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Table of Content

    28 April 2021, Volume 42 Issue 2
    Previous Issue    Next Issue
    Petroleum Geology
    Impacts of the closure of eastern Mongolia-Okhotsk Ocean on formation and evolution of Songliao Basin
    Zhiqiang Feng, Li Dong, Ying Tong, Zihui Feng, Shun Zhang, Gaokui Wu, Qiang Ren
    2021, 42(2):  251-264.  doi:10.11743/ogg20210201
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    Songliao Basin is a large non-marine petroliferous basin of the Cretaceous in Northeast China.It is generally believed that its tectonic evolution is mainly controlled by the westward subduction of the Western Pacific Plate.What is the dynamic mechanism in forming a large depression basin only in Northeast China within the Western Pacific tectonic belt? Why are there two stages of volcanism (Huoshiling Formation and Yingcheng Formation) developed in the rifting stage of Songliao Basin? Why are the Mesozoic magmatic rocks prone to get developed in Northeast China? Obviously, it's difficult to explain these questions merely by Pacific Plate activities.The closure of Mongolia-Okhotsk Ocean is the most important tectonic event in Northeast Asia in the Mesozoic.There is no doubt that the plate subduction, collision, subsequent compression, extension and magmatic activity would have significant effects on the formation and evolution of the basins and mountains in Northeast Asia.Based on existing research results, this paper analyzes the influence of the closure of the eastern Mongolia-Okhotsk Ocean on the formation and evolution of Songliao Basin from the perspectives of regional dynamics, temporal and spatial distribution characteristics of volcanic rocks and tectonic evolution of sedimentary basins.Besides, a genetic model of bidirectional subduction between the Mongolia-Okhotsk Ocean and the West Pacific Plate is proposed for the formation of Songliao Basin, under the effect of which multiple transitions between compression and extension mechanisms in Northeast China are delineated.In all, the formation and evolution of Songliao Basin are re-recognized from brand-new perspectives, while reasonably interpreting the differential magmatic activities between Northeast China and southern West Pacific during the Late Jurassic and Early Cretaceous.

    Integrated 3D hydrocarbon exploration in sedimentary basins of China
    Dengfa He, Desheng Li, Xiaoguang Tong, Xiaozhi Wu
    2021, 42(2):  265-284.  doi:10.11743/ogg20210202
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    Oil and gas exploration in China has entered the peak of discovery with breakthroughs made in mature regions, frontier areas, new plays, new types of rock assemblages and new domains for both conventional and un-conventional hydrocarbon reservoirs in recent 15 years.Discussing the nature and guiding notion of the exploration during this period is of significant referential value to the on-going hydrocarbon exploration in China.Viewing from sedimentary basin system theories and petroleum exploration methodology, the paper puts forward exploration scenarios and methodology for the coming era based on a combination of oil and gas geologic condition analyses of sedimentary basins with current exploration activities in China.It shows that the sedimentary basins in China are chiefly of multi-cycle superimposed type under the joint control of the adjoining plate tectonics, the intra-continental structural deformation, and the deep mantle geologic processes, which together have resulted in large-scale oil and gas accumulation in multiple reservoirs or plays.The sedimentary basins in China are rich in variety of resources, including conventional oil and gas, low-maturity oil, bio-methane gas, tight gas, shale oil and gas, coal, coal-bed gas, salt and gypsum, hydrate, and metals etc.Petroleum exploration in China, after stages for structural oil and gas pools and lithologic-stratigraphic oil and gas pools, is now entering a stage for both conventional and unconventional oil and gas—also called the "integrated 3D reservoir exploration" stage.The stage is characterized by a full-chain and integrated exploration that scans the basin as a whole and takes multiple plays, multi-scale reservoirs, hybrid resources as its targets.It drills into subsurface over 10, 000 meters deep and explored the deep waters.It is based theoretically upon petroleum geology advancements in multi-cycle superimposed basins, deeper understanding to the dynamics of oil and gas accumulation, and progress in unconventional petroleum geology.It employs technologies including the multi-scale (micro-to-nano meter scale) analytical methods for reservoirs, the high-resolution seismic exploration featuring wide-frequency, wide-azimuth, and high-density, as well as the massive fracturing in horizontal wells.It is performed by integrating geology and engineering, exploration and production.It thus guarantees an integrated exploration of both conventional and unconventional oil and gas as well as other resources like coal, salt and metals in the basins.

