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Abstract
The faults of different occurrence have differential oil-gas reservoir controlling mechanisms. There are two kinds of fault systems including deep antithetic fault system and shallow consequent fault system in Fushan Depression. The deep antithetic fault system was favorable to form various kinds of traps and control the most petroleum accumulation distribution in Fushan Depression, because of its early occurrence, short activity time and proper fault throw. The shallow consequent faults were unfavorable to the petroleum accumulation and preservation. The oil-gas reservoir controlled by the deep antithetic fault system was located on the down block of antithetic fault. The oil-gas reservoir controlled by the shallow consequent fault was located on the upper block of consequent fault. Oil and gas were mainly accumulated in the down block traps of the deep antithetic fault system.
... Fushan Depression in the Beibuwan Basin of the South China Sea has been well-studied and plenty of work has been done in the last two decades (e.g., Ding, Wang, & Ma, 2003;He & Gao, 2006;Li et al., 2007Li et al., , 2008Li et al., , 2009Luo & Pang, 2008;Zhang, He, Gong, Zhang, & Huang, 2012;Yu, Liao, Lin, & Wang, 2012;Ma, Zhao, Liao, & Lin, 2012;Xie et al., 2012;Sun et al., 2013;Zhao, Wang, Gan, Chen, & Wang, 2014;Zhao et al., 2015;Lu et al., 2016;Wang et al., 2016;Liu, et al., 2017), including the sequence stratigraphy, sedimentary infill architecture, palaeoclimate condition, structural feature, regional tectonic evolution have been investigated in detail, as well as the reservoir properties and petroleum systems. One of the typical research findings among them is the Huangtong Sag (west area) and the Bailian Sag (east area) which, separated by the Huachang Transfer Zone, show distinctive differences in sedimentary fill pattern, source rock potential, and reservoir characteristics (Jin et al., 2014;Liu et al., 2014Liu et al., , 2015Liu, Sun, Yue, Ma, & Fang, 2013;Liu, Wang, Lin, Li, & Ma, 2012;Yu et al., 2012). ...
... It is one of four petroliferous basins with an area of approximately 3.98 × 10 4 km 2 in the northern continental shelf area of SCS and the main part of the Beibuwan Basin is in the sea area with~1.6 × 10 4 km 2 (Lin, 2011;Zhu, 2009). Previous studies have shown that the structural evolution of the basin can be largely divided into two stages: a syn-rifting stage from Palaeocene to Early Eocene and a post-rifting stage from Late Eocene to Oligocene (Jin et al., 2014;Li et al., 2008;Luo & Pang, 2008). However, recent research indicates that the Beibuwan Basin also experienced an intensive extension during the second episode of Zhu-Qiong Orogenic Event, and a combination of the proto South China Sea (SCS) oceanic crust subduction and clockwise rotation of Indo-China Plate along the Red River Fault caused the extension and strike-slip as a transtensional tectonic setting (Huang, Tian, Wilkins, Xiao, & Li, 2013;Zhao et al., 2014Zhao et al., , 2015. ...
... The Depression forms a half-graben faulted in the south and overstepping in the north (Figure 2c). Two kinds of fault systems were colloquially developed as a "double layer structure" from deep antithetic faults to shallow consequent faults (Liu et al., 2014;Luo & Pang, 2008). The Huachang Uplift in the central region is interpreted to be a transfer zone which separates the Fushan Depression into two independent sags with different structures and sedimentary patterns, the Huangtong Sag in the west and the Bailian Sag in the east (Figures 1b and 2a,b;Cao, Jin, Sun, & Wang, 2016), resulting in different tectonic styles (extensional in the western and transtensional setting in the eastern) and sedimentary patterns as well. ...
The lacustrine Beibuwan Basin contained large petroleum reserves in the northern continental shelf of the South China Sea. Due to the great sedimentologic complexity of the lacustrine basins, a significant financial risk associated with exploration needed to be reduced by specific techniques or significantly increasing knowledge of the lake system. Sedimentary infill, sequence stratigraphy, and organic geochemistry of the Fushan Depression in the Beibuwan Basin were studied, and enhanced our comprehension of the genetic relations between the sediment + water supply rates (climate change) and potential accommodation (tectonic subsidence and inherited topography) change. Here, we report two different lake‐type evolution cycles developed in two separated sags in the Fushan Depression, which are characterized by: (a) overfilled, (b) balanced‐fill, (c) overfilled, (d) balanced‐fill, and (e) overfilled in the Huangtong Sag, and (a) overfilled, (b) balanced‐fill, and (c) overfilled in the Bailian Sag and the centre Huachang Transfer Zone. Climate change is the dominant factor in the first three phases of the lake‐type evolution in both two sags, and transtensional tectonic setting is the controlling factor in the fourth and fifth phases of the lake‐type evolution in the Huangtong Sag. The same lake‐type with similar source rock and reservoir properties and observations indicate that these associations of source rock and reservoir play elements occur in a wide variety of tectonic settings. Moreover, the discovery of the cyclical process of the lake‐type evolution in the Fushan Depression can be a reference in the study of other similar lacustrine basins under an analogous tectonic and climate background.
... The Fushan Depression, located in the southeast of Beibuwan Basin, is a "dustpan" shape depression featured by zonary distribution in EeW direction and blocking distribution in NeS direction (Li et al., 2008Shi et al., 2011). Bounded by the Lingao Fault to the west, the Anding Fault to the south and Changliu Fault to the east (Figs. 1 and 2), accumulating sediment is supplied through fan deltas mainly from the southern margin (Li et al., 2008Luo and Pang, 2008;Ma et al., 2012). The unique tectonic setting represented by two kinds of fault systems vertically, including a deep antithetic fault system and a shallow consequent fault system, is colloquially referred to as a "double layer structure" (Luo and Pang, 2008;Liu et al., 2012;Ma et al., 2012). ...
