine facies controlled by multiple sources. There are four major sources around the sag and the sediment supply is mainly from the southwestern Bachu Uplift and the western Keping Fault Uplift,then from the northern Tabei Uplift and the southeastern Tazhong Uplift. The braided river delta plain subfacies are mainly distributed at the southern and southwestern edge of the sag,mainly composed of coarse grained sandstone and sandy gravel sandbodies of braided distributary channel microfacies. The center of the lake basin is located in the eastern part of the sag. Within the sag,there are braided delta front subfacies and lacustrine facies,mainly composed of underwater braided distributary channels and beach bar sandbodies. During the Triassic,the lake basin experienced stages of early initiation,intermediate growth and peak,and finally filling and extinction. The early stage of the Lower Triassic(SQ1-SQ2 period),corresponding to the development of the lower part of the Ohobluk Formation,is the initiation stage of the Awati lake basin in the Triassic. Sediments mainly came from the Bachu Uplift in the south and the Keping Fault Uplift in the west. The sedimentary facies were distributed in an asymmetric belt from south to north,with the development of braided delta plain and front subfacies and shore-shallow lacustrine subfacies. Moreover,during that stages,the lake basin is shallow in the south and deep in the north,shallow in the west and deep in the east. The middle to late stage of the Lower Triassic(SQ3-SQ5 period),corresponding to the development of the upper member of the Ohobluk Formation,is the growth and peak period of lake basin. With the continuous progradation of sources from the southwest and west towards the basin,fan delta facies were formed. At the same time,the lake basin expanded to the north and northeast,forming a braided delta plain and front subfacies that continuously migrated from south to north,while the northeast is a shore-shallow to semi-deep lacustrine subfacies. The early-middle period of the Middle Triassic(SQ6-SQ7 period),corresponding to the development of the Karamay Formation,is the filling and extinction period of the lake basin. The migration of the subsidence center to the northeast region resulted in the continuous progradation of the sources,causing the lake basin to rapidly shrink and eventually died out.
2024 Vol. 26 (4): 763-778 [
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779
LIU Mingcheng, WU Shenghe, WAN Xiaolong, LI Zhen, FAN Jianmin, CHEN Zhaohui, WANG Zifeng, LIU Haixu
Sequence stratigraphy and sedimentary facies characteristics of the shale strata of Yanchang Formation in Qingcheng Oilfield,Ordos Basin
The shale succession of Yanchang Formation in Ordos Basin is rich in hydrocarbon resources,but its sequence framework and sedimentary facies distribution are controversial,which affects the exploration and development of shale hydrocarbon. Taking the Xi233 area of Qingcheng Oilfield as an example,this paper reconstructs the sequence stratigraphic framework and sedimentary facies distribution using core,logging,and the latest 3D seismic data. Research has shown that in the original Chang 8
1
-Chang 3
2
interval of the Yanchang Formation,the strata represent a progradational superposition pattern towards the center of the lake basin,which can be divided into two long-term base-level cycles and ten mid-term base level cycles. In each mid-term cycle,the paleogeomorphology towards the basin is followed by shallow-water area,slope area and basin floor area. The shallow-water area mainly develops deltas. The slope belt develops gravity flow channels,which are distributed in narrow bands along the slope; the gravity flow lobe deposits are developed in the proximal basin floor,and the sand bodies are sandwiched in mud shale. The distal basin floor is mainly deep-lake shale interbedded with slump deposits. The original “Chang 7 interval” shale strata is a set of diachronous strata,which is composed of sublacustrine fan sand bodies and semi-deep to deep lake shale in the lower part of the slope belt and basin floor belt of multiple mid-cycles. This study not only plays an important guiding role in the effective development of interbedded shale oil in the Ordos Basin(optimization of development areas and deployment of horizontal wells),but also promotes the development of the theory of sequence and sedimentary evolution in depression lake basins.