    Framework evolution of North Tarim Paleo-uplift from the Late Ordovician to Middle Devonian
    Jiajun Chen, Dengfa He, Fangyuan Sun, Fanglei Tian
    2021, 42(2):  285-298.  doi:10.11743/ogg20210203
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    The discovery of the Ordovician giant oil and gas fields in the North Tarim Uplift has triggered heated discussion on the Early Paleozoic tectonic framework evolution.An analysis on the early Paleozoic unconformity in terms of geography, geometry, and temporal variables will facilitate the understanding of North Tarim Paleo-uplift development.Based on quantities of drilling and seismic data, this study analyzed the Silurian unconformities at the base and on top, determined the distribution and geometry of the paleo-uplift at the end of the Late Ordovician and the end of the Middle Devonian, and discussed the evolution of tectonic stress field.Both the Silurian unconformities at the base and on top are characterized by "truncation and onlap", that is, the stratum below the unconformity was truncated while younger onlaps on the surface.The paleo-uplift struck nearly E-W at the end of the Late Ordovician, while NE-SW in the south slope region at the end of the Middle Devonian developed as three individual paleo-lower uplifts (i.e., Yingmaili Lower Uplift, Lunnan Lower Uplift, and nose-shaped Korla Salient).The tectonic framework evolution (the near NS-trending structures superimposed by NE-SW folding) indicates that the compressive stress field changed from NS striking at the end of the Late Ordovician to NW-SE striking at the end of the Middle Devonian.In all, the results serve to provide essential evidence and detailed reasoning for the Late Caledonian tectonic framework of the North Tarim Uplift.

    Structural characteristics and exploration prospects of the southwestern margin of Bachu Uplift, Tarim Basin
    Fei Ning, Jinbiao Yun, Jianjiao Li, Haiming Song, Liangdong Zhao
    2021, 42(2):  299-308.  doi:10.11743/ogg20210204
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    Structural faults of various styles are developed in the Bachu Uplift of western central uplift zone in the Tarim Basin.To pinpoint potential exploration targets in the southwestern margin of this petroliferous uplift, the geometry and distribution of the faults were defined through a fine interpretation of available seismic data and the connection between the faults and hydrocarbon pooling patterns was analyzed based on known reservoir characteristics.The results reveal two typical faulting styles in the margin: transpression, mostly recognized in NW-trending faults, and thrust nappe, largely found in near EW-trending faults.Both of the faults show a multi-phase composite superposition feature, which is made more complicated with the occurrence of some decollement layers.Traps formed by the EW-trending faulting is comparatively better than the NW-trending faulting-related anticline traps in terms of integrity.The structural traps in thrust nappe footwall under the gypsolyte are relatively favorable for hydrocarbon preservation, and together with the overlying gypsolyte and the Cambrian source rocks, they form a complete petroleum system.It is then concluded that the relatively integrated traps in the Cambrian pre-salt and inter-salt structures as well as the Caledonian paleo-tectonics in the margin are potential targets for further exploration with their quality source rocks and excellent preservation conditions.

    Exploration directions on the Kelasu and East-Qiulitag fault hanging walls, Kuqa Depression, Tarim Basin
    Shiwei Yi, Mingpeng Li, Tuzhi Fan, Fan Yang, Hui Fang, Fuxi Huang, Wudi Jin
    2021, 42(2):  309-324.  doi:10.11743/ogg20210205
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    The foreland thrust belts come in favorable accumulation conditions in the Kuqa Depression, Tarim Basin.Of the belts, the Kelasu and East Qiulitag belts, in particular, contain a giant gas field of over trillion-cubic-meter reserve consisting of Keshen, Kela 2 and Zhongqiu 1 gas reservoirs.Horizontally, most of the reservoirs concentrate on the footwalls of Kelasu and East-Qiulitag Faults, only Kela 2 is on the hanging wall.Vertically, the reservoirs mainly occur in the Cretaceous Bashijiqike Formation.Analyses show that the somehow continuous gas reservoir on the footwall of Kelasu Fault were trapped by rows or belts of large-scale anticlines and faulted anticlines formed by a series of nearly EW-trending thrust faults.The faults that get connected to hydrocarbon source rocks downward and to gypsum-salt cap rocks upward without cutting them through, serve as effective migration channels while keeping the integrity of the trap untouched, thus making a perfect pooling combination.While on the hanging walls, gas traps with similar condition were mostly destroyed by the Kelasu and East-Quilitag Faults cutting through the gypsum-salt cap rocks, leaving only the Kela 2 reservoir preserved by salt on the top-up structure.Meanwhile, analysis of the Kelasu and East-Qiulitag Faults' control on the reservoirs shows various impact upon different reservoir-cap assemblages and strata in a vertical direction.In all, the study dissected the accumulation model on the hanging walls of the Kelasu and East-Qiulitag Faults through composite hydrocarbon accumulation theory while considering the abundant surplus resources and the exploration imbalance of the hanging walls and the foot walls, and suggested that the hanging walls of the Kelasu and East-Qiulitag Faults have great exploration potential and the Jurassic and Paleogene strata are targets of future exploration on the foot walls of the faults.