... Bounded by the Lingao Fault to the west, the Anding Fault to the south and Changliu Fault to the east (Figs. 1 and 2), accumulating sediment is supplied through fan deltas mainly from the southern margin (Li et al., 2008Luo and Pang, 2008;Ma et al., 2012). The unique tectonic setting represented by two kinds of fault systems vertically, including a deep antithetic fault system and a shallow consequent fault system, is colloquially referred to as a "double layer structure" (Luo and Pang, 2008;Liu et al., 2012;Ma et al., 2012). The transfer zone developed in the central region divides the whole depression into two independent sags (the Huangtong Sag in the west and the Bailian Sag in the east), resulting in different tectonic styles and sedimentary patterns between the western and eastern areas (Liu et al., 2012) (Fig. 2). ...
... On the basis of 3D seismic, drilling, logging, geochemistry and palaeontological data, the sequence stratigraphic framework of the Paleogene in the Fushan Depression has been constructed, dividing the Paleogene into three formations (Changliu Formation, Liushagang Formation and Weizhou Formation) and seven third-order sequences (SQEch, SQEls3, SQEls2, SQEls1, SQEwz3, SQEwz2 and SQEwz1) from bottom to top (Li et al., 2008Luo and Pang, 2008;Liu et al., 2012;Ma et al., 2012) (Figs. 2 and 3). As the primary oil and gas reservoir, the Liushagang Formation (SQEls) reaching a maximum thickness of 3000 m is the major depositional unit and the primary oil and gas reservoir of the Fushan Depression. ...
In sedimentary basins, a better understanding of the controlling effect of accommodation/transfer zones on stratal architecture and facies distribution can improve the success rate of locating hydrocarbon reservoirs. The Fushan Depression is a half-graben rift sub-basin located in the southeast of the Beibuwan Basin, South China Sea. In this study, comparative analysis of seismic reflection, palaeogeomorphology, fault activity and depositional facies distribution indicates that three different types of inner-basin slopes (i.e. multi-level step-fault slope in the western area, slope flexure zone in the accommodation zone area and gentle slope in the eastern area) were developed along the southern slope of the Fushan Depression, together with a large-scale accommodation zone located at the intersection of the western and eastern fault systems. Further analysis shows that the accommodation zone played an important role in controlling not only stratal architectural variability in the southern slope but also depositional facies distribution in the accommodation zone area. During the high-stand stage, the deposition of depositional systems was mainly controlled by sediment supply and the NW-trending transfer faults. Major drainage systems entered the Fushan Depression through the accommodation zone along the direction of sediment supply, and sedimentary flow paths were parallel to the accommodation zone axis with sedimentary flows constrained to the adjacent areas of the NW-trending transfer faults. By contrast, during the low-stand stage, the transfer faults had little control over depositional facies distribution, and the sublacustrine fan sediments diverted away from the accommodation zone and flew down along the northeast, oblique to the accommodation zone axis. The flow division in the lowstand stage might be greatly influenced by flow type and topography. In addition, the accommodation zone area demonstrated unique hydrocarbon accumulation models different from the western and eastern areas, suggesting that the future exploration should be conducted at different levels in the Fushan Depression.
... The Fushan Depression, located in the southeast of Beibuwan Basin, is a "dustpan" shape depression featured by zonary distribution in EeW direction and blocking distribution in NeS direction (Li et al., 2008Shi et al., 2011). Bounded by the Lingao Fault to the west, the Anding Fault to the south and Changliu Fault to the east (Figs. 1 and 2), accumulating sediment is supplied through fan deltas mainly from the southern margin (Li et al., 2008Luo and Pang, 2008;Ma et al., 2012). The unique tectonic setting represented by two kinds of fault systems vertically, including a deep antithetic fault system and a shallow consequent fault system, is colloquially referred to as a "double layer structure" (Luo and Pang, 2008;Liu et al., 2012;Ma et al., 2012). ...
... Bounded by the Lingao Fault to the west, the Anding Fault to the south and Changliu Fault to the east (Figs. 1 and 2), accumulating sediment is supplied through fan deltas mainly from the southern margin (Li et al., 2008Luo and Pang, 2008;Ma et al., 2012). The unique tectonic setting represented by two kinds of fault systems vertically, including a deep antithetic fault system and a shallow consequent fault system, is colloquially referred to as a "double layer structure" (Luo and Pang, 2008;Liu et al., 2012;Ma et al., 2012). The transfer zone developed in the central region divides the whole depression into two independent sags (the Huangtong Sag in the west and the Bailian Sag in the east), resulting in different tectonic styles and sedimentary patterns between the western and eastern areas (Liu et al., 2012) (Fig. 2). ...
... On the basis of 3D seismic, drilling, logging, geochemistry and palaeontological data, the sequence stratigraphic framework of the Paleogene in the Fushan Depression has been constructed, dividing the Paleogene into three formations (Changliu Formation, Liushagang Formation and Weizhou Formation) and seven third-order sequences (SQEch, SQEls3, SQEls2, SQEls1, SQEwz3, SQEwz2 and SQEwz1) from bottom to top (Li et al., 2008Luo and Pang, 2008;Liu et al., 2012;Ma et al., 2012) (Figs. 2 and 3). As the primary oil and gas reservoir, the Liushagang Formation (SQEls) reaching a maximum thickness of 3000 m is the major depositional unit and the primary oil and gas reservoir of the Fushan Depression. ...