2024 Vol. 26 (4): 779-795 [
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796
CHEN Hongkui, ZHU Xiaomin, XIAO Kunye, DU Yebo, YUAN Zhiyun, ZHANG Xinshun, WANG Xiaoxue
Sequence stratigraphic framework and sedimentary system of the Lower Cretaceous Kedeni Formation in Eastern Depression of Doseo Basin,Chad
A significant breakthrough has been made in hydrocarbon exploration in the Kedeni Formation of the Doseo Basin in the Central African Rift System. However,the stratigraphy and sedimentary infill of this dominant formation are still unclear,which has become a key constraint for the next step of hydrocarbon exploration. In this study,the Kedeni Formation in the Eastern Depression of Doseo Basin is taken as the research object. Based on the theory of seismic stratigraphy and sedimentology,combined with the core,logging and seismic data,the sequence stratigraphic framework and sedimentary system distribution of Kedeni Formation are clarified. The research results demonstrate that the Kedeni Formation exhibits three identifiable sequence interfaces,dividing into two third-order sequences(SQⅠ and SQⅡ),in response to two ‘transgressive-regressive’ cycles. The sediments of Kedeni Formation mainly came from four directions,and six types of sedimentary facies were developed. Distribution of sedimentary facies was characterized by east-west differentiation and features of vertical inheritance. Normal delta were distributed in the southeastern gentle slope zone and the central low uplift zone,whereas the braided river delta were primarily distributed in the long axis direction of the northeast and the southern steep slope zone. The fan delta and the nearshore subaqueous fan developed in the downthrown side of the boundary fault in the northern steep slope zone. The sublacustrine fan were mainly distributed in the deep lake area of the western sag. Vertically,the framework sandstone scale of the SQⅠ was larger than that of the SQⅡ,and in each third-order sequence,the delta-subaqueous fan was more developed in the lower unit composed of a lowstand system tract and a transgressive system tract. The sequence and sedimentary evolution of the Kedeni Formation were primarily controlled by factors such as the structural activity of major faults,differential sediment supply from the south to the north,and shifts in climate from semi-arid to humid conditions. Under the guidance of sequence-sedimentary model,it is considered that the small-scale fan delta front-nearshore subaqueous fan sandstone in the northern steep-slope zone,the large-scale normal delta front and braided river delta front sandstone in the south and northeast,and the sublacustrine fan sandstone in the deep lake area of the western sag can be regarded as favorable exploration targets within the region.
2024 Vol. 26 (4): 796-815 [
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LITHOFACIES PALAEOGEOGRAPHY AND SEDIMENTOLOGY
816
LI Jin, LIU Zhen, ZHANG Zhongmin, LIU Jianping, SUN Xiaofan, LIU Huimin, YANG Guang, ZHU Maolin, WANG Wenbin, XIAO Hong
Formation mechanism,development regularity and exploration implica-tion of oblique fan delta in steep slope zone of rift lacustrine basin in the early stage of rifting: a case from northern belt of Dongying sag
As one of the most important types of oil and gas basins in the world,the rift lacustrine basin can often form glutenite sedimentary formations with a thickness of thousands of meters in the early stage of filling and evolution process,which has become an important field of oil and gas exploration. However,there are still many controversies regarding the sedimentary genesis of this extremely thick glutenite. In this study,Yanjia area in the steep slope zone of the Dongying sag,a typical rift lacustrine basin in Jiyang Depression,is selected as an example. Utilizing high-resolution 3D seismic data,core samples,logging,and well logging,we conduct a systematic analysis of the glutenite sedimentary formations in the steep slope zone during the early rifting stage. This study applies seismic stratigraphy,structural geology,sedimentology,and rheology analysis methods comprehensively. The following understandings have been obtained: (1)The gravel content in the sediments of oblique fan delta is relatively high,which is formed by traction flow and gravity flow. The average quartz content is about 40%-55%. Fan delta plain,fan delta front and pre-fan delta subfacies are developed. The seismic response characteristics include(oblique,imbricate and broom-shaped)progradation facies along the direction of sediment source,and filling facies perpendicular to the sediment source direction. (2)The oblique fan delta mainly developed in the early stage of rifting,and disappeared in the middle stage of rifting. The fan and the boundary fault are obliquely intersected at a low angle,typically not exceeding than 45°. The development scale of the fan is affected by the location of its root,and its disappearance is not isochronous. (3)A new mechanism for the formation of the large-scale oblique fan delta in the steep slope zone of the rift lacustrine basin during the early stage of rifting is proposed. Relay structures related to the segmentation characteristics of boundary faults provide transport conduits for the development of oblique fan deltas. The paleo-troughs controlled by the boundary fault activity provide new accommodation for the development of oblique fan deltas during the early stage of rifting. The fracture zone formed by the interaction between faulted blocks provide material basis for the development of oblique fan deltas,and the connection of the faulted block leads to the disappearance of the oblique fan delta and the transformation to the nearshore subaqueous fan. (4)It has been found that most of the deep glutenite reservoirs in the steep slope zone of the rift lacustrine basin belong to the oblique fan deltas. This study points out that the relay zones at all levels in the steep slope zone can be key target areas for exploration,and the well layout direction follow the oblique boundary fault. This study not only enriches the sedimentary theory of steep slope zone in rift lacustrine basin,but also provided a new idea and theoretical basis for deep petroleum exploration of the continental rift lacustrine basin in China.
