Table of Content

01 June 2023, Volume 44 Issue 3

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Petroleum Geology

Predictive stratigraphy： From sequence stratigraphy to source-to-sink system

Changgui XU, Chenglin GONG

2023, 44(3):  521-538.  doi:10.11743/ogg20230301

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Predictive stratigraphy with the ability to predict sedimentary fills and high-quality reservoirs has been widely applied to basin analysis and hydrocarbon exploration， and has undergone the evolutionary process from sequence stratigraphy to source-to-sink system for reservoir quality prediction. In response to the challenge of predicting favorable play elements （i.e. reservoir and seal）， geologists of ExxonMobil established the sequence stratigraphic methodology and theory. To answer why the sequences do not necessarily control sandstone development and lowstand systems tract does not necessarily result in fan deposits， the geologists adopted the source-to-sink hypothesis in predicting high-quality reservoirs， creating the source-to-sink-based sandstone mapping methodology. The present study reviews the status quo of sequence stratigraphy and major advances in application to marine and lacustrine sequences， and introduces major progress in the application of source-to-sink-based methodology to predict high-quality reservoirs in both continental-marine and continental-lacustrine systems. A source-channel-sink-diagenesis coupling technique to predict high-quality reservoirs is proposed in the study to solve the difficulty encountered in exploring why the source-to-sink system serves to control sandstone development， but not necessarily determine reservoir formation.

Quantitative reconstruction， hierarchical division and coupling mode establishment for ancient source-to-sink systems in continental basins

Hongtao ZHU, Changgui XU, Xiaofeng DU, Qianghu LIU, Zhongheng SUN, Zhiwei ZENG

2023, 44(3):  539-552.  doi:10.11743/ogg20230302

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Quantitative reconstruction， hierarchical division and coupling mode establishment are important means to restore the erosion and sedimentation processes for depth-time ancient source-to-sink systems in continental basins. The main difficulties in quantitative reconstruction of provenance lie in paleo-geomorphology reconstruction and paleo-drainage reconstruction. New techniques for paleo-geomorphology reconstruction use a “three-zone and five-step” restoration method based on the framework of denudation， overlap-denudation and overlap zones as well as steps of residual geomorphology reconstruction， differential subsidence correction， overlap-denudation zone restoration， denudation zone restoration and depositional paleo-geomorphology restoration. In terms of paleo-drainage system reconstruction， a quantitative restoration and picking method is resorted to based on the ArcGIS system. The division of ancient source-to-sink systems in continental basins is usually realized by using the “three-line and three-level” method， of which， picking up watersheds， water dividing lines and ridge lines are very critical before a further division of first-order， second-order， and third-order source-to-sink systems. According to the area， geometric configuration and coupling relationship of the drainage unit and sedimentary body within the reconstructed ancient source-to-sink system， this paper puts forward four types of source-sink coupling modes， i.e.， “dumbbell”， “racket”， “trophy” and “javelin” modes， and thus may serve as a valuable reference for quantitative study of source-to-sink system and prediction of deep exploration targets.

Hierarchical division and fine architectural depiction of the interior of deep-water channel deposits

Dongwei LI, Chenglin GONG, Lin HU, Xiaohu HE, Quanyuan LUO

2023, 44(3):  553-564.  doi:10.11743/ogg20230303

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Deep-water channel deposits are of an important type of deep-water hydrocarbon reservoirs. Given their complicated depositional processes and variable facies， it is urgent to establish a method for the hierarchy division and fine architectural depiction of the interior of deep-water channel deposits. The study proposes an efficient method to characterize and evaluate the interior hierarchy of deep-water channel deposits by using seismic data while considering the characteristics of diverse deep-water channel deposits. If the bottom boundary of seismically resolvable channel-complex set is recognized， the methodology of “three boundaries plus six facies belts” can be applied to hierarchical division and fine architectural depiction of the interior of deep-water channel deposits. According to three types of geological interfaces （i.e.， the bottom boundary of channel sedimentation system， the bottom boundary of channel-complex set， and the interface of sedimentary facies belts）， six facies belts （including basal lags， axial fills， marginal fills， slumps and debris flows， levee， turbidite mud） can be delineated in this type of deep-water channels. While， if not， the methodology of “two boundaries plus two facies belts” can be used， where two facies belts （i.e. channel fills and levee） can be delineated by two kinds of boundaries （i.e. bottom boundary of channel sedimentation system and boundary of levee） in this type of deep-water channel deposits. “Basal lags and axial fills of the six facies belts” and “channel fills of the two facies belts”， among others， could have quality reservoirs developed， the sand-rich property of which can be recognized by top convexity （that is， relatively sand-rich property indicated when the top convex seismic reflection is visible， and relatively mud-rich property when this is invisible）. Applying the methodology of “three boundaries plus six facies belts” and “two boundaries plus two facies belts” in the investigation of the deep-water channel deposits in the Qiongdongnan Basin， we can conclude that it could better achieve the hierarchical division and architectural depiction of the interior of deep-water channel deposits. This also shows that the methodology can be applied to deep-water channel deposits in other regions， and is promising in industrial application.