Drilling has shown that there is a large petroleum reserve in the Liushagang Formation of the Fushan Depression, South China Sea from reservoirs of the sublacustrine fans. To reduce the risk associated with exploration, the sedimentary characteristics of the sublacustrine fans should be identified and described. Analysis of the drilling cores, well data and 3D seismic data was conducted to identify and interpret the sublacustrine fans which formed in a lacustrine succession as the low-stand fan of the 2nd member of Liushagang Formation (SQEls2). The reservoirs were commonly associated with pebbly conglomerates, mudstone shivers, muddy debris, various deformation structures including micro-faults, liquified deformation structure, ball and pillow structure, slump and stress-release deformation. Further contrastive analysis of sedimentary characteristics and palaeogeomorphology between the western and eastern sublacustrine fans indicates that they belong to two different types of sublacustrine system in continental basins. More specifically, the western fan, controlled by the multi-level step-fault belt, is a typical gravel-rich sublacustrine system which is characterised by structurally controlled, high gradient (2.8-3.3°), admixtures of muddy debris and pebbly conglomerates and poor reservoir quality, while the eastern fan, controlled by the flexure slope-break belt, is a sand-rich sublacustrine system dominated by relatively low gradient (2.0-2.5°), general fining-upwards trend with good reservoir quality. Therefore, the development of future exploration should be conducted at different levels in the Fushan Depression.
... In 1977, the first well in the Beibuwan Basin was drilled, showing that the Palaeogene Liushagang Formation (E2L) had good source rocks, and the industrial oil flow was tested in the third member of the Liushagang Formation (E2L3) [8]. After that, the source rock evaluation in the Beibuwan Basin started to gain increasing attention, and it is believed that the source rocks of the Paleogene Liushagang Formation function as the main source rocks in the Beibuwan Basin [9][10][11][12], However, since there are multiple sets of source rocks, like seven sub-members of the Liushagang Formation that have developed in the depression [13][14][15], the quality and hydrocarbon seepage characteristics are still unclear. This has limited the understanding of the resource potential and the selection of favorable resource fields. ...
... The Fushan Depression has a total area of 2920 km 2 and is located on the southern margin of the Beibuwan Basin on the continental shelf of the South China Sea and the slope of the Haizhong uplift. The shape of the nearly triangular depression in the Fushan Depression is closely related to the left-lateral strike-slip of the NW-trending Red River Fault and the clockwise rotation of the Palaeogene Hainan Island [11,[44][45][46]. On the western side, it is adjacent to the Lingao uplift, and on the eastern side, the Yunlong uplift is visible [45,47]. ...
This study presents an integrated approach using organic geochemistry and incident-light organic petrographic microscopy techniques to characterize the kerogen type, hydrocarbon potential, thermal maturity, and effective depositional environment of the Eocene Liushagang Formation intervals in the western Huangtong Sag, eastern Bailian Sag, central Huachang Sub-uplift, and Southern Slope Zone area in the Fushan Depression, Beibuwan Basin. The results show that the hydrocarbon potential of these organic-rich lacustrine shale areas is mainly dependent on the depositional environment and the present-day burial depth of sediments. Oscillations and transitions between (i) rocks with dominant allochthonous organic matter (including primary/reworked vitrinite and inertinite macerals and terrestrial debris particles) representing a large influence of continental sediments (e.g., source supply direction) and (ii) rocks with dominant autochthonous organic matter (e.g., alginite) indicate a distal and stable lacustrine basin depositional environment. The source rock thickness ranges from 40.1 to 387.4 m. The average TOC of the Liushagang Formation in the Fushan Sag is between 0.98% and 2.00%, with the highest organic matter abundance being in the first and second sections of the Liushagang Formation, presenting as high-quality source rocks. The organic matter is predominantly Type II1 and Type II2. The highest vitrinite reflectance (1.14%) is in the Huangtong and Bailian Sags. The source rocks of the second section of the Liushagang Formation are primary hydrocarbon generators, contributing 55.11% of the total generation. Hydrocarbon sequestration peaks at %Ro 0.80%, with a maximum efficiency of 97.7%. The cumulative hydrocarbon generation of the Liushagang Formation is 134.10 × 108 tons, with 50.52 × 108 tons having been expelled and 83.58 × 108 tons remaining. E2L2X and E2L2S have maximum hydrocarbon displacement intensities of 184.22 × 104 t/km² and 45.39 × 104 t/km², respectively, with cumulative displacements of 52.99 × 108 tons and 15.58 × 108 tons. The oil and gas accumulation system is highly prospective, showing significant exploration potential.
... Vertically, however, crude oils in the shallower reservoirs have lower 4-/1-MDBT and 1-/4-MDBF values ( Table 1). The Chaoyang oilfields are situated in the northern fault step belt and the widely developed consequent fault may be the principal petroleum migration pathway for accumulations in the Chaoyang oil fields (Figure 14) [64]. The Hongguang and Meitai oilfields are located in the southern slope area, mainly developed the braided river delta in Els 3 [4,5,23]. ...
... Vertically, however, crude oils in the shallower reservoirs have lower 4-/1-MDBT and 1-/4-MDBF values ( Table 1). The Chaoyang oilfields are situated in the northern fault step belt and the widely developed consequent fault may be the principal petroleum migration pathway for accumulations in the Chaoyang oil fields (Figure 14) [64]. ...