2024 Vol. 26 (4): 816-833 [
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834
XU Peng, LIAN Haojie, BI Lige, HU Hewei, GAO Zhenghui, JIANG Fujie, SHAO Longyi
Depositional systems and genetic types of sand bodies of the Paleogene in Chengbei sag,Bohai Bay Basin
Chengbei sag is a half graben basin in the Bohai Bay Basin.Its exploration potential is limited by the understanding of the genetic type of sand bodies and the distribution of reservoirs.This paper has analyzed the sedimentary characteristics,sand body genetic types and reservoir distribution patterns of the Paleogene in the Chengbei sag,using the drilling cores,log curves,and 3D seismic reflection data. The Paleogene in this sag has been developed with fan delta,braided river delta,sub-lacustrine fan sedimentary facies.The fan delta is developed in the steep slope zone on the northeast side of the sag,with lithology dominated by massive gravelly coarse-grained sandstones.The seismic reflections show lenticular and wedge progradation and retrogradation morphologies and chaotic and the logging facies mainly show low-amplitude jag-shaped,jagged box-shaped,and bell-shaped.The braided river delta is developed in the gentle slope zone on the southwest side of the sag,with lithology mainly consisting of cross-bedded sandstones.The seismic reflection shows S-shaped progradation and imbricated progradation patterns. The logging facies mainly show box-shaped,bell-shaped and funnel-shaped patterns. The sub-lacustrine fan is controlled by gravity flow and developed in the deep lacustrine area of the sag center.The seismic reflection is featured by worm-like progradation. The lacustrine shoal-bars are developed at the edge or central uplift of lakes,controlled by wave action,and are characterized by sheet-like sandy deposits surrounded by mudstone.The seismic reflection is featured by lens-shaped reflection,and the logging facies mainly show finger-shaped and funnel-shaped.Vertically,the Member 3 of the Shahejie Formation is mainly composed of braided river delta,fan delta,and lacustrine sedimentary facies. The Members 1 and 2 of Shahejie Formation are mainly composed of braided river delta,fan delta and lacustrine sedimentary facies. The Member 3 of Dongying Formation is mainly composed of braided river delta and fan delta sedimentary facies,while the Member 2 of Dongying Formation is mainly composed of braided river delta sedimentary facies,with the scale of fan delta development being small. The generation and development of Paleogene sand bodies in the Chengbei sag are mainly influenced by tectonic activity,sediment supply,palaeo-microgeomorphologies,and lake level changes.The predominant genetic types of these sand bodies include fan delta sand bodies,braided river delta sand bodies,sub-lacustrine fan sand bodies,and lacustrine shoal-bar sand bodies.
2024 Vol. 26 (4): 834-845 [
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846
WEN Zuchao, SHEN Yulin, LI Zhuangfu, JING Yuhong, DANG Zhiying, ZHU Yulin, MENG Yan, LI Linchao
Datailed characterization of mixed sedimentation: a case study of the Cambrian Mantou Formation in Xuzhou,Jiangsu Province
The Cambrian Mantou Formation in the Xuzhou area of Jiangsu Province is characterized by mixed siliciclastic-carbonate sedimentation. In this study,a set of typical mixed siliciclastic-carbonate sequences from the Mantou Formation were systematically analyzed for their petrological characteristics based on section and microscopic features. By analyzing the mixed sedimentation characteristics in various sedimentary environments,a mixed siliciclastic-carbonate sedimentation model was established in the epicontinental sea environment. The different types of mixing,mixing processes,and controlling factors of various mixed siliciclastic-carbonate rocks were investigated. Research has shown that there are mainly 11 types of mixed sedimentary combination types in the sequence,including sandy-bearing oolitic limestone,sandy limestone with intraclast,fine-grained limy sandstone,and limy-bearing siltstone. The overall sequence is a result of frequent fluctuations in secondary sea-level variation,accompanied by multiple periods of storm activity,within the context of epicontinental transgression. It mainly evolves from an inshore mixed carbonate-siliciclastic tidal flat to a mixed restricted platform. The sedimentation in the study area consists of facies mixing and punctuated mixing,with localized occurrences of in situ mixing and source mixing. Climate,terrestrial input,sea level changes,and carbonate yield rate serve as the primary factors influencing facies mixing,with additional influences from biological activities and hydrodynamic conditions. The multi-stage storm activity can dominate the formation of punctuated mixing,and promote the development of facies mixing and source mixing to some extent. Under the superimposed influence of various factors,the study area is characterized by the development of a combination of multiple mixing types. Clarifying the distribution of various mixing types and mixing process can provide reference for the restoration of the Cambrian paleoenvironment and sedimentary evolution of the North China Plate. It also offers typical examples for the study of mixed sedimentation in the‘deep time' epeiric platform.