Source-to-sink system and hydrocarbon source rock prediction of underexplored areas in rifted lacustrine basins： A case study on northern lows in Zhu Ⅰ Depression， Pearl River Mouth Basin

Hao LIU, Changgui XU, Yangdong GAO, Heming LIN, Xinwei QIU, Yongtao JU, Xudong WANG, Lei LI, Jun MENG, Xiaoming QUE

2023, 44(3):  565-583.  doi:10.11743/ogg20230304

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Hydrocarbon source rock is one of the key factors controlling oil/gas accumulation in petroliferous system of a sedimentary basin. Focusing on the challenges facing hydrocarbon source rock prediction in basins or lows with relatively low exploration maturity， sparsely distributed drilling wells and insufficient source rock index， we take the underexplored areas in northern Zhu Ⅰ Depression in the Pearl River Mouth Basin （PRMB） as an example to systematically analyze the source-to-sink system and predict source rocks from the perspectives of “searching for lake （deep-to-semi-deep lake）”， “recognizing mudstones （deep-to-semi-deep lacustrine mudstones）”， and “identifying hydrocarbon （hydrocarbon source rock prediction and evaluation）”. First， the original basin features and location of the paleo-lacustrine basin are illustrated by restoring multi-stage tectono-palaeogeomorphology to “find the lake”； and the deep-to-semi-deep lake is identified in the underexplored area together with preliminary selection of potential lows with well-developed hydrocarbon source rocks in combination with analyses including “identifying the lacustrine basin boundary by seismic progradational reflection， determining the scope of the deep-to-semi-deep lake by slope break system， illustrating paleo-environment characters by biochemical index， and defining the lacustrine basin scale by tectonic intensity”. The results indicate that five lows formed during the Wenchang depositional period， and five lows formed during the depositional period of the lower Enping Formation have the potential of developing deep-to-semi-deep lacustrine mudstones. Second， the reconstruction of source-to-sink system and the geological analysis of deep-to-semi-deep lacustrine mudstone development are conducted through analyses of paleo-provenance， paleo-environment and depositional systems， as well as tectono-palaeogeomorphology features； various configuration elements simulation results of the source-to-sink system show that basins characterized by medium sediment supply intensity， fine-grained source materials， high lake level， large accommodation space and under-equal or equal compensation are favorable for the deposition of mud-rich lacustrine facies. Finally， based on data of exploratory wells encountering high-quality hydrocarbon source rocks in mature exploration areas， we analyze four types of deep-to-semi-deep seismic facies and their geologic backgrounds corresponding to hydrocarbon source rock intervals， establish the “hydrocarbon source rock facies （seismic facies of hydrocarbon source rocks）” of the mature exploration areas. Coupled with the systematic summary of hydrocarbon-enrichment factors in main hydrocarbon-rich lows of the Zhu Ⅰ Depression， we assess and rank the high-quality hydrocarbon source rocks in the underexplored areas with multiple factors and from multi-dimensional points of view. The research results also indicate that the first-order potentially hydrocarbon-rich lows include LF22， HF33， and HZ24 （with the lower and upper Wenchang Formation as source rocks）， as well as the LF7 and HF10 （with the lower Wenchang Formation as source rocks）； the second-order potentially hydrocarbon-rich ones are HZ5 and HZ11 （with the lower Enping Formation as source rocks）， with the hydrocarbon source rock prediction and evaluation results of some lows having been testified in practical exploration.

Characteristics and evolution of the source-to-sink system of the Paleogene Wenchang Formation in Baiyun Sag， Pearl River Mouth Basin

Yangdong GAO, Guangrong PENG, Xiangtao ZHANG, Xudong WANG, Hui SUN, Taixun LIU, Fengchun SUN

2023, 44(3):  584-599.  doi:10.11743/ogg20230305

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The Pearl River Mouth Basin （PRMB） is a typical marginal sea basin to the east of the South China Sea. The Paleogene strata of the Baiyun Sag， a deep and large sag with well-developed Mesozoic-to-Cenozoic strata in the Zhu Ⅱ Depression of the PRMB， are the major targets for recent hydrocarbon exploration in the deep-water area. The Wenchang Formation is the hydrocarbon source rock sequence verified in exploration in the Baiyun Sag， and the provenances around the sag are also favorable for reservoir development. The quantitative characterization of the source-to-sink system， and the analysis of its characteristics and evolution of the formation provide a basis for predicting the distribution of the main hydrocarbon source rocks and large-scale sand bodies in the Baiyun Sag. A combination of zircon dating and heavy mineral analysis serves to determine the provenance system and its evolution. The types and distribution of transport channels are characterized by PSDM seismic data and palaeo-geomorphology. The characteristics of sedimentary system and sedimentary facies are depicted by well-tied seismic interpretation， while the source-channel-sink system is quantitatively coupled with the calculation results of palaeo-geomorphic relief， denudation thickness and sedimentary accumulation. The research results show that during the deposition of the Paleogene Wenchang Formation， there are four proximal provenance systems developed， that is， the Panyu low uplift， Yunli low uplift， Yundong low salient and Yunkai low salient， resulting in four complete suites of“source-channel-sink”systems with fault-related ditches and ravines as channels. The source-to-sink system tends to undergo significant changes in different sedimentary periods. Under the effect of differential provenances in different periods， near-provenance braided river delta and fan delta deposits of small-scale lacustrine facies are developed in the Wenchang Formation of the Baiyun Sag as controlled by paleo-grooves and large-scale tectonic transition zones. The deposits of shore-shallow lake facies are widely developed in the 5^th， 4^th and 3^rd members of the Wenchang Formation under the control of the source-to-sink system of the Baiyun Sag. While the occurrence of semi-deep to deep lake facies is sparsely seen in the 5^th member of the Wenchang Formation， and seen in the central of the 4^th， 3^rd， and 2^nd members， which provide a basis for predicting the distribution of the main hydrocarbon source rocks. Under the control of wide and gentle rift structure and large provenances， the braided river delta of sand-rich sedimentary system is developed on the north slope of Baiyun Sag that controls the planar distribution of large-scale sand bodies； the braided river deltas and fan deltas in the east of the Baiyun Sag are small in scale with sand bodies of favorable facies widely seen.