The Paleogene sandstone reservoir in the Huangtong Sag of the Fushan Depression is one of the most commercially prolific oil accumulations in the Beibuwan Basin, South China Sea. In this study, twenty-seven crude oil samples from four oilfields in the sag were geochemically analyzed and classified into three oil groups. Oils in the Meitai and Hongguang oilfields show similar geochemical characteristics, with relatively abundant C30 4α-methyl-24-ethylcholestanes and low contents of oleanane and C19+20 tricyclic terpanes (TT), and therefore may be derived from the same source kitchen. Oils from the Yong’an oilfield have a greater proportion of oleanane and C19+20 TT compounds. Oils from the Chaoyang oilfield have intermediate contents of these biomarkers and proportionate values of other related parameters. The unimodal distribution pattern of homogenization temperatures of fluid inclusions found in all the Paleogene reservoirs indicates a single episode of charging. Combining this understanding with the reconstruction of stratigraphy-burial and geothermal histories by 1-D basin modeling, the petroleum charging time was determined to be between 8–2 Ma (the end of the middle Miocene to the early Pliocene). Hydrocarbon migration orientation and charging pathways were traced using molecular indicators (4-/1-methyldibenzothiophene and 1-/4-methodibenzofuran). In the Chaoyang oilfield, the values of these geochemical parameters decreased with decreasing burial depth, indicating vertical oil migration along faults, which are plentiful in the sag. Sandstone bodies with deeper burial depths may therefore be the most promising exploration targets in the Chaoyang area. The Yong’an oils generally migrate from the northwest toward the southeast, suggesting that the source kitchen for the Yong’an oilfield is in the northwest. By similar inference, the hydrocarbon source kitchen for the Hongguang-Meitai area is likely to be on the north side of the Hongguang-Meitai area. By tracing these hydrocarbon charging pathways, the northwest of the Yong’an area and the northern part of the Hongguang-Meitai area are identified as the most promising areas for exploration.
... Basing on the investigation of fault zones and the laws of faults controlling reservoirs [26][27][28][29][30][31][32] , the No. 2 fault zone in the dominant oil and gas enrichment area is taken as the research object. This study focuses on the echelon faults and structural traps in the No. 2 fault zone, and the research data includes relevant data of faults and traps on different survey lines( ...
The Weixinan Sag in Beibuwan Basin is a pilot area of the offshore exploration and development integrated technology. The main oil bearing layers, first member of Liushagang formation and third member of Weizhou formation, develop many fault systems and structural traps. The key factors of the offshore exploration and development integrated technology are”the fault research”and “the effectiveness of structural trap”. Fault research includes the study of the geometry, dynamics, kinematics characteristics of fault, and the division of fault system. The effectiveness of structural trap include analogical studies on the structural characteristics of traps, quantitative studies on fault lateral sealability, fault vertical sealability, and prediction technology of oil column height. Synthesizing the relationship of the law of oil and gas distribution,the differential fault system in the internal of the No. 2 fault zone, and the effectiveness research of traps, the fault-controlled reservoir laws and the dominant fault-controlled reservoir mode are obtained. The research on fault and trap of offshore integrated exploration and development has been successfully applied in the Weixinan Sag. The 37 of 45 development evaluation wells have been drilled successfully and the drilling success rate is 82%. It has promoted the ODP(original drilling project) implementation of 5 oilfields, the adjustment implementation of 8 oilfields and made important contribution to the increase of reserves and production in the Weixinan Sag.
... Basing on the investigation of fracture zones and the laws of faults controlling reservoirs (Campagna.1994;Deng et al.2017;Gong et al.2007;Guo et al.1996;Luo et al. 2008;Wei et al.2001;Woodcock et al. 1986;Zhao et al.2019), the No. 2 fracture zone in the dominant oil and gas enrichment area is taken as the research object. This study focuses on the echelon faults and structural traps in the No. 2 fracture zone, and the research data includes relevant data of faults and traps on different survey lines. ...
The Weixinan Sag in Beibuwan Basin is a pilot area of the offshore exploration and development integrated technology. The main oil bearing layers, one member of Liushagang formation and third member of Weizhou formation, develop many fracture systems and structural traps. The key factors of the offshore exploration and development integrated technology are"the fracture research"and "the effectiveness of structural trap". Fracture research includes the study of the geometry, dynamics, kinematics characteristics of fracture, and the division of fracture system. The effectiveness of structural trap include analogical studies on the structural characteristics of traps, quantitative studies on fault lateral sealability, fault vertical sealability, and prediction technology of oil column height. Comprehensive the relationship of the law of oil and gas distribution,the differential fracture system in the internal of the No. 2 fracture zone, and the effectiveness research of traps, fault-controlled reservoir laws and the dominant fault-controlled reservoir mode are obtained. The research on fracture and trap of offshore integrated exploration and development has been successfully applied in the Weixinan Sag. The 37 of 45 development evaluation wells have been drilled successfully and the drilling success rate is 82%. It has promoted the ODP implementation of 5 oilfields, the adjustment implementation of 8 oilfields and made important contribution to the increase of reserves and production in the Weixinan Sag.
... The migration of hydrocarbons controls hydrocarbon accumulation, and it has been widely accepted that faults are the secondary migration pathway for hydrocarbons. This can become the main controlling factor of oil and gas accumulation in faulted basins, in particular [15][16][17][18][19][20]. However, the controlling factors of oil and gas distribution in structurally complex reservoirs such as the Wang Guantun area in the Bohai Bay Basin with well-developed growth faults are still unclear and require further studies. ...
Mechanisms that lead to different quantities of hydrocarbon accumulation in complex fault blocks are a major subject that impacts further development plans of oil and gas fields. To better understand such mechanisms, fault activity has been interpreted along with existing electron micrographs from the fault zone, petrophysical data, and the occurrence of the seismic pump. This enabled us to investigate the controlling mechanisms of the growth faults and other associated faults with the main growth fault in the Wang Guantun area that could have impacted hydrocarbon distribution. The results showed that the activity of Kongdong growth fault is periodic and intermittent, which produced strong seismic pumping action. Furthermore, a series of secondary faults were generated in adjacent strata due to the fault activity, which could have led to the formation of a secondary seismic pump source. A combination of these two incidents is believed to influence the differential distribution of hydrocarbons in the area, in fault-associated reservoirs. Ultimately, we correlated the activity of the growth fault to the strength of the pumping force causing the distribution of hydrocarbons in the active parts of the faults (pump source position) on the horizontal plane and vertically located reservoirs to be more dominant.