    Geometrical and kinematical characteristics of the Mazartag fault zone in Bachu Uplift, Tarim Basin
    Yong Zhang, Menglin Zheng, Jiajun Chen, Mingzhe Deng, Fanglei Tian, Weikang Zhang, Yingqiang Li
    2021, 42(2):  325-337.  doi:10.11743/ogg20210206
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    The Mazartag fault zone at the southeastern margin of the Bachu Uplift has drawn wide attention because of its partial exposure and dents of intensive Quaternary activities.The study on the geometric and kinematical characteristics of the zone may provide a basis for exploring the evolution characteristics of the stress field in the Bachu Uplift and even the Tarim Basin, and also for an indirect observation of intensity variations of stress fields (of the basin) at the zone through various stages.The geometry of the Mazartag fault zone was then characterized and combined with seismic data and outcrop observation to establish 3D sections via 3D modeling, and to restore the evolution process with balanced section restoration.The results show that the Mazartag fault zone is characterized by layered deformation and the structural style along the fault zone is relatively consistent.The outcrops are shaped by the Quaternary activities.Vertical overlapping fault planes of each structural layer are easily identified in the 3D sections.All these suggest that the Mazartag fault zone was initially a weak normal fault zone formed in the basement that then went through the Early Ordovician inversion, Silurian reactivation, and Late Permian decollement along the bottom of the Middle Cambrian in succession.At the end of the Paleogene, the zone was activated further and started to form the Cenozoic decollement fault zone along the Paleogene in the early Eocene.The decollement faults have been active from the early Eocene, Miocene, Pliocene to Quaternary, and thrusted the Cenozoic strata to the surface to take the present relief.

    Distribution and origin of tectonic fractures in ultra-deep tight sandstone reservoirs: A case study of Keshen gas field, Kuqa foreland thrust belt, Tarim Basin
    Ke Wang, Ronghu Zhang, Junpeng Wang, Xiongwei Sun, Xuejun Yang
    2021, 42(2):  338-353.  doi:10.11743/ogg20210207
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    Observations of cores and thin sections combined with logging data interpretation were used to analyze the basic properties and forming sequence of tectonic fractures in the ultra-deep tight sandstone reservoirs of the Bashijiqike Formation in Keshen gas field.The differences in the fracture properties, including occurrence, mechanical characteristics, density, aperture, filling and fills, were then studied and the causes for the differences were also investigated to select suitable reservoir stimulation measures and determine exploration targets for the field.The results show that the fractures were developed in 3 stages (early, middle and late) in Keshen gas field and constitute reservoirs of distinctive characteristics due to the control of tectonic deformation stage and aqueous diagenesis environment.The Keshen-5 and -6 gas reservoirs in the north of the field are dominated by vertical fractures or high-angle tensile fractures which are nearly parallel to the long axis of the anticline under the effect of slow tectonic deformation at the early stage; the Keshen-2 gas reservoir in the center was under the influence of the middle deformation stage and contains both near NS-trending shearing fractures and near EW-trending tensile fractures with the former being relatively superior in number; the Keshen-8 and -9 gas reservoirs in the south of the field is dominated by vertical shearing fractures of nearly NS trending under deformation of the late stage.In general, from the north to the south, both the major differential stress to rock compressive strength ratios and the fracture density show a rise-fall tendency; meanwhile, the dominant strike of fractures gradually runs parallel to the present major principal stress from a high-angle intersection along with shrinking facture forming time, which in turn reduces filling rate and results in greater effective aperture.In addition, the Bashijiqike Formation (K1bs) was deposited in a saline water as a whole: the Keshen-5, -6, and -2 blocks in the north of the field were formed in a fresh-brackish setting with fractures mainly filled by calcites, making acidification a suitable reservoir stimulation choice; and the Keshen-8, and -9 blocks in the south of the field were formed in a alkaline-prone water setting with fractures primarily filled with anhydrite and dolomite, making fracturing a more applicable stimulation option.The Keshen-8 block ranks first in terms of tectonic fracture effectiveness, followed by the Keshen-2 and -9 blocks, and then by the Keshen-5 and -6 gas pools.The Keshen-18, -19, -20, and -24 blocks are suggested to have well-developed tectonic fractures and shall be targeted in future exploration.