2024 Vol. 26 (4): 846-862 [
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863
XU Ganxiao, QI Yong'an, HE Wenyi, DU Yaogang, LIU Xiaomei
Characteristics and formation mechanism of iron-bearing ooids of the Cambrian Mantou Formation in Weihui area,northern Henan Province
Oolitic limestone is an important type of carbonate rocks in the Cambrian period,with calcite and dolomitic ooids being the most common. While there have been numerous studies on the formation of these ooids,the occurrence of iron-bearing ooids in the Cambrian is rare and the formation mechanism remains unclear. The iron-bearing ooids of the Cambrian Mantou Formation in the Weihui area of Henan Province were systematically analyzed using sedimentology,sedimentary paleogeography,mineralogy and geochemistry. They can be divided into microsparry iron-bearing ooids and sparry iron ooids,which were formed in the transition zone between the oolitic shoal and back-shoal microbial bioherm of a limited sea in Shanxi and Henan Provinces. The geochemical tests and microscopic observations of iron-bearing ooids indicate that the iron minerals are idiomorphic crystals of hematites with irregular morphology and serrated edges,with no other iron minerals present. The contents of major elements such as Ca,Fe,Al and Si in the samples are high,showing a significant positive correlation between Fe and Al. The abundance and variation trend of trace elements resemble those found in the Earth's crust. The total amount of rare earth elements is high,with a deficiency in light rare earth elements,and an enrichment in heavy rare earth elements. There is no obvious Eu anomaly. These characteristics indicate that the iron minerals in iron-bearing ooids originate from terrestrial detrital input. The analysis of mineralogy and sedimentary palaeogeography of iron-bearing ooids reveals that the iron-bearing minerals likely formed in the study area as a result of the precipitation of Fe(OH)
3
colloidal solution from the Ordos land with runoff. Three conditions are necessary for the formation of iron-bearing ooids in the Mantou Formation of the study area: continuous supply of Fe(OH)
3
colloidal solution from terrestrial source,iron enrichment in a semi-enclosed coastal bay environment,and continuous oxidation of iron minerals under the hydrodynamic conditions of regular agitation. During oolitic formation,the intermittent input of iron elements from terrestrial weathering leads to the development of alternating laminae of hematite and illite-bearing calcite,which interbed with calcite lamina lacking hematite crystals to form microsparry iron-bearing ooids. If the iron supply is sufficient,sparry iron ooids are formed,characterized by hematite lamina interbedded with calcite lamina. These findings are crucial for understanding the paleo-marine environment and atmospheric environment during the geological period,with significant theoretical and practical implications for the development and utilization of iron deposits.
2024 Vol. 26 (4): 863-879 [
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880
WEI Liubin, WANG Zongyan, LI Yi, PANG Zhichao, WANG Qianping, BAO Hongping, YANG Qiqi, WANG Zhen, SU Zhongtang, ZHANG Lijun
Classification and genesis of leopard skin(spot)carbonate rock: a case study of the Member 4 of Majiagou Formation in Ordos Basin
Leopard skin (spot) limestone, a type of marine carbonate rock with unique morphology, is widely distributed in the Cambrian-Ordovician strata of North China. It has attracted significant attention due to its distinct characteristics and formation mechanism. Abundant leopard-skin (spot) carbonate rocks developed in Member 4 of the Middle Ordovician Majiagou Formation in the Ordos Basin. Eight species of 7 genera have been identified, including
Thalssinoides
network,
Thalassinoides
boxwork,
Planolites
isp.,
Palaeophycus tubularis
,
Phycodes
isp.,
Chondrites
isp.,
Skolithos
isp.,?
Zoophycos
isp. Two morphologic types of
Thalassinoides
(horizontal enigmatic network and three-dimensional boxwork) are predominant, belonging to the Glossifungites ichnofacies under firm ground condition. Based on detailed macroscopic and microscopic characteristics of outcrops and drilling cores, this paper systematically summarizes the types of carbonate rocks caused by bioturbation. By examining the morphological characteristics of bioturbation and burrow, as well as the calcite/dolomite content, this paper proposes a classification for leopard-skin (spot) carbonate rocks formed by bioturbation. These rocks are categorized into leopard-skin limestone and leopard-spot dolomite, corresponding to the
Thalassinoides
network (two-dimensional horizontal mystery palace) and the
Thalassinoides
boxwork (three-dimensional box-like structure), respectively. The leopard-skin (spot) carbonate rocks in Member 4 of the Majiagou Formation in the Ordos Basin are primarily controlled by the interaction of bioturbation and early diagenesis during high-frequency sea level fluctuations. The bioturbated burrows in the firm substrates creat pathways for Mg
2+
rich brines, promoting dolomitization within the burrows. Two distinct types of leopard-skin (spot) carbonate rocks, induced by bioturbation, were generated under varying bioturbation types, parasyngenesis, and buried dolomitization.