Sedimentary transformation and source-to-sink response to the Eocene rifting-detachment in the western Baiyun Sag， Pearl River Mouth Basin

Jun LIU, Guangrong PENG, Jinyun ZHENG, Song CAI, Dingwei ZHU, Ziyi WANG

2023, 44(3):  600-612.  doi:10.11743/ogg20230306

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During the Eocene rifting period， the Pearl River Mouth Basin （PRMB） was in an alternating pattern of uplift and depression. The transition of tectonic activity from brittle rifting to ductile detachment in the deep water area of the continental margin is mostly manifested as the change of sag from deep to shallow and the migration of depositional center. However， the lack of discussion on the sedimentary responses in the depression has restricted the prediction of the distribution of sedimentary bodies formed in the process of transition. The study serves to restore the sedimentary palaeogeomorphology during the Eocene rifting and detachment periods based on the drilling and 3D seismic data of full coverage in the western Baiyun Sag， PRMB. The results are shown as follows. First， during the rifting period， the lacustrine basins were narrow and deep， generally with multiple depositional centers developed， while during the detachment period， the lacustrine basins were wide and gentle， with water converging toward the detachment center. Second， combined with well-tied seismic interpretation of depositional framework and seismic calibration of depositional configuration， it is believed that minor-axis provenances existed in the western Baiyun Sag due to the control of pre-existing fault blocks， leading to the development of fan delta， semi-deep lake and braided river delta （fan/braid） depositional systems during the rifting period； while during the detachment period， as a result of ductile uplifting， the overall terrain slope got lowered （<10°）， the water body got shallowed and the range of high-energy water body expanded. Major-axis provenances appeared in the uplifted zone， and braided river delta-beach bar （beach/fan） depositional systems were widely developed within the structural lows. Third， the coupling of drainage zoning parameters in Panyu low uplift area reveals that the sediment supply efficiency of beach/fan sedimentary systems was high， with an increasing proportion of sand in the sediments， and the sedimentary system developed in the major-axix direction of the ductile uplift zone can form a favorable source-reservoir configuration.

Source-to-sink system during rifting-depression transition period and its exploration significance： A case study of the Upper Enping Formation at southeastern margin of Huizhou 26 sub-sag， Pearl River Mouth Basin

Guangrong PENG, Xucheng WANG, Weitao CHEN, Yaoyao JIN, Fei WANG, Wenyong WANG, Han QUAN

2023, 44(3):  613-625.  doi:10.11743/ogg20230307

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Rifting-depression transition period is a significant stage of transition for a rifted basin from extension to subsidence， during which the tectonic activities get stable along with declining extension， affecting the sedimentary system and favorable reservoir distribution in the late rifting period. With the guidance of the “source-to-sink” coupling ideology and based on the analysis of petrological characteristics， logging and seismic facies， we propose the idea of “tectonic activity-controlling facies， and valley/fault-controlling reservoir” to analyze the origin and distribution pattern of favorable reservoirs according to the differential activities of boundary faults during the rifting-depression transition period of the Upper Enping Formation at southeastern margin of Huizhou 26 sub-sag. The major conclusions can be drawn as follows. First， the Mesozoic intrusive rocks of the Dongsha uplift serve as the provenance to the upper member of the Enping Formation in the study area， with five major catchment units developed during the rifting-depression transition period. The supply capacity can be characterized by the quantitative analysis of the area and head of these catchment units. Second， the sediment-transport pathways are composed of the valleys and boundary faults. The morphology and scale of valleys within provenance affect the water kinetic energy of each drainage， and the planar combination pattern of boundary faults （i.e. the concave corner type， the straight and flat slope type， and the straight and flat fault type） and their vertical throws serve to influence the way of sediments converging in lake， as well as the distribution of sand bodies. Third， the “source-to-sink” coupling with four elements including source， valley， boundary fault and sedimentary system helps establish a sedimentary framework of “fan deltas in the south and braided deltas in the north” resulting from the differential activities of boundary faults at the tectonically quiet stage of the rifting-depression transition period. Sedimentary characteristics are finely depicted to disclose favorable facies zone and quality reservoir distribution， that is， the coarse-grained deposits of channel sub-facies， braided river delta plain facies， that is formed by sediments transported along the “valley-boundary faults” to lake， and characterized by well-developed primary pores and high permeability. These new understandings serve to promote the exploration progress of the Enping Formation and manifest exploration direction of the Paleogene deep strata at the southwestern area of Huizhou Sag.