... The Fushan Depression was developed on the basement of the Paleozoic strata and the Mesozoic intermediate acidic igneous rocks (Chen et al., 1991). It is a NE-E trending depression bounded by the Changliu Fault to the east, the Anding Fault to the south and the Lingao Fault to the northwest, respectively, which is filled with over 9000 m of Cenozoic sediments with an area of 2920 km 2 ( Fig. 3) (Luo and Pang, 2008;Ma et al., 2012). ...
Carbazole (CA) and alkylated carbazoles are common nitrogen-containing heterocyclic aromatic compounds widely found in crude oils and sedimentary rocks extracts. Their geochemical significances in petroleum exploration have been widely investigated in previous studies. Using density functional theory (DFT) and molecular dynamics (MD) simulation, we calculated the polarity of alkylated carbazole isomers/homologues and their adsorption energy with water and α-quartz, respectively. The calculated results show that there are significant differences in migration fractionation effects for alkylated carbazole isomers/homologues. This study confirmed the validity of previously proposed carbazole migration indicators, i.e. 4-/1-MCA (the relative abundance of 4-methylcarbazole to 1-methylcarbazole), 2,5-/1,8-DMCA (2,5-dimethylcarbazole to 1,8-dimethylcarbazole) and 2,4-/1,8-DMCA (2,4-dimethylcarbazole to 1,8-dimethylcarbazole) on the basis of quantum chemistry and molecular simulation. All these three parameters have been successfully applied in a lacustrine sandstone reservoir in the Fushan Depression, Beibuwan Basin (South China Sea). The calculations of Gibbs free energy show that the differences in thermal stability between the isomers of alkylcarbazoles can be negligible. The source input and depositional environment have no significant effect on these indicators for oils from a same oil family. Hence, the values of these parameters are mainly controlled by subsurface petroleum migration processes. Therefore, the molecular parameters relative to alkylated carbazoles can be used as practically effective geochemical markers to trace the oil filling orientation and pathways for subsurface oil migration.
... The depression is bounded by the Lin'gao Fault, the Anding Fault, and the Changliu Fault in the northwest, south, and northeast, respectively. A unique Bdouble-layer structure^was identified in the depression as it represents two kinds of fault systems, i.e., a deeper antithetic, south-dipping fault system and a shallower north-dipping fault system (Lin et al. 2015;Liu et al. 2012;Luo and Pang 2008;Ma et al. 2012) (Fig. 1c). The transfer zone, formed in the central region, separated the depression into two sags, i.e., the Huangtong Sag in the west and the Bailian Sag in the east (Liu et al. 2012) (Fig. 1b). ...
The Fushan Depression is a hydrocarbon-bearing half-graben rift sub-basin located in the southeast of the Beibuwan Basin, South China Sea. The sublacustrine fan systems in the Paleogene Liushagang Formation have recently become important targets in this depression, while the depositional process and detailed characterization of these systems are little known. Analysis of drilled cores, wire-line logs, and 3-D seismic survey suggest that the Fushan Depression develops two different types of sublacustrine fan systems. The fine-grained sublacustrine fan system located in the western area is mainly composed of fine- to medium-grained light gray sandstones interbedded with layers of silt-rich shales. This kind of sediment slumped from the distal bar of the delta front due to high sediment pore pressures at the slope edge as well as the activity of the Meitai Fault. By contrast, the coarse-grained sublacustrine fan system formed in the eastern area is characterized by medium to coarse gray sandstones and conglomerates with the development of various deformed lenticular beddings, deformed pillow structures, and micro-faults. Further study suggests that its sedimentation process is closely related to the geomorphology of flexure slope-break belt. The slope changed abruptly from 3°~5° to 7°~9° across the flexure slope point, increasing the accommodation space under the flexure belt. The coarse-grained sublacustrine fan formed when coarse-grained sediment spillover the slope-break toward the increased accommodation space. As the reservoir quality of these sublacustrine fan systems is well controlled by the sedimentary characteristics and sedimentation processes, our results should be of great significance for the development of future exploration.
... Pervious seismic-and well-based investigations have been mainly focused on the slope and marginal areas of the Fushan sub-basin [16]. As a result, slope studies have been more extensive than that for the sub-lacustrine areas [17]. With an increasing demand on petroleum resources in China, the Liushagang formation has become a significant hydrocarbon-bearing formation in the Fushan Oilfield. ...
The Fushan Depression is a half-graben rifted sub-basin located in the southeast of the Beibuwan Basin, South China Sea. The Paleogene Liushagang sequence is the main hydrocarbon-bearing stratigraphic unit in the sub-basin. Using three-dimensional (3-D) seismic data and logging data over the sub-basin, we analyzed structural styles and sedimentary characteristics of the Liushagang sequence. Five types of structural styles were defined: ancient horst, traditional slope, flexure slope-break, faulted slope-break and multiple-stage faults slope, and interpretations for positions, background and development formations of each structural style were discussed. Structural framework across the sub-basin reveals that the most remarkable tectonic setting is represented by the central transfer zone (CTZ) which divides the sub-basin into two independent depressions, and two kinds of sequence architectures are summarized: (i) the western multi-stage faults slope; (ii) the eastern flexure slope break belt. Combined with regional stress field of the Fushan Depression, we got plane combinations of the faults, and finally built up plan distribution maps of structural system for main sequence. Also, we discussed the controlling factors mainly focused on subsidence history and background tectonic activities such as volcanic activity and earthquakes. The analysis of structural styles and tectonic evolution provides strong theoretical support for future prospecting in the Fushan sub-basin and other similar rifted basins of the Beibuwan Basin in South China Sea.
... The Fushan Depression, situated in the south of the Qiongzhou Strait, north of Hainan Island, is a Mesozoic -Cenozoic rifting half-graben (Li et al., 2008a(Li et al., , 2009Shi et al., 2011). This NE-SW trending depression, forming the southeastern part of the BBWB, is bounded by the Changliu Fault in the southeast, the Anding Fault in the south, and the Lingao Fault in the northwest (Li et al., 2008a(Li et al., , 2009Luo and Pang, 2008;Ma et al., 2012;Liu et al., 2014). The Fushan Depression is filled with over 9000 m of Cenozoic sediments in an area of approximately 3000 km 2 , and one third of this depression lies offshore. ...