2024 Vol. 26 (4): 880-894 [
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895
WU Yuting, NING Meng, XIA Pan, QIAO Zhanfeng, YU Zhou, WEN Huaguo
Research on dolomitization process in dolomite-evaporite syngenetic system of the Majiagou Formation in Ordos Basin
The dolomite reservoir is developed in the typical dolomite-evaporite paragenetic system(DEPS)of the Ordovician Majiagou Formation in the Ordos Basin. Many researchers have conducted numerous studies on the characteristics and formation mechanism of dolomite reservoirs in the Majiagou Formation. However,limited research has been done on the dolomitization process and evolution characteristics from the perspective of DEPS. In this study,two dolomite-evaporite sedimentary sequences(S1 and S2)from the Xiweikou section on the southeast margin of the Ordos Basin were selected for high-precision continuous sampling. Through sedimentology and petrology analysis,as well as carbon-oxygen isotopes,rare earth elements(REEs)and trace elements,two types of dolomite-evaporite paragenetic assemblages were identified. A detailed study was conducted on the properties,sources,evolution of dolomitization fluid,and the dolomitization process. The results show that: (1)in the S1 sampling interval,a Type I paragenetic assemblage is present,consisting mainly of intraclast grain dolomite-dissolved gypseous breccias dolomite-dolomicrite bearing gypsum pseudocrystals-mudstone. This indicates a supratidal environment with strong evaporation. In the S2 sampling interval,a Type II paragenetic assemblage is observed,mainly composed of thin-medium thick layers of finely crystalline dolomite-medium(grain)dolomite-argillaceous dolomite bearing gypsum pseudocrystals-mudstone,representing an intertidal-subtidal environment. (2)The REE distribution patterns of dolomite in two types of paragenetic assemblages are similar to those of pore water,both showing enrichment of MREE,with slightly negative anomalies-normal of Ce and Eu. There is no significant difference between
δ
13
C and
δ
18
O,and the contents of Fe
carb
and Mn
carb
are relatively high. It is speculated that the sources of dolomitized fluid are the same,originating from weakly reducing pore water derived from seawater. (3)The sedimentary sequence in the studied geological section changes from type I to type II from bottom to top,reflecting the typical dolomitization process of paragenetic systems in evaporative tidal flat environments. As sea level rose from S1 to S2,penecontemporaneous sabkha dolomitization in the supratidal zone evolved into seepage reflux dolomitization in the intertidal-subtidal zone. The concentrations of MREE and Fe
carb
in dolomite of the type I paragenetic assemblage are higher than those in type Ⅱ,indicating the evolution of dolomitization fluid properties from rich Fe
2+
-Mn
2+
and MREE to poor Fe
2+
-Mn
2+
and MREE. The research has enriched the basic theory of dolomite genesis under paragenetic systems and provides theoretical guidance for oil and gas exploration in the Majiagou Formation in the Ordos Basin.
2024 Vol. 26 (4): 895-910 [
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911
HUANG Yougen, XU Jie, ZHENG Xiaopeng, LIU Yan, ZHONG Shoukang, LI Lin, ZHANG Daofeng, TAN Xiucheng
Sedimentary characteristics and sedimentary models of the Lower Permian Taiyuan Formation limestone in mid-eastern Ordos Basin
The Taiyuan Formation limestone is extensively distributed in the central and eastern parts of the Ordos Basin,with successful gas flow tests conducted in several wells. It has significant exploration potential and has gained increasingly attention in recent years. However,research on the sedimentary microfacies types and sedimentary modes of Taiyuan Formation limestone in the central and eastern parts of the basin remains undeveloped. Based on a detailed analysis of numerous drilling cores,thin sections,and well logging data from the Taiyuan Formation limestone in the central and eastern parts of the basin,the sedimentary characteristics and sedimentary processes were examined,including rock types,sedimentary facies,typical sedimentary sequences and sedimentary modes. The results indicate that the rocks of Taiyuan Formation in the central and eastern parts of the Ordos Basin can be classified into three categories: marine carbonates,peperite and terrigenous clastic rocks,which can be further subdivided into 15 types. The marine carbonate platform facies can be further categorized into seven sedimentary microfacies,including clastic beach,interbeach sea,supratidal zone(mud flat)and intertidal zone(mud flat). Six typical sequences in the transition environment from land to sea were identified based on detailed characterization of strata section. Sequences 1,2 and 3 consist of marine carbonate sedimentary sequences,and sequences 4,5 and 6 are composed of continental delta sedimentary sequences. The comprehensive analysis indicates that the Lower Permian Taiyuan Formation in the Ordos Basin underwent sedimentary evolution from a continental delta(clastic deposits)to a marine continental transition(mixed deposits)to a marine carbonate platform(carbonate deposits). Influenced by the sedimentary landform,the overall sedimentary filling pattern is characterized by “sandstone in the north and limestone in the south,overlaid with peat”. The research findings not only enhance the understanding of the sedimentary geology of Taiyuan Formation in the central and eastern parts of the Ordos Basin,but also establish an important theoretical doundation for the subsequent exploration of natural gas in the Taiyuan Formation.