    Characteristics and geological implications of seismic reflections of deep and ultra-deep layers in central Tarim Basin
    Fanglei Tian, Miao Peng, Jun Han, Jiajun Chen, Debo Ma, Danfeng Mao
    2021, 42(2):  354-369.  doi:10.11743/ogg20210208
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    The deep and ultra-deep layers of central Tarim Basin are currently important targets for hydrocarbon exploration in the area.The understanding of seismic traits of shallower strata and their geological implications established decades ago can no longer meet the needs of the present time.To deal with the problem, the characteristics and stratigraphic significances of seismic reflections of deep and ultra-deep layers were accurately calibrated and analyzed through synthetic seismogram, seismic-well tie calibration, and well-tie cross-section interpretation in line with the latest drilling data of ultra-high and high-precision 3D seismic data, from which mark layers were identified.The focus was on the exploration issues concerning the Middle Cambrian gypsum-bearing caprocks, the Middle-Lower Cambrian source rock-reservoir-caprock combinations, and the "bead-like" reflections in the Shuntuoguole area.The results show that: 1) The Paleozoic deep and ultra-deep layers dominating the basin downward from the middle can be summarized as six layers with strong reflection, seven layers with weak reflection, three layers with normal reflection, and one layer with confusing reflection, which are in consistency with the vertical stratification features of stratigraphic lithology and velocity of the layers.2) The existence of the Middle Cambrian gypsum-bearing rocks results in higher vertical heterogeneity of wave velocity.More gypsum means higher amplitude of seismic reflections.The Tazhong area has most of the rock while areas around Well Luntan1 contain the least or no such rock at all.As for the Shuntuoguole and Shunnan areas, the rock mainly exists in the Shayilike Formation featuring high-amplitude or cloud-like reflections with parts thickening, folding or remarkably thinning along dislocated faults.3) The Middle-Lower Cambrian source rock-reservoir-caprock combinations are distributed in the south slope of Tabei, the Shuntuoguole Lower Uplift and the Shunnan area.However, the source rock and reservoir quality are comparatively poorer in some parts of Tazhong Uplift.4) Three types of "bead-like" reflections occur in the Shuntuoguole area.

    Syncline development and tight sandstone gas accumulation model in Shigouyi area at western margin of Ordos Basin
    Dengfa He, Fangyuan Sun, Yonghe Zhai, Hongping Bao, Jinghui Ma, Baize Kai
    2021, 42(2):  370-390.  doi:10.11743/ogg20210209
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    Syncline as a basic type of folds is one of the main structures hosting coal-bed gas, tight gas, shale gas, salt and gypsum, and uranium, etc.The study on the formation mechanism of syncline means significantly to analyzing the tectonic evolution and resource distribution in fold zones.The deep structure and forming mechanism of the syncline in Shigouyi area at the western margin of Ordos Basin were analyzed based on recent high-resolution seismic profiles and deep borehole data.It is shown that the syncline was formed during the end of the Middle Jurassic and Late Jurassic and located at the transitional area between the northern and central segments of the western foreland margin of the Ordos Basin.Based upon the large-scale imbricate thrust system detached along the coal measures of the Upper Carboniferous Yanghugou Formation and the Lower Permian Shaanxi Formation, the syncline is the upper folding system of a large-scale tectonic wedge underthrusted by underlying earlier domino-style normal fault blocks.A positive reverse fault named Hui'anpu in the area can be used to define the tectonic and depositional domains.The Upper Carboniferous and Lower Permian tight sandstones in the syncline and adjacent areas were the trinity of source rocks, reservoirs and cap-rocks for gas during the Early Cretaceous.The syncline, coupled with the fact that it was formed prior to gas pooling peak period, is therefore considered rich in gas and a good exploration target.

    Wushen-Jingbian Paleo-uplift and its control on the Ordovician subsalt deposition and reservoirs in Ordos Basin
    Liubin Wei, Hongde Chen, Wei Guo, Ting Yan, Zhenghong Cai, Lixia Zhou
    2021, 42(2):  391-400, 521.  doi:10.11743/ogg20210210
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    A detailed study on stratigraphic correlation based on drilling data and of seismic sections, the Cambrian system thickness measurements, observation of superjacent pinch-out boundaries of intervals in the system and the pre-Ordovician paleo-geologic mapping of the Ordos Basin shows that, apart from the Yimeng and Lyuliang Uplifts as well as the Central Paleo-uplift, another uplift, the Wushen-Jingbian Paleo-uplift was also developed in the basin during the Early Paleozoic.Though completely absent from the core of the paleo-uplift, the Cambrian intervals are successively (from old to young) overlapping the uplift.Further paleo-structural analysis indicates that inherited from its Cambrian predecessor, Wushen-Jingbian was still a low-amplitude underwater uplift during the Ordovician sedimentary period and controlled the subsalt deposition and reservoir development in the central and eastern Ordos Basin.As a result, the uplift is of distinctively differentiated sedimentary facies in the east-west direction, and has played an indispensable role in joining the Central Paleo-uplift and Lyuliang Uplift to form the Mizhi Salt Depression in the east of the basin.Furthermore, with well-developed mound-shoal complexes that are typical of shallow-water deposition and prone to generate dissolved pore structures due to intermittent exposure during sea regression, the uplift is considered to be highly promising in terms of the Ordovician subsalt gas exploration.