Structural evolution and its control on source-to-sink system of Panyu 27 Sag in Pearl River Mouth Basin during rifting

Ziyi WANG, Hongbo LI, Jinyun ZHENG, Dingwei ZHU, Sa YU, Zhaoming CHEN, Zhensheng LI

2023, 44(3):  626-636.  doi:10.11743/ogg20230308

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Sag Panyu 27 is located in the Panyu low uplift area in the middle of the central uplift zone of the Pearl River Mouth Basin. It is a Cenozoic sag developed on the basement of the Mesozoic continental margin magmatic arc， which is ‘faulted in the south and overlapped in the north’. Oil samples and extracted hydrocarbon from wells drilled in the sag can be traced back to the intermediate to deep lacustrine source rocks in the Wenchang Formation. However， none of the wells actually reached the formation， making the specific location and period for the development of source rocks still uncertain. Through the study of the controlling fault and the structural style of the sag， it is shown that the development and evolution of the sag has a ternary coupling genesis of ‘pre-existing structural constraints， dextral extension control， and magmatic underplating transformation’， which controls the structural pattern of ‘left stepping asymmetric graben-left lateral en echelon semi-graben-wide and gentle semi-graben’ in the three-stage rifting period. The differential activation of sag-controlling faults in the three stages of rifting period affects the migration of the sedimentary center， and the differential evolution of the sag architecture affects the sedimentary filling pattern of the sag. With the rifting， the sedimentary center gradually migrated from east to west， and from both ends to the middle. In the early stage of Wenchang Formation deposition， the source-to-sink sedimentary systems are mainly distributed in the central transition zone along the NW-trending axis， while in the late stage， they mainly occurred on the northern gentle slope， and a large source-to-sink sedimentary system with long provenance appeared in the northwest during deposition of the Enping Formation. As a sedimentary center for a long time， the western sub-sag is the right place for the development of high-quality source rocks. The middle stage of rifting when a large semi-graben lake basin was formed and accompanied by strong magmatic activity was the time for the development of high-quality source rocks. The long-term development of large source-to-sink sedimentary system in the northwest during the middle-late rifting stages provided favorable conditions for the formation of large Paleogene reservoirs.

Factors controlling the formation and distribution of Mesozoic buried hill reservoirs in the Qiongdongnan Basin

Mingcai HOU, Xiaohu HE, Qiuyue JIN, Haiyang CAO, Liwen HE, Youyuan QUE, Anqing CHEN

2023, 44(3):  637-650.  doi:10.11743/ogg20230309

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Buried hills are emerging as an important exploration target in the sedimentary basins offshore China. Despite the successful drilling of several gas-bearing prospects in the Mesozoic buried hills in Qiongdongnan Basin， there is a lack of research on the primary factors controlling oil and gas accumulation， and no large-scale gas reservoirs have been discovered so far. Clarifying the development characteristics and genetic mechanisms of these reservoirs is crucial for advancing buried hill exploration in this region. This paper examines the development characteristics of buried hill reservoirs across different structural belts in the Qiongdongnan Basin using existing drilling， core， thin section， and geophysical data. The results indicate that structural fractures are the primary reservoir space， with thrusting event of the pre-Cenozoic Indosinian and Yanshanian playing a key role in fracture formation. The presence of preexisting main faults provided the foundation for the formation of the buried hills and the Cenozoic extension activities reshaped the fracture systems. Bedrocks dominated by felsic minerals are the favorable reservoir rocks， with multi-stage fluid action effectively improving the quality of buried hill reservoirs and contributing to their vertical zoning. The Mesozoic buried hill structural belt of Qiongdongnan Basin can be classified into a double-layer type of buried hill intensively modified by tectonism in the south， a three-layer type of buried hill modified jointly by weathering and tectonism in the middle， and a single-layer type of buried hill weakly modified by weathering in the north. The study also shows that ancient bedrocks that experienced the Indosinian tectonic activity are more favorable for the development of high-quality buried hill reservoirs， while the buried hills near the periphery of the depression are more conducive to the formation of large gas reservoirs.

Deep architecture of hyperextended marginal basin and implications for hydrocarbon exploration：A case study of Qiongdongnan Basin

Keqiang WU, Xinong XIE, Jianxiang PEI, Jianye REN, Li YOU, Tao JIANG, Yongbin QUAN

2023, 44(3):  651-661.  doi:10.11743/ogg20230310

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The passive continental margin basin in the northern South China Sea is an important oil and gas base in China’s offshore area. This paper reveals depositional filling characteristics and stratigraphic patterns of hyperextended continental margin rift basins based on coupling analysis of the deformation of deep crust and basin filling in the passive continental margin through comprehensive analyses of massive seismic and drilling data from the Qiongdongnan Basin of South China Sea. The results show that the Qiongdongnan Basin， located on the western tip of the northwest sub-basin of the South China Sea， is once a failed continental margin rift basin and presents now as a neck and distal zone along the central depression belt after lithospheric detachment and thinning. The rifting period contains three stretching stages. During the early stage， the formation of isolated rifted basin is characterized by high-angle positive faults. During the detachment active stage， the formation of detachment rifted basin is characterized by low-angle detachment faults with their active time obviously characterized by an east-west migration. And during the late rifting-depression stage， the depocenter is located in the center of the depression. The migration characteristics of differential deformation tectonic movement define the unique deep structure pattern of the Qiongdongnan Basin and lead to obvious differences in depositional fillings and stratigraphic patterns in different tectonic units， especially in depositional systems， where fan delta deposits dominate the main detachment fault side and braided delta deposits occupy the opposite rolling anticline. These factors restricted the distribution of source rocks and deep reservoirs during the rifting period， which in turn constrained geological conditions of hydrocarbon generation and accumulation. Therefore， the deep architecture of hyperextended continental marginal rift basins established based on non-transient breaking-up process of the lithosphere and depositional filling characteristics have important guiding significance for hydrocarbon exploration in the Qiongdongnan Basin， and may serve as reference for the study of deep stratigraphic pattern and depositional filling in passive continental margin basins.