2024 Vol. 26 (4): 911-925 [
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926
ZHANG Xiting, FAN Kunyu, GUO Pei, LI Pengzhen, MIAO Rulin, DENG Bin
Collaborative controls of the Early-Eocene greenhouse climates and transgression on deposition of salt-bearing sequence of Kumugeliemu Group in Kuqa Depression
The thick salt-bearing sequence in Paleogene Kumugeliemu Group(54-46 Ma)of the Kuqa Depression acts as the high-quality regional caps for oil and gas accumulation. However,the original temporal-spatial distribution,palaeogeographic environment,and sedimentary controlling factors of the salt-bearing sequence remain unclear,which causes great difficulties for the drilling of the subsalt hydrocarbon reservoirs. Through the outcrop measurement at the basin margin and the logging element analysis within the basin,the correlation of sedimentary cycles between the lake-basin center and the margin was carried out to explore the main controlling factors of the salt-bearing sequence deposition of Kumugeliemu Group. The Kumugeliemu Group consists of three third-order cycles. The first third-order cycle Ⅰ
1
is characterized by the lithofacies assemblage of conglomerates,sandstones,mudstones and gypsums at basin margins,and of mudstones,gypsums,halites,carbonates at basin center,indicating an palaeogeographic-environment transformation from a terrigenous basin to a continental-margin basin. The second third-order cycle Ⅰ
2
is characterized by the lithofacies assemblage of conglomerates,sandstones,mudstones,and gypsums at basin margins and the assemblage of thick halites and minor mudstones at basin center,corresponding to a depositional model of a terrigenous epeiric basin. The third third-order cycle Ⅰ
3
has similar lithofacies assemblage at the lake-basin margin and center,characterized by thin layer gypseous mudstones and mudstones,typical depositional model of terrigenous playa lakes. Combined with the Early-Eocene global climates and sea-level changes,it is believed that the deposition of thick gypsum rocks of Ⅰ
1
(ca. 54-51 Ma)were related to the Early Eocene Climatic Optimum(53-51 Ma)and global sea-level rise. The deposition of thick halite rocks in Ⅰ
2
(ca. 51-48 Ma)was controlled by the global dry climate and sea-level rise. The absence of thick salt-bearing sequence deposits in Ⅰ
3
(ca. 48-46 Ma)is related to the global dry climate and sea-level drops.
2024 Vol. 26 (4): 926-940 [
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SPECIAL ISSUE ABOUT DEEP.WATER SEDIMENTATION
941
FENG Youliang, ZOU Caineng, YANG Zhi, JIANG Wenqi, ZHANG Tianshu, ZHANG Hong, LIU Chang, WANG Xiaoni
Fine-grained gravity flow deposits and their significance for shale oil within high-accommodation sequences in faulted and down-warped lacustrine basins of China
Gravity flows,including coarse-grained and fine-grained gravity flows,are the most important sedimentation processes in deep-lake environments. Fine-grained gravity flow sediments hold significant potential as unconventional shale oil “sweet spot intervals”. The distribution characteristics of fine-grained gravity flow sediments within sequence stratigraphic frameworks in lacustrine basins are of great significance to the prediction of shale oil “sweet spot intervals”. Through systematic comparative study of gravity flow deposits and sequence frameworks in major lacustrine basins in China,it is found that gravity flow sedimentary systems are mainly deposited in high accommodation sequences. In the lowstand systems tracts(LST)of the faulted/rifted lacustrine basins,coarse-grained gravity flow deposits are developed at proximal sublauctrine fans and thin fine-grained gravity flow sediments are deposited at distal sublaucustrine fans. The distribution of sublacustine fans is controlled by valley and syn-sedimentary structural slope breaks. Fine-grained channel-levee-lobe systems are developed in transgressive systems tracts(TST). Unconfined-channel sublacustrine fans are developed in highstand systems tracts(HST),and fine gravity flow deposits are distributed in distal sublacustrine fans. Large fine-grained channel-levee-lobe systems influenced by flooding occurred in transgressive systems tracts(TST)of down-warped lacustrine basins. On the steeper slope of syn-depositional flexural breaks,fine-grain sublacustrine fan systems are present. Both fine-grained sublacustrine fan systems and fine-grained channel-levee-lobe systems can be deposited in highstand systems tracts(HST)and lowstand systems tracts(LST). Fine-grained gravity flow deposits at distal sublacustrine fans and channel-levee-lobe systems can form “sweet spot intervals” of shale oil. Mudflow deposits form “sweet spot intervals”of foliation type,fine-grained debris flow,fine-grained turbidity current,fine-grained hyperpycnal flow and fine-grained transition flow deposits form “sweet spot intervals”of lamination type and bedding type,while fine-grained concentrated density flow deposits only form “sweet spot intervals”of bedding type. Fine-grained gravity flow deposits are favorable “sweet spot intervals” for shale oil exploration and development.