    Analogue modeling insights to foreland basin growth: A case study on the Longmenshan thrust belt in western Sichuan Basin
    Bin Deng, Yu He, Jiaqiang Huang, Qiang Luo, Rongjun Yang, Hao Yu, Jing Zhang, Shugen Liu
    2021, 42(2):  401-415.  doi:10.11743/ogg20210211
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    Analogue modeling of the formation and evolution of Longmenshan belt and foreland basin system in western Sichuan Basin reveals a strong impact of tectonics, erosion, and sedimentation as well as of the interactions between the later two processes, especially those changing along the the strike of the system.The modeling shows a negative relationship between wedge geometry (i.e., length and height) and surface erosion magnitude.Heavier erosion makes it easier for faults at the back of the wedge to thrust in a random sequence and be reactivated, thus stopping the wedge from further forelandward propagating.While surface sedimentation, that adds to the layers overlying the thrusts and increases the possibility of fault locking, serves to faciliate the forelandward propagation of the wedge.Changing (e.g., increasing) erosion-sedimentation process along the strike of the wedge results in a direction shift of the strike of deep strata and steeper faults at the back of the wedge, some evidently shrinking forelandward faults with smaller dips and increasing backthrusting and the formation of oblique ramps in the foreland basin.We therefore conclude that it is this changing or differential erosion-sedimentation process in the Longmenshan belt and foreland basin system since the Late Triassic that controls the development of a rejuvenated foreland basin with a possible oblique ramp (Longquanshan fault) from the Late Cretaceous to Ceonozoic.

    Characteristics and genetic mechanism of tectonic deformation in basin-mountain transitional zone: A case study of Sangmuchang Anticline in southeastern Sichuan Basin
    Jinhe Yang
    2021, 42(2):  416-429.  doi:10.11743/ogg20210212
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    The Sangmuchang Anticline, sitting at a crucial location where the front of "trough-like" fold belt in western Hubei and Hunan Provinces superimposing the "barrier-style" fold belt in southeastern Sichuan Basin, was chosen to be studied to gain insight into the superimposition relationship between the belts as well as the tectonic deformation characteristics and genetic mechanisms of this mountain-basin transitional zone from Xuefeng Mountains to the Sichuan Basin.Available seismic and logging data together with DEM digital elevation and other related geological information were combined with fault-related fold theories to perform a fine interpretation of multiple 2D seismic sections crossing the anticline for architectural characterization and to establish a kinematics model with the balanced section restoration technique for tectonic evolution analysis.The study has concluded that the deformation of the anticline involves layered decollement with a mutual structural superposition in vertical direction, and that with some consistently deformed cap-rocks and bedding shear deformation dominating along the decollement surface, the overall structure of the anticline is under the joint influence of basement occurrence and Qiyueshan Fault.The fault is a concealed system formed by the combination of several basement thrusting faults striking near south-east, and the deep basement wedge and imbricated structures are thereby in superimposition, jointly controlling the anticline deformation.Further analysis shows that the anticline deformation initiated since the middle of Yanshan period, and its tectonic evolution has experienced the superposition of multiple tectonic movements.Under the westward thrusting of Xuefeng Mountains, the east-to-west progressive tectonic deformation of the west of Hunan and Hubei "trough-like" fold belt controls the activity of the deep tectonic wedges and the imbricated structures, which in turn control the main tectonic form of the anticline.The influence of the "partition-style" fold belt in the interior of the southeastern Sichuan Basin on the tectonic deformation of the dorsal basin-mountain transitional zone is mainly reflected in the anterior flank of the dorsal slope, resulting in the broad gentle fore wing and the steep narrow back wing of the anticline.

    A new perspective on the features of the Caledonian unconformity in Dagang exploration area, Bohai Bay Basin
    Pinlong Han, Dunqing Xiao, Jufeng Wang
    2021, 42(2):  430-442.  doi:10.11743/ogg20210213
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    To restore the post-Caledonian paleogeomorphy of the Dagang exploration area, an innovative "scaled stratigraphy" method, in which a regional substratum division coupled with the substratum correlation based on lithological composition and electric logs is conducted, is used to investigate the substratum outcrops of the Ordovician Fengfeng Formation in the area.The result shows that the Caledonian unconformity is of low-angle substrata of ten to dozens of meters thick and its paleo-geomorphology shows an overall northward and eastward rising, and that the outcropped substrata have a tendency of aging upward.This low-angle feature altered the conventional understanding of the Caledonian unconformity in the area and highlighted the influence of Caledonian movement on the Ordovician strata.This study also reveals that, within the Fengfeng Formation, the Substratum V controls the vertical distribution of carbonate reservoirs, which are considered quality reservoirs as they are sandwiched between karst with lumpy dolomite at the top and layered dolomite at the bottom, and continuously occur in central and southern Huanghua Depression.The Ordovician Fengfeng Formation bounded by the Caledonian unconformity as a whole, together with overlying coal seam sedimentation and underlying Ordovician reservoirs, forms a good source rock-reservoir combination for later generated oil or gas.