Dibenzothiophenes (DBTs) and benzo[b]naphthothiophenes (BNTs) are important sulfur heterocyclic aromatic compounds in oils and sedimentary rock extracts. Based on both migration fractionation effects and differences in thermal stability of different isomers, DBTs and BNTs can be used to trace oil migration orientations and filling pathways. They are present in significant abundances in oils, including light oils and condensates, in which biomarkers are commonly absent or present only in extremely low concentrations. Molecular biomarker compositions indicate that all oils and condensates in the Eocene clastic reservoir of the Fushan Depression (Beibuwan Basin, South China Sea) belong to a single oil population. In this study, three geochemical indicators relating to DBTs and BNTs, namely; (1), 4-/1-methyldibenzothiophene (4-/1-MDBT）; (2), total content of DBTs; and (3), benzo[b]naphtha[2,1-d]thiophene/(benzo[b]naphtha[2,1-d]thiophene +benzo[b]naphtho[1,2-d]thiophene) ([2,1]BNT/([2,1]BNT+[1,2]BNT)) were applied to trace oil migration orientations and filling pathways. The results show that these parameter values gradually decrease from northeast to southwest, towards the Huachang Uplift, and from north to south towards the Bailian Faulted-Belts. Therefore, the source kitchen for identified oil accumulations in the Huachang Uplift is likely to be located in the Bailian Sag, which lies to the northeast of the Huachang Uplift. Zones in the Bailian Sag, situated upstream of the preferred oil filling pathways are likely to be the most favorable prospecting regions. It is concluded that relative parameters of DBTs and BNTs are practical molecular indicators for tracing oil migration orientations and filling pathways.
Beibuwan Basin is an important offshore oil and gas bearing basin of China. However, the distribution of oil and gas in different sags is uneven, and the rule of hydrocarbon accumulation and main controlling factors have not been clearly understood in this basin. Based on abundant seismic, well and analytical data, the differences and main controlling of tectonic evolution, sedimentary filling and source rock quality in sags are analyzed, and combined with the study of geothermal filed, the main controlling factors of differential hydrocarbon generation in sags of Beibuwan Basin are clarified. On the basis of the research above, the hydrocarbon enrichment of the basin is clarified through the systematic analysis of reservoir‐caprock assemblage and accumulation characteristics of different structural units. The results showed that there are four NE trending sag zones in the basin, and under the influence of fault activity, the scale of sags is regularly distributed. The scale of rifts in different sag zones decreases gradually from northwest to southeast and the scale of rifts in the middle of a single sag zone is normally larger than that at two ends. Under the control of the activity sag‐controlling faults, paleoproductivity and organic matter preservation conditions, the quality of source rocks in sags of Beibuwan Basin decreases from northwest to Southeast. The hydrocarbon‐rich sags in Beibuwan Basin were formed under the control of source rocks and geothermal field, and the distribution of large‐medium scale fields is controlled by the favorable structural traps in the rift strata of inner gentle slope zone. The understanding of the hydrocarbon enrichment and main controlling factors in Beibuwan Basin also provides a significant guidance for the oil and gas exploration in other rift basins.
The control effects of different occurrence faults on oil and gas accumulation and distribution in the outer slope area of oil and gas reservoirs were studied taking the south-central Wen'an slope of the Jizhong depression in the Bohai Bay Basin as an example. Based on 3D seismic data and the distribution of oil and water, the controlling differences between consequent fault and antithetic fault were analyzed and compared from the formation and evolution rule of faults and the formation mechanism of fault traps, including development positions of the consequent fault traps and antithetic fault traps, oil and gas distribution horizon adjusted by fault and formation period of fault traps. The differences between consequent faults and antithetic faults in controlling reservoirs have three main aspects: (1) Consequent fault traps and antithetic fault traps are in different positions, the consequent fault traps are at the segmented growing point in the hanging wall of “hard-linkage” faults, while the antithetic fault traps are developed in the position with the largest throw in the footwall because of tilting action; (2) The two kinds of faults result in different oil and gas distribution vertically, oil and gas adjusted by consequent faults is distributed in a single layer or multi-layers, while oil and gas adjusted by antithetic faults occur in single layers; (3) The two kinds of fault traps are formed in different periods, the consequent fault traps are formed at the time when the related faults enter the stage of “hard-linkage” while the antithetic fault traps are formed at the beginning of the fault active period. © 2018 Research Institute of Petroleum Exploration & Development, PetroChina
The oblique transfer zone in the Fushan Sag, a syndepositional dome sandwiched between the Bailian and Huangtong sub-sags, has been the most important exploration target. The major oil observation occurs in the E2l
1L+M and the E2l
3U. 46 oil and rock samples reveal that the oil in the transfer zone is mostly contributed by the Bailian sub-sag, though the source rock conditions, hydrocarbon generation and expulsion histories of the Bailian and Huangtong sub-sags are similar. The E2l
3U oil, characterized by high maturity, Pr/Ph ratio and oleanane/C30-hopane ratio, shows a close genetic affinity with the E2l
3b source rocks, while the E2l
1L+M oil, characterized by lower maturity, Pr/Ph ratio and oleanane/C30-hopane ratio, is suggested to be derived from the E2l
1+2b source rocks. The homogenization temperatures of aqueous fluid inclusions, taking the burial history of the reservoirs into account, reflect that the oil charge mainly occurred from mid-Miocene to Pliocene in the oblique transfer zone. The oil transporting passages include connected sand bodies, unconformities and faults in the Fushan Sag. Of these, the faults are the most complicated and significant. The faults differ sharply in the west area, the east area and the oblique transfer zone, resulting in different influence on the oil migration and accumulation. During the main hydrocarbon charge stage, the faults in the west area are characterized by bad vertical sealing and spatially dense distribution. As a result, the oil generated by the Huangtong source rocks is mostly lost along the faults during the vertical migration in the west area. This can be the mechanism proposed to explain the little contribution of the Huangtong source rocks to the oil in the oblique transfer zone. Eventually, an oil migration and accumulation model is built in the oblique transfer zone, which may provide theoretical and practical guides for the oil exploration.