2024 Vol. 26 (4): 941-961 [
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LI Hua, HE Youbin, YAO Fengnan, HE Yiming, JIANG Chunwei, ZHANG Xiankun, WU Jize, XU Yanxia, LIANG Jianshe
Contour current deposits in the Ordovician Kelimoli Formation,northwestern margin of Ordos Basin
Contour current deposits are a common type of deep-water deposits. However,limited work has been done on identification and formative mechanisms of contour current deposits in the rock record. Here,a new type of contour current deposits(lenticular contour current deposits)is documented in the Ordovician Kelimoli Formation of the study area in the Ordos Basin. The results show that There are four types of lithofacies and lithofacies associations in the study area,including lenticular micrite(lithofacies 1),mainly lenticular silt-sized silt-crystalline limestone(lithofacies 2),micrite and sparry calcarenite or calcirudite(lithofacies 3),and bedded calcirudite(lithofacies 4). It is found that lenticular micrite,silt-sized silt-crystalline limestone,and calcarenite or calcirudite are present as fine-grained through coarse-grained to fine-grained facies sequences. These deposits are characterized by relatively thin bed thickness,wavy surface,lenticular geometry,and association with muddy lamina,stripe,and spot,which are attributed to the persistent effect of contour current. Silt-sized silt-crystalline limestones are usually mottled. Bedded calcirudites with imbricate structures and the inverse Chinese character of “small”are likely to be storm deposits. In addition,carbon and oxygen isotopes indicate that relative sea level and paleotemperature show two rising to falling cycles and a continously-rising half cycle. It is also found that the contour current in the study area are low energy thermohaline circulation and that the strength of the contour current is positively correlated with eustatic sea level and salinity change. Finally,the energy of the contour current increases gradually upsection with occassional transient extremes,which might be also effected by storms in the Kelimoli stage.
2024 Vol. 26 (4): 962-971 [
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PAN Feng, JIANG Yuqiang, JIANG Liwei, WANG Yue
Discovery and geological significance of large landslide bodies using borehole image logs in the Longmaxi Formation of Da'an area, southern Sichuan Basin,China
Large landslide bodies are discovered for the first time in the Longmaxi Formation from the Da'an area,southern Sichuan basin,based on high-resolution borehole image well logs. Landslide bodies are distributed in the Long1
2
Sub-member and Long2 Member. They are both composed of black shales,and thus display similar features to background shale deposits on conventional logs. On borehole image logs,however,the landslide bodies can be identified by slide and slump structures,which have sharp contrasts with normal shale. Multi-layered landslide bodies are found in 6 drilling wells distributed in the Da'an area(~1000 km
2
). The landslide bodies are layered and separated by normal shales. The number of layers and thickness of landslide bodies in different wells are markedly different,making it challenging to correlate between wells. The landslide bodies are overall thick in the north and thin in the south. A single landslide body varies in thickness from 0.5 to 32.4 m,while composite landslide bodies in a single well can reach a thickness of 41 to 194.5 m. It is believed that landslide bodies are widely distributed,occur at multiple stratigraphic levels with large bed thickness,and have the characteristics of earthquake origin. They were the product of short distance transportation and rapid deposition,which was controlled by the southeast slope of the Leshan-Longnvsi palaeo-uplift. Different areas have different sedimentary responses to the same earthquake event. A single landslide body may be a response to one earthquake event,or it may be the result of the superposition of multiple earthquake events. The discovery of the landslide bodies provides a new perspective on stratigraphic correlation of the Longmaxi Formation in the southern Sichuan Basin. At the same time,the landslide bodies are thick with well-developed fracture networks,and feature favorable reservoir and seal conditions. These landslide bodies may thus represent potentially important prospects for the future shale gas exploration.