Characteristics of the source-to-sink system for the Paleogene Sha 2 Member of northern Liaoxi Sag， offshore Bohai Bay Basin and its control on beach bar sands

Xiaogfeng DU, Xiaojun PANG, Xiaobo HUANG, Bingjie WANG

2023, 44(3):  662-674.  doi:10.11743/ogg20230311

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The beach-bar deposits developed in the 2^nd member of the Paleogene Shahejie Formation （Sha 2 Member， Es₂） in the northern Liaoxi Sag， have great exploration potential as proved by the testing results of exploratory wells. However， there is little insight obtained on the mechanism of beach-bar deposition， seriously restricting the exploration and evaluation of this kind of deposits. Therefore， it is necessary to analyze the controlling factors of beach-bar sand deposition from the perspective of source-to-sink system. The source-to-sink system of the Es² in the northern Liaoxi Sag and its control on beach-bar deposition are studied by integrating the data of core observation， logging， grain size and casting thin section， combined with 3D seismic and regional geological data. The results are shown as follows. First， during the deposition of the Es² in the study area， the Xingchenghe river system and the Yuehe river system in the Yanshan uplift transported sediments to the braided river deltas， which in turn served to supply sediments for the beach bar. Because the relatively small catchment area of the two river systems together with the shorter length of main river， the braided river deltas formed are relatively small in scale， and their capability to provide sediments for beach bars is thereby relatively weak. Second， during the deposition of Es²， a wide and gentle palaeo-uplift was developed in the northern Liaoxi Sag， providing favorable sedimentary palaeo-geomorphic conditions for beach bar deposition. Third， during the same period， the study area was characterized by shallow water body and weak sediment supply capacity， resulting in the transportation of sand from the braided river delta front towards the lacustrine basin with waves. Blocked by the higher landform of the palaeo-uplift zone， the sand bodies deposited in the palaeo-uplift zone and formed fine-grained sandy beach bars， dominated by fine-grained sandstone and siltstone. As the lake level gradually rises， the development of the beach bars gets weakened. Fourth， Yanshan uplift， Xingchenghe-Yuehe river systems， palaeo-uplift in the central Liaoxi Sag and lake level fluctuation jointly control the lithology and scale of beach-bar sands， and they are the key factors in making breakthrough in hydrocarbon exploration in the study area. The understanding obtained in the study is of referential significance to the prediction of beach-bar sandstone reservoirs in the Paleogene hydrocarbon exploration in the Bohai Sea.

Geological model and construction process of the Mesozoic volcanic edifices in Bozhong Sag， Bohai Sea

Xuanlong SHAN, Wei WANG, Xintao ZHANG, Jian YI, Chunqiang XU, Junpei YUE, Pengcheng LIU, Mao FU

2023, 44(3):  675-688.  doi:10.11743/ogg20230312

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Fine depiction of volcanic edifices is of great value to mapping volcanic reservoirs. A variety of volcanic edifices under fierce later modification are developed in the Mesozoic of the Bozhong Sag， Bohai Sea. There is， however， a lack of documents on the types， structure and construction process of the volcanic edifices in the area at present. The data of core observation， logging and seismic interpretation are applied to divide the Mesozoic volcanic edifices in the Bohai Sea into four types， that is， lava dome， stratovolcano， pyroclastic sheet and shield volcano. The study serves to depict the eruptive units， lithology and lithofacies within the four types of volcanic edifices， with a geological model established and based on the morphological characteristics and internal structure， the diagrams of four types are built to show the volcanic construction processes. Among others， the lava dome characterized by a high aspect ratio is composed of thick-bedded acidic lava of massive structure within； the stratovolcano widely seen in the Bohai Bay Basin and large in scale is mound-shaped in terms of external morphology， smaller than the lava domes in terms of aspect ratio， and characterized by complex lithological types， most of which are of alternating lava and pyroclastic rocks； and the shield volcano is of gentle low-mound shape， and mainly composed of basic rocks， featuring low-viscosity magma and large flow range， resulting in large-scale volcanic edifices of sheet-like structures inside to a large extent. The pyroclastic sheet is in the shape of sheet and filling， with very small aspect ratio and layered structure caused by multi-stage eruption and superimposition. In addition， the comparison of the physical properties of different volcanic edifices shows that the reservoirs of lava dome and stratovolcano types in the study area are of the best physical properties， which can be viewed as favorable targets for future oil/gas exploration， followed by the pyroclastic sheet and then the shield volcano coming at the worst.