Based on the interpretation of the 3D seismic data, the sequence patterns and sedimentary characteristics in the sedimentary period of Liushagang formation were studied taking the transfer zone of the Fushan sag as the research project. The results show that the transfer zone developing above the Huachang in the centre of Fushan sag, plays a crucial role in adjusting the structure differences between the western and eastern areas. There are significant differences on the strike of faults and structure styles between two extensional structure systems, which develops at two lateral sides of the transfer zone respectively. The multi-level step-fault belt type of sequence pattern develops in the western area, while the gentle slope type of sequence pattern in the eastern area. The flexure slope break type of sequence pattern develops in the transfer zone with huge-scale braided channel delta in the high system tract and large-scale turbidite fan in the low system tract. Structural transfer zone, relative lake level changes and paleogeomorphy control the sequence architecture and depositional filling of the Fushan sag together, and this controlling effect varies among different system tracts to some extent.
Types of found reservoirs and its distribution characteristics of Putaohua oil layer in Sanzhao Sag of Songliao Basin were analyzed. The controlling factors of hydrocarbon distribution were investigated. Sanzhao Sag is Sag-wide Oil-Bearing, but its distribution of oil and water is extremely complicated. The reservoir types are mainly fault block reservoirs, low amplitude structure reservoirs, fault-lithologic reservoirs and lithologic reservoirs. The distribution of reservoirs is mainly controlled by five geological factors: Distribution of mature source rocks control ranges and styles of hydrocarbon migration; hydrocarbon is mainly vertical migration in mature source rock; and laterally migration in crudity source rock. The distribution of (submerged) distributary channel sandbodies control spatial distribution of quality reservoir, quality reservoir formed by channel sandbodies accounted for more than 95%. Fault is predominant transporting pathway of hydrocarbon migration; the combination of faults and sandbodies controls ranges and distribution of hydrocarbon migration. Long-term inherited nose-like structure is predominant direction of petroleum migration, it induced oil and gas migration at a critical period of hydrocarbon accumulation and formed oil-gas accumulation area. Many factors temporal and spatial coupling control the final accumulation and distribution of hydrocarbon.
Based on the analysis on tectonic evolution of Fushan sag and prototype basin morphology in different evolutionary stages, the formation mechanism of the internal structure in Fushan sag was studied here. It is believed that the morphology evolution of Fushan sag not only results in internal dual faults, east-west division and tectonic transform zone, but also influences the development of depositional system. The results show that the tectonic formation of Fushan sag is mainly controlled by the activity of the master control fault, i. e., Lingao fault. The unbalanced activity characteristics of Lingao fault on the Paleogene period, which represent the larger extension in the eastern region than in the western region, lead to the Paleogene stratum of Fushan sag presenting triangular shape. The morphology evolution of Fushan sag in each evolutionary stage makes the sag evolve from the no-rotating half-graben morphology to the ramp-flat half-graben morphology. The sag morphology evolution coupled with the toughness characteristic of high plasticity mudstone in the second layer of Liushagang formation (E2ls2) results in the formation of dual faults, which consist of the lower reverse fault and the upper synthetic fault. The study also indicates that the master control fault in the sag is characterized by migration of activity rate in different evolutionary stages. This migration leads to that the rotational extension direction of the sag adjusts in different evolution periods, thus morphology evolution is unbalanced in the eastern and western regions of the sag. The east-west division and tectonic transform zone are mainly controlled by the imbalance of morphology evolution. ©, 2014, China University of Mining and Technology. All right reserved.
The hydrocarbon distribution was summarized and the main control factors of spatio-temporal differences were discussed through statistics of 250 boreholes in the isochronous sequence stratigraphic framework in Fushan Sag. The results show that hydrocarbon resources of Fushan Sag mainly distribute in SQEls3HST, SQEls2LST and SQEls1HST on vertical, which also have notable regional spatial difference in space, enrichment in the central transfer zone during the high system tract period while in the eastern area in the low system tract. There is some relation between hydrocarbon distribution and sequence (or system tracts) development styles. The main control factors of spatial differentiations include sedimentary facies, the development of transfer zone and different sequence models. According to these conclusions, two thoughts of three elements coupled controlling favorable exploration traps have been put forward, which will have significance to guide hydrocarbon exploration in Fushan Sag and enrich theories of other similar continental basins hydrocarbon exploration.
The present paper studied characteristics of the fault development and activities in the Beibuwan Basin and discussed control factors of the fault formation and evolution on hydrocarbon reservoirs. The study showed that four types of faults developed in the Beibuwan Basin, including the early rift fault, middle strike-slip extension fault, late extension fault and long-term active fault. In Cenozoic, the Beibuwan Basin experienced both evolutionary stages, i.e. the rifting stage in Paleogene and the subsidence stage in Neogene. There were three major periods of fault activities in the extensional rifting stage. There formed NE- and NEE-direction extension faults during Paleocene and Eocene. In Oligocene, NEE-direction strike-slip extension faults and approximately EW-direction sub-faults were formed under the influence of strike-slip extension. The formation and evolution of faults controlled the distribution of hydrocarbon generative depressions, accelerated the thermal evolution of source rocks to generate various traps of hydrocarbons and provided major pathways for hydrocarbon migration. Zones with long-term active faults adjacent to hydrocarbon generative depressions and central uplift zones in depressions are the most favorable area for hydrocarbon migration and accumulation.