    Geological characteristics and shale oil exploration of Es3(1) in Qikou Sag, Bohai Bay Basin
    Lihong Zhou, Guomeng Han, Fei Yang, Jianying Ma, Liangang Mou, Kejia Zhou, Changli Wang, Lina Meng
    2021, 42(2):  443-455.  doi:10.11743/ogg20210214
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    The high and stable industrial shale oil flow from the first submember of the third member of Shahejie Formation (Es3(1)) in Qikou Sag, Bohai Bay Basin, reveals a possible 100 million tons of oil resource in the lacustrine shale of the member and serves as a significant reference for shale oil exploration in the whole Shahejie Formation of the basin.Based on a comprehensive study of sedimentary setting, lithology, reservoir properties, brittleness and oil-bearing property of the shale series in the member, we found that the shale series are of semi-deep lake-deep lake environment and characterized by well-developed pores and fractures, high brittleness, low reservoir sensitivity and high oil-gas bearing potential.They are favorable for hydrocarbon generation and accumulation and can be easily stimulated.Longitudinally, the member can be divided into 6 sweet-spot sections, namely sections C1 to C6 downwards of 7 to 96 m thick and 434 m in total.Horizontally, the sections are distributed in areas from 87.4 to 194.3 km2 with a cumulative area of 256 km2.Preliminary assessment using volumetric oil content method indicates the sections have a resource of 4.1×108 t.Guided by the development strategy of "using old wells to control the range of sweet spots and horizontal wells to increase production", two highly deviated wells, Wells Bin 60-56 and F38x1 were tested with industrial oil flows.Deploy horizontal well exploration for C1 sweet spot, Well QY10-1-1 was tested with 100 equivalent tons of hydrocarbon per day.Furthermore, Bin56-1H produced 3 043.11 t in total after 206 days of production, and the reverse drainage rate was 7.77%, indicating a major breakthrough in the shale oil exploration of Es3(1) in Qikou Sag, Bohai Bay Basin.

    Geologic architecture and tectonic evolution of Nanyang Sag, Nanxiang Basin
    Zhi Li, Zhiye Zhang, Yunfei Yang, Wenjian Guo, Yan Li, Teng Li, Jian Xiong
    2021, 42(2):  456-468.  doi:10.11743/ogg20210215
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    Nanyang Depression, a secondary structural unit to the Nanxiang Basin, is a petroliferous fault depression developed in the Qinling-Dabie fold orogenic belt.The study on its geological structure features is of great significance to the hydrocarbon exploration in the depression and the understanding of basional structural-sedimentary responses by the uplifting and denudation of Qinling-Dabie fold orogenic belt.Based on 2D and 3D seismic survey results as well as drilling and log data of the area, this study identified regional unconformities, divided structural-stratigraphic sequences, and characterized the main faults as well as the planar and sectional features of the depression.The evolution process of the depression was also rebuilt with balanced section restoration.The results show that the Nanyang Sag has four regional unconformities at the bottom of Hugang (Tg), Yuhuangding (T7), Hetaoyuan (T5) and Shangsi Formations (T1), and five recognizable structural-stratigraphic sequences, namely the basement, Hugang Formation, Yuhuangding-Dacangfang Formations, Hetaoyuan-Liaozhuang Formations and Shangsi-Pingyuan Formations.Controlled by several basement faults, i.e.the Xinye Fault, Yutong Fault and Shigang Fault, the Sag is characterized by a tectonic pattern of NS zonation, EW partition and alternate uplift and sag.Five tectonic evolution stages are suggested to have successively shaped the Sag from the initial rift depression to continuous fault depression, strong fault depression, fault-depression transition and final depression.It is therefore concluded that the Nanyang Sag is structurally controlled by its complex basement structure, boundary fault variations and sectional subsidence differentiations.