Provenance transformation and sedimentary response of ramp facies in downfaulted basins： A case study on the Paleogene source-to-sink system in Lixian slope， Raoyang Sag， Bohai Bay Basin

Hehe CHEN, Xiaomin ZHU, Ruisheng SHI, Zili ZHANG, Qi LI, Zhenjun ZHU, Zehao YAN

2023, 44(3):  689-706.  doi:10.11743/ogg20230313

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The quantified coupled response of intra-basin sedimentary system to extra-basin sediment flux variation is still a weak link in source-to-sink system research， and also an important topic in the framework of source-to-sink system research in downfaulted basins. Based on the 3D seismic and log data， as well as core observation of the Lixian slope in the Raoyang Sag， combined with detrital zircon U-Pb dating， the study carries out quantitative characterization of the provenance system during the deposition of the second and first members of the Paleogene Shahejie Formation （that is， Sha 2 and Sha 1 members）， and establishes a quantitative characterization method for shallow-water delta by means of quantitative sedimentary structure interpretation and quantitative seismic geomorphic measurements. The results show that the sediment flux of the paleo-Yongding River branch was seized by the paleo-Daqing River， resulting in the sediment flux decrease from 37 % to 26 % in zone A of Lixian slope during the sedimentation of Sha 1 to Sha 2 members， the corresponding area of the delta in zone A declining from 144 km² to 38 km² and a reduction of the average subaqueous distributary channel width from 104 m to 47 m. While the sediment flux of the paleo-Tang River increased from 31 % to 39 %， corresponding to an increase in the area of delta in zone B from 71 km² to 94 km² and an increase in the average subaqueous distributary channel width from 77 m to 88 m. No significant changes are observed in the sediment flux of the paleo-Dasha River， and the corresponding area of the delta in zone C decreased from 206 km² to 185 km²， and the average subaqueous distributary channel width decreased from 120 m to 81 m. Allogenetic forces such as tectonic evolution， climate change， and lake-level fluctuation tend to generate larger scope of influence， causing the synchronous response of sedimentary systems. In contrast， changes in sediment flux will affect specific sedimentary systems and are the primary driving mechanism for the sedimentary response of shallow-water deltas in the study area.

Genesis and evolution of pore-fractures in deep sandy conglomerate reservoirs in Bohai Bay Basin： Taking the Paleogene Kongdian Formation in Bozhong 19-6 structure as an example

Xintao ZHANG, Xiyu QU, Peng XU, Qingbin WANG, Xiaojian LIU, Tao YE

2023, 44(3):  707-719.  doi:10.11743/ogg20230314

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Mapping of quality reservoirs is critical to the successful exploration and discovery of deep- to ultra-deep clastic oil and gas reservoirs. This paper studies the genesis and evolution of storage space in high-quality and deep sandy conglomerate reservoirs in the Paleogene Kongdian Formation， Bozhong 19-6 structure， and establishes pore-fracture evolution models through physical simulation experiments and other means. The results show that： （1） pores formed by dissolution of feldspar particles are usually high-quality reservoir space. The dissolution of potassium feldspar intensifies or even exceeds that of plagioclase with rising temperature， indicating that potassium feldspar has greater potential for increasing porosity of deep layers. （2） Gravel-sized particles， felsic composition and low matrix content are more conducive to the generation of induced fractures， which are more highly developed in feldspar than in quartz. Vertically， induce fractures of the first stage appear at a burial depth of about 2 500 m， that of the second stage appear at a depth larger than 3 000 m with their number peaking at about 4000 m deep， and that of the third stage appear at a depth greater than 4 500 m with their amount increasing with the increasing depth. （3） Vertically， porosity is damaged by early compaction and late cementation but improved by dissolution occurring in between： the early compaction reduced the porosity by 17.38 % and the late ankerite cementation reduced the porosity by 7.76 %， while organic acid dissolution between the two processes increased the porosity by 5.45 %. （4） Hydrocarbon charging is well timed relative to the second-stage dissolution with porosity enhancement， and the first- and second-stage induced fracture development， thus， hydrocarbons enrich in reservoirs at about 3 000 m deep. In addition， high-quality reservoirs may occur in the deep sandy conglomerate at 4 000 m deep， where the mass development of induced fractures， together with the dissolution such as potassium feldspar， greatly enhance reservoir quality.

Zonation and dynamic genetic mechanism of the Indosinian thrust nappe system in Bohai Sea

Dingyou LYU, Haifeng YANG, Haibo YU, Pengbo LIU, Hui DENG, Shen ZHANG

2023, 44(3):  720-734.  doi:10.11743/ogg20230315

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The Indosinian orogeny is a key period shaping the intracontinental deformation pattern in eastern China. Current understanding of the Indosinian tectonism period is mainly based on the study of the orogenic belt around the North China Craton and the continental area of the Bohai Bay Basin. However， little research attention has been paid to the Mesozoic tectonic evolution in the Bohai Sea. Based on the latest three-dimensional seismic reflection data， we present a detailed structural analysis of the Indosinian deformation in the Bohai Sea and propose that the Indosinian thrust nappe system can be divided into the three thrust zones as follows. The first thrust zone along Shaleitian salient， Bozhong 19-6 structural zone and the Bonan sub-salient， is a high-angle trust structure as shown by the Archaean basement thrust at high angle along reverse fault with part outcropped and subject to long-term weathering and denudation during the Indosinian period， resulting in the absence of the Paleozoic. The high-angle thrust faults turned into strike-slip faults as shown by the present-day Bohai Bay section along the Zhangjiakou-Penglai fault zone. The second one， the central thrust-fold zone located in the Bozhong and Bodong sags， is mainly characterized by fault-related fold deformation in the Paleozoic strata， especially in the Bodong Sag， where typical imbricates and duplex structures can be identified. The third thrust zone， the front belt of the thrust nappe located in the Liaodong Bay Depression， is mainly characterized by extensive and gentle anticlines and synclines under the Paleozoic deformation， typical fault-propagation or fault-detaching folds， which are associated with the hidden thrust nappe faults of slope-plateau type in the basin basement during the Indosinian. We may conclude through the seismic section across the entire Bohai Sea and previous research data， that the spatial association of three thrust nappe zones is controlled by the three-step basement detachment layers during the Indosinian period. The dynamic genetic mechanism of the Indosinian thrust nappe system in the Bohai Sea may be related to the differential basement features in the North China Craton， that is， the thrusting of Luxi basement against Yanliao basement along the decollement layer during the Indosinian， leading to the overlying Paleozoic caprock characterized by fold deformation in zonation.