2024 Vol. 26 (4): 972-984 [
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WANG Xiaoni, YANG Zhi, FENG Youliang, JIANG Wenqi, LONG Guohui, WU Kunyu, ZHANG Hong, HOU Mingqiu, XING Haoting, ZHANG Tianshu, LIU Chang, WEI Qizhao
Fine-grained gravity flow deposits and their petroleum significance in saline lacustrine basins: a case study from the Upper Member of Lower Ganchaigou Formation in Yingxi area of Qaidam Basin, China
Fine-grained gravity flows can transport clastic sediments and particulate organic matter from shallow water to deep lakes,forming distinctive sediments finer than 0.0625 mm. During the depositional period of the upper section of the Lower Ganchaigou Formation(E
2
3
),a sub-lacustrine fan system and chemically precipitated carbonates developed in a saline semi-deep to deep lake in the Yingxiongling area of the Qaidam Basin. Seven types of lithofacies associations(LA)are identified in the fan fringe sub-environment on the basis of core observations: LA1: association of mud flow,mud turbulent flow,and suspension fallout deposits in quiet water;LA2: association of chemical deposits interbedded with event deposits in semi-deep lake;LA3: association of slump,fine-grained debris flow,and mud flow deposits;LA4: fine-grained hyperpycnal flow deposits;LA5: fine-grained concentrated density flow deposits;LA6: association of fine-grained debris flow,fine-grained transitional flow,and fine-grained turbidity current deposits;LA7: association of fine-grained concentrated density flow,fine-grained turbulent flow,fine-grained transitional flow,and mud flow deposits. These lithofacies associations are attributed to various fine-grained gravity flows and their transformations which are suggestive of complex flow processes. Our study also shows that the fine-grained deposits are influenced by paleoclimate,tectonic activity,and lake-level fluctuations. In an arid environment and during humid periods,lake-level rise promotes the formation of favorable source rocks and fine-grained gravity flow deposits. Conversely,in times of drought,lake-level decrease is more conducive to the development of chemically precipitated carbonates and slump-derived fine-grained gravity flow deposits. Mud flow,mud turbulent flow,and fallout deposits in quiet water(LA1)and chemical deposits interbedded with event deposits(LA2)are the most favorable “sweet sections”in the saline semi-deep to deep lake.
2024 Vol. 26 (4): 985-1004 [
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LIU Yuxing, CHEN Yuhang, FAN Guozhang, LI Lin, LU Yintao, ZUO Guoping, CAO Quanbin, DING Qunfeng
Sedimentary characteristics of lobes formed by interacting bottom currents and gravity flows: a case study from the Miocene in Rovuma Basin,East Africa
The interplay of bottom currents and gravity flows is an important deep-water sedimentary process,which plays an important role in redistributing submarine sediments and reshaping seafloor bedforms. At present,research on bottom currents and gravity flows interaction in deep-water channels is relatively mature,but little work has been done on the sedimentary characteristics of lobes influenced by bottom currents. Based on an integrated study of seismic facies and seismic attributes using 3D seismic and logging data,this study documents the distribution and lithological characteristics of the Miocene lobes in the Rovuma Basin(East Africa). The effect of bottom currents and gravity flows interaction on lobe sedimentation is revealed through quantitative restoration of gravity flow velocity,combined with regional ocean current data analysis. The results indicate that,early channels migrated southward due to the northward-directed bottom current,and their northern levees are well developed with a negative terrain present to the south of the channels,which led subsequent lobes to deposit to the south of these channels. Deposition of the lobes is influenced by bottom current flow direction and gravity flows forming the lobes. Velocity of the gravity flows is much higher than that of the bottom currents. Therefore,when the directions of bottom currents and the lobe propogation are opposite and oblique,the influence of bottom currents is suppressed on the side against bottom current flow direction(south side),and the lobe gradually thins from the axis to the margin,accompanied by increasing mud content;On the side downstream of bottom current flow direction(north side),gravity flows intersect obliquely with bottom currents,and the mud at the top of turbidity currents is stripped off by the bottom current,leaving sand deposited in the lobe. When bottom current flow direction and lobe propogration direction are close to orthogonal,the influence of bottom current on the lobe is consistent on the whole,and the mud at the top of gravity flows is stripped. Therefore,the lobe is mainly composed of thick sandy deposits at the side against the bottom current flow direction,and the sand content decreases towards the side downstream of the bottom current,accompanied by an increasing mud content.
2024 Vol. 26 (4): 1005-1016 [
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