    Fault development and its analogue modeling in the Paleogene Funing Formation in Gaoyou Sag, Subei Basin
    Xu Tang, Yonghe Sun, Huiting Hu, Wenquan Yu, Haiqing Tang
    2021, 42(2):  469-480.  doi:10.11743/ogg20210216
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    The study of fault development characteristics of the Funing Formation in Gaoyou Sag is of great significance to the understanding of the sag evolution and oil-gas enrichment.There are a large number of domino-type faults dipping NW in the east of the sag, and Ⅴ-shaped faults dipping NW and SE in the west of the sag.The difference in fault development characteristics indicates different fault formation processes, and the fault formation mechanisms in different zones are yet to clarified.Through three-dimensional seismic interpretation, unconformity and residual stratum identification, combined with displacement-depth curve and structural evolution analyses, it is proven that the boundary faults in the Funing Formation of Gaoyou Sag are non-syndepositional, thus having no obvious control on the formation of secondary faults in the sag.All faults of different assemblages in the east and west of the sag are formed under uniform stress field striking NW-SE, with main activities occurring at the end of the Funing Formation deposition.Two sets of physical simulation experiments were therefore designed according to the analytic conclusion of faults and the principle of similarity.The experimental results show that the Ⅴ-shaped fault assemblage in the west is formed under bilateral extension, while the domino fault assemblage in the east is controlled by unilateral extension and has no link to the eastern slope.The results of the two sets of experiments can reasonably explain the causes of the differential fault assemblage patterns.Geographically, before the deposition of Dainan Formation, Gaoyou Sag was in the same sag with Linze Sag in the north of the basin that is characterized by a narrow west and wide east.During the extensional deformation period, the stress propagation velocity on both sides of the basin to the center was different, leading to the differential overlapping zones with faults striking to various directions in different areas.

    Controlling factors and distribution pattern of effective tight gas pools in blocky pyroclastic rocks in the Cretaceous Shahezi Formation in Wangfu gas field, southern Songliao Basin
    Fancheng Zeng, Changmin Zhang, Zhongcheng Li, Guoyi Zhang, Chi Zhang, Yongjun Wang, Wentie Sun, Qingjie Deng
    2021, 42(2):  481-493.  doi:10.11743/ogg20210217
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    Tight gas pyroclastic rock reservoirs are special unconventional gas reservoirs with complex lithology that call for in-depth research and more exploration.Those developed in the Shahezi Formation of Wangfu Fault Depression, Songliao Basin, are found to be challenging with fault blocks as their major control coupled with confusing features of sedimentary pyroclastic rocks and tight gas reservoirs.Limited understanding of the controlling fault blocks has hindered the exploration of the reservoirs.In this study, various techniques were used to evaluate the unique features of the reservoirs, including lithology, storage space, pore structure, physical properties and reservoir effectiveness, with a view to revealing the impact of source rock-reservoir interaction, hydrocarbon pooling elements, interlayer and intra-layer heterogeneities, and micro-diagenesis on the development of effective reservoirs from macroscopic (i.e.gas pools), mesoscopic (i.e.sand bodies), and microscopic (i.e.reservoirs) perspectives.We concluded that the traps, migration channels, gas pool distribution, gas-water contact and sweet spot distribution of the reservoirs in the area are conditioned by structures, faults, sand bodies, intra-layer heterogeneity and diagenesis, respectively.In addition, the lateral distribution of the reservoirs can be likened to branches of a tree along gentle slope zones from east to west; while vertically, the reservoirs are blocky and anti-rhythm distributed under the control of some hummocky rocks.The conclusion was applied to the well emplacement and mature well reevaluation in the Chengshen 6 well-block and yielded positive results, thus can be used for reference for the exploration, deployment, and classification of similar tight gas reservoirs in other areas.

    Petrologic features and diagenetic evolution of fine-grained sedimentary rocks in continental lacustrine basins: A case study on the Lower Jurassic Da'anzhai Member of Yuanba area, Sichuan Basin
    Yixiu Zhu, Zhenkui Jin, Ke Jin, Qiheng Guo, Huan Wang, Pin Lyu, Xinyao Wang, Yuan Shi
    2021, 42(2):  494-508.  doi:10.11743/ogg20210218
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    Core observation, thin section identification, scanning electron microscopy (SEM) and X-ray diffraction analysis were used to finely describe the petrological characteristics and diagenesis of fine-grained sedimentary rocks as well as to discuss rock types with hydrocarbon potential and their genesis in the Lower Jurassic Da'anzhai Member of Yuanba area in the Sichuan Basin.The results show that there are three types of rocks, namely mudstone (mudstone, conchoidal shale and calcareous mudstone), siltstone and limestone (conchoidal limestone).The main mineral components are quartz, clay minerals (dominated by illite/smectitem (I/S) mixed layer, illite, kaolinite and chlorite), calcite, feldspar and pyrite; the rock assemblages are mainly conchoidal shale and mudstone interbedded with conchoidal limestone.The fine-grained rocks, currently at the B sub-stage of the mesodiagenesis, experienced compaction, cementation, dissolution, metasomasis, recrystallization of clay minerals and hydrocarbon generation from organic matters, of which, compaction and cementation were the major factors controlling the physical properties of shale and limestone.It also shows that, controlled by sedimentation and diagenesis, the organic-rich shale and the conchoidal shale are two most promising rock types for hydrocarbon reservoirs.Also as a result of diagenesis processes, the conventional reservoirs of conchoidal limestone and limestone are too tight to hold any oil or gas, but the fine-grained sedimentary rock and conchoidal shale of less tight are wonderful unconventional shale gas reservoirs.This assumption was verified in field tests during which large volume of gas flew out of conchoidal shale, but none from the adjacent conchoidal limestone.The result also suggests that rocks with well-developed laminae and micro-laminae (observed under microscope) or with siliceous and calcareous compositions of directional arrangement are potentially high-quality reservoir rocks.