Source-to-sink coupling and temporal-spatial evolution in the Lishui Sag of East China Sea Shelf Basin during the Paleocene

Zhenjun ZHU, Qi LI, Hehe CHEN, Jian LI, Weiping ZHANG, Fengfan YANG, Yingzhao ZHANG, Jun QIN, Fengxun LI, Shuaiqiang SHAN

2023, 44(3):  735-752.  doi:10.11743/ogg20230316

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The source-to-sink systems in rifted basins are generally complex as a result of multi-stage tectonic activities and multiple provenances and drainage systems both within and outside the basins. The coupling relationship and spatial-temporal evolution of these systems in different tectonic stages are in need of in-depth study. Based on analyses of 3-D seismic data， well logging， cores， thin sections and heavy minerals of the Paleocene Lishui Sag in the East China Sea Shelf Basin， this study attempted to reconstructed the source-to-sink system for the Sag through sediment provenances analyses， transport channels identification and （quantitative） tracing， depositional system mapping and （half-quantitative） characterization. The results show that the early syn-rift stage was characterized by isolated sinks， small depositional centers， low tectonic activity rate， and inadequate sediment supply， with small and numerous fan deltas well-developed in gentle slope zones and small braided river deltas developed in steep slope zones. The main syn-rift stage witnessed sufficient sediment supply， larger but fewer fans developed in steep slopes and the braided river delta enlarged and combined into larger ones in gentle slopes， due to intensive faulting activities. The late syn-rift stage experienced the gradual disappearance of fault-related source-to-sink systems and the development of large floor fans at the slope break zones due to the ever-incrementing river incision as the gentle slope zones were under the influence of rapidly dropping sea level. Under the joint control of tectono-sedimentary pattern， parent rock types and boundary conditions， five source-to-sink systems were developed during different tectonic stages： ① metamorphic rock-dominated parent rocks—SE-trending U-shaped paleo-valley—braided river delta in the northern gentle slope zone， ② volcanic rock-dominated parent rocks—NE-trending faulted-trough /SE-trending V/U-shaped paleo-valley—braided river delta/basin floor fan in the southern gentle slope zone， ③ intrusive rock-dominated parent rocks—SW-trending paleo-valley—fan delta in the northern part of the Lingfeng Uplift， ④ volcanic rock （in early stage）/metamorphic rock （in late stage）—SW-trending paleo-valley/NE-trending faulted-trough—proximal fan delta system in the southern section of Lingfeng Uplift， and ⑤ intrusive and metamorphic rocks—SW/S-trending paleo-valley—fan delta/braided river delta in the Yandang Uplift. Quantitative statistical results of sediment routing systems show that the paleo-valleys and faulted trough in gentle slope zones are wide and shallow， whereas those in the steep slope zones are narrow and deep. The cross-sectional areas of the sediment routings in the early and late syn-rift stages display a positive correlation with the areas of corresponding fan/delta complexes in the Sag.

Characteristics and forming mechanisms of transform zone in the Tiantai slope， Xihu Sag， East China Sea Shelf Basin

Lang YU, Yixin YU, Yiming JIANG, Wei ZOU, Shi Chen, Xianjun TANG, Xinxin LIANG, Xinjian HE, Dongxia CHEN

2023, 44(3):  753-763.  doi:10.11743/ogg20230317

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The Tiantai slope in the Xihu Sag， East China Sea Shelf Basin， has a special tectonic background with a deep NW-trending fault zone cutting through and adjusting/transforming the tectonic features of the Cenozoic in the southern and northern parts of the slope. Based on the detailed interpretation of 3D seismic data and restored balanced cross sections， this study establishes models for the development of deep and shallow faults， determines the tectonic attribute of the NW-trending transform zone and discusses the evolution history and genetic mechanisms of the transform zone against regional geological settings. The results show that the NW-trending extensional detachment faults， including the Tiantaixi fault and the Gushan fault， were developed in the Mesozoic of the Tiantai slope， and have played an accommodating and transforming role in shaping the geological framework and controlling fault development and extension intensity of the Cenozoic， resulting in a gentle and a steep slope respectively on the south and more stretched north parts of the slope. The faults of the Cenozoic in the northern slope obviously outnumber those in the southern slope. Arc fault systems are found to be developed in the hanging wall of the Gushan fault. The NW-trending tectonic transform zone bounded by the Tiantaixi fault and the Gushan fault differs greatly from that formed in the extensional setting， indicating a discrete type of tectonic transform zone. Influenced by the post-orogenic collision and extension between the Qinling and Dabie orogenic belts， and the collision and subduction of the Eurasian and Pacific plates， the transform zone in the Tiantai slope has experienced four evolutionary stages， including the gestation period from the Late Jurassic to the Early Cretaceous， the embryonic period from the middle Early Cretaceous to the Early Eocene， the maturation period from the middle Late Eocene to the Early Oligocene， and the quiescence stage from the Late Oligocene to present.

Sequence filling and evolutionary model of the Lower Cambrian Maidiping-Qiongzhusi formations in Sichuan Basin and on its periphery

Dong WU, Hucheng DENG, Liang XIONG, Kaixuan CAO, Xiaoxia DONG, Yong ZHAO, Limin WEI, Tong WANG, Ruolong MA

2023, 44(3):  764-777.  doi:10.11743/ogg20230318

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The Lower Cambrian Qiongzhusi Formation in the Sichuan Basin and on its periphery is rich in shale gas resources and has been proven to be one of the important shale gas pay zones in the basin. Due to the deep burial depth， few data and complex geological conditions， the research on its sequence stratigraphy has been relatively weak， having seriously restricted the follow-up research and development. The study on the Early Cambrian tectono-sedimentary setting of the Sichuan Basin and on its periphery， coupled with lithology， rock color， logging curve， trace elements， paleontology， seismic reflection event and other characteristics， is carried out to further explore the sequence stratigraphy of the Maidiping-Qiongzhusi formations， with the idea for the sequence division proposed. As a result， the Maidiping-Qiongzhusi formations are divided into four third-order sequences from bottom to top. By revealing the spatial distribution pattern of the sequence stratigraphy， we establish a sequence evolutionary model of “differential sedimentation in early narrow basin， and stable sedimentation in later wide basin” for the Maidiping-Qiongzhusi formations in the Sichuan Basin and on its periphery under the control of global plate extensional activity. The sequence evolutionary model will be of help to deepening understanding of the Cambrian tectono-sedimentary setting in the Sichuan Basin， promoting geological research on sedimentary reservoirs， and serving the exploration and development of deep shale gas.

Sweet spot assessment and its significance for the marine-continental transitional shale gas of Permian Longtan Fm. in southeastern Sichuan Basin

Dongjun FENG

2023, 44(3):  778-788.  doi:10.11743/ogg20230319

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The marine-continental transitional deposits of Permian Longtan Formation in southeastern Sichuan Basin have been targeted for shale gas but without any significant success. By resorting to analyses on petrology， organic geochemistry， and geology as well as the results of X-ray diffraction， organic petrology analysis， argon ion milling scanning electron microscopy observation， helium porosity testing， and joint measurement， the organic geochemical characteristics， reservoir characteristics， gas-bearing property and their vertical variations of 9 clay mineral-rich（42.69 % on average） layers in the Longtan Formation from some key wells （including Well S1） in southeastern Sichuan Basin were studied to locate potential shale gas enrichment intervals. The results show that the marine-continental transitional shale deposits in the Longtan Formation are thick （the cumulative thickness of shale with TOC>2 % is 50.11 m） and mainly composed of coal seams， carbonaceous shale， shales， silty mudstone， calcareous mudstones and argillaceous limest one. Their average TOC is 4.63 % and average porosity is greater than 2.0 %， both showing a decreasing trend from coal seam， shale， silty mudstone to calcareous mudstones. The organic matter is mainly of type Ⅲ， with an average R_o of 2.07 %. Reservoirs are dominated by clay mineral pores and micro fractures. The combinations of coal with organic-rich shale within layers ② to ④， ⑥ and ⑧ characterized by large thickness， high organic matter abundance， high porosity and gas-bearing property are deemed as potential exploration targets for making shale gas discoveries.

Characterizing hydrothermal siliceous nodules to guide shale oil exploration in the Middle Permian Lucaogou Formation， Bogda area， Junggar Basin

Jiaquan ZHOU, Yue WANG, Ziyi SONG, Jiting LIU, Sainan CHENG

2023, 44(3):  789-800.  doi:10.11743/ogg20230320

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Siliceous rocks are frequently included in reconstructing paleoenvironment， tracing hydrothermal circulation and understanding silicification process， aiming at using the results to guide the exploration and development of continental shale oil. Siliceous rock samples from the Lucaogou Formation in Bogda area， Junggar Basin， were observed through X-ray diffraction and scanning electron microscope and analyzed with major and trace elements， vitrinite reflectance， Rock-Eval and total organic carbon data to identify their genetic mechanisms and hydrocarbon significance. The results show that the siliceous nodules in the samples host highly-deformed soft-sediments mainly composed of crypto-microcrystalline silica， indicating a synsedimentary product. Hydrothermal minerals such as barite， mangano-siderite， trevorite and azurite are developed inside the nodules with low total content of rare earth elements and relative high content of heavy rare earth elements as well as positive Eu anomalies， all indicating a hydrothermal origin and intermittent hydrothermal activities in the Bogda area during the Permian. The nutrients and metal cations， brought by the hydrothermal fluid， improved the paleo-productivity and paleo-salinity， and enhanced the reducibility of the bottom water， which was conducive to the enrichment of organic matter. Well-developed pores and fractures with enriched quartz greatly improved the brittleness of the rocks， making the reservoirs in the formation more responsive to fracturing. The hydrothermal activities associated with siliceous rocks participated in the evolution of hydrocarbon generation of organic matters， resulting in reduced crude viscosity and enhanced crude mobility which are conducive to shale oil development.