    Controlling effects of syn-depositional reverse fault associated folds on the deposition in alluvial fan sheet glutenites and braided channels: A case study of the Triassic Karamay Formation in Huwan area, northwestern margin of Junggar Basin
    Qinyu Xia, Shenghe Wu, Wenjie Feng, Zhongbao Liu
    2021, 42(2):  509-521.  doi:10.11743/ogg20210219
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    This study applied the flume simulation to the deposition of alluvial fan controlled by syn-depositional reverse fault associated folds and conducted a sectional observation on the distribution of sheet glutenites and braided channels in the fan after the simulation.The result was then used as a guidance in an identification of sheet glutenites in the lower Triassic Karamay Formation and braided channels in upper Karamay Formation of the Huwan area in northwestern margin of Junggar Basin that comprehensively took into consideration the core, outcrop, infill well network and seismic data as well as information gained through studies on the intensity of syn-depositional reverse fault associated folds in the area.Individual subsurface single configured unit was also dissected and investigated according to quantitative outcrop scale and well log marks to study the process, scale, direction and superimposing styles of the deposition of sheet glutenites and braided channels under the control of the folds.The results show that sheet glutenites are preferentially deposited in locations near provenance and with underdeveloped folds.They are mostly large in size—at least 1 000 m wide and 1 400 m on average.They usually migrate from main stream pathway to both sides and are less laterally superimposed.In locations at the edge of folds or where folds are less active, the sheet glutenties and braided channels would flow around the folds and migrate from an obliquely crossing provenance way to a direction parallel to the provenance way.With the smallest ones of only 450 m in width and an average of 1000 m, the glutenites in these locations are small in size and laterally superimposed.The braided channels in these locations are interlaced or banded and about 150 m to 350 m wide and 1.5 m to 8.1 m thick.In locations where the folds are active, sheet glutenite is not observed and braided channels are narrow isolated bands of about 80 m to 120 m wide and 1 m to 4 m thick.

    New understanding and tapping effect of remaining oil reservoirs in Xijiang oilfield, PRBM, South China Sea
    Yi Tu, Zhenghe Yan, Jianwen Dai, Yahui Wang, YiDong Zhu, Yong Yang, Jiao Yang, Weifeng Wang
    2021, 42(2):  522-532.  doi:10.11743/ogg20210220
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    To tackle problems in understanding the distribution and mechanisms of remaining oil reservoirs in mature offshore oilfields of middle or later stages of development, this study focuses on the migration direction and migration-accumulation time of remaining oil in various types of reservoirs in Xijiang oilfield, PRBM, South China Sea, while combining "potential control theory" with "reservoir architecture interface" and taking single sand body in the reservoirs as research unit.It concludes that the remaining oil mainly concentrates in low-potential trapped areas, and thereby constructs a "migration-accumulation regenerated reservoir model".The results show that: (1) Sand bodies in major pay reservoirs are mainly superimposed with rare interbeds; the reservoirs are mostly high-porosity and high-permeability and conducive to generate preferential seepage channels.After shutting wells down for about one year, the remaining oil migrates and accumulates in small amounts to high parts of structures of low potential energy and beyond well area.These wells tend to yield high but only for a short time after the 1 year shut-in.(2) Sand bodies in non-major pay reservoirs are usually laterally overlapping or completely isolated with the well developed third- and fourth-order architectural interfaces in the reservoirs.The dynamic migration and accumulation time and direction of remaining oil are related to reservoir rhythm, interlayer order and fluid property.After shutting wells down for more than 3 years, the remaining oil in these reservoirs that once were controlled by the fourth-order interfaces gradually flows around and accumulates into low-potential closed areas; those once controlled by the third-order interfaces would accumulate in large scale in low-potential closed areas by vertical permeation.These wells perform good after 3-year shut-in with high production for a long time.The research results are of technically supportive value to the rejuvenation of mature offshore oil fields in medium-high water cut stage and to the realization of the company's annual production target.