The conditions of source rocks, reservoirs, transporting, sealing and traps were analyzed. The types and distribution of subtle reservoirs with various palaeogeomorphology backgrounds of slope-break in various sedimentary periods were predicted. The results show that affected by continuous uplift of the Hainan Uplift, continuous settlement of the depression area in the central of Fushan Sag and asymmetric tensile stress, two different structural slope-break types, including fault ramp zone affected by the Meitai Fault and flexure slope-break, are formed in the west and east of the gentle slope in south Fushan Sag respectively. The slope-breaks in the gentle slope are inner basin slope-breaks. Their control on sedimentary is affected by variations of the lake level and source supply significantly, which is weak in Liu 3 Member, and increased significantly in Liu 2 Member. The formation mechanism, scale and distribution of turbidite sandstone bodies are also different in different sedimentary period. Because of the discrepancy in gradient, extending range of the slope-breaks and accommodation space size, different kinds of slope-breaks show controllable variance on development scale of fan bodies and vertical distribution characteristic of sedimentary facies zone. The slope-breaks are favourable to form subtle reservoirs. The research results are of great significance to the deployment of oil and gas exploration.
Based on the regional tectonic analysis of Beibu Gulf basin, in this paper, the tectonic activities of Fushan sag is studied, the dynamic formation mechanism of the sag is discussed, and the formation mechanism of double-layer structure in Paleogene strata of the eastern area is analyzed. It is regarded that Fushan sag was under the dual effect of sinistral strike-slip of Red River fault and clockwise rotation of Hainan uplift. During the deposition period of the third member of Liushagang Formation (Els3), intense horizontal extension occurred during the rotation and pull-apart of the sag, which provided dynamic source for the antithetic faults of the lower tectonic layer in the eastern area. The rapid uplift of Hainan uplift led to the formation of gravity slumping growth faults in the upper tectonic layer. In the Early Oligocene, the intrusive igneous rocks in the second member of Liushagang Formation (Els2) formed a natural barrier. Finally, there formed the double-layer structure. The forming process of the double-layer can be divided into five major stages as follows: rifting embryonic period, extending faulted period of the lower tectonic layer, development period of middle plastic layer, formation period of tectonic system, and finalizing period of double-layer structure.
The Fushan Sag is a secondary tectonic unit in the southeastern coast of the Beibuwan Basin which is located in the northwestern of the South China Sea. Based on the interpretation of the 3D seismic data and the contrast of tectonic characteristics between the western and eastern area, a transfer zone is found in the centre of the sag. The transfer zone was mainly developed in the Palaeogene Liushagang Formation and located above the Huachang low uplift at the intersection of the western and eastern areas. Two extensional structure systems were developed respectively at two lateral sides of the transfer zone. There were significant differences on the strike of faults and structure styles between the two systems. The formation of the transfer zone was caused by the existence of strike-slip process with the development of flower structure. The transfer zone is the entrance of source and controls the distribution of sand and deposition system through the counseling function of regulation faults. The deep antithetic fault system is favorable to form various kinds of traps and control the most petroleum accumulation distribution. The complex structure system contributes to the formation of the fault block reservoir and fault-nose pool and improves the quality of sandstone reservoir. The transfer zone holds distinct hydrocarbon accumulation with two oil resources and various types of reservoir. In all, the transfer zone has a good control effect on hydrocarbon accumulation and has good prospects for hydrocarbon exploration.
Distributary channel sandstones of the Lower Cretaceous Helvetiafjellet Formation and underlying prodelta shales and thin-bedded sandstones of the Upper Jurassic Janusfjellet Formation exposed along the east coast of Spitsbergen are cut by syndepositional planar and listric faults forming collapse scars with depths of 35-90 m and widths up to 1.5 km. The fault zones mostly have a dip of 40-60°, a width of up to 2.3 m, and contain fault-parallel 3-5-m-long overlapping sandstone sheets with widths of 5-40 cm, and up to 2-m-thick fault-parallel sandy mudstones. The intrafault sandstones show fault-parallel banding resulting from differences in detrital clay content and grain size. The banding has been enhanced by selective, late diagenetic quartz cementation of the clay-poor bands. Thin clay laminae, now developed into fault-parallel stylolites, occur along the margins of the intrafault sandstones. The clay laminae do not emerge from clay layers in the fault blocks and are not clay smears. The laminae probably formed during faulting when fluidization within the fault zones allowed clay particles to move laterally and accumulate along the margins of the fault zones. There is no enhanced cementation or cataclastic deformation within the fault zones. The ability of the fault zones to act as capillary seals or barriers to fluid flow is therefore mostly determined by the clay laminae rimming the intrafault sandstones. Fractures filled by quartz and calcite cement containing oil inclusions that homogenize at 58-73°C probably were not produced by the syndepositional faulting and may have formed during uplift of the area.
A fault-seal study was performed for part of the Gullfaks field (North Sea) as an aid to improve reservoir management of this complexly faulted structure. The operator's (Statoil) map data were used to build a three-dimensional model of the fault network. Together with petrophysical logs, this model was used to compute the variation of fault-seal potential (shale gouge ratio or fault-zone percent shale) on each fault surface. Pressure data from exploration and production wells have been projected onto the modeled fault surfaces. The preproduction pressure differences at sealing faults (separating different hydrocarbon columns) provide information about the capillary seal at the faults. Across-fault pressure drops at particular times during production also have been displayed. These dynamic pressure drops provide a guide to the permeability of the fault zones after flow has started. The calculated fault parameters (displacement and shale gouge ratio) can be converted to fault-zone thickness and permeability and can be used to derive fault transmissibility modifiers for reservoir simulations. Maps have been produced showing reservoir juxtaposition areas and calculated fault permeabilities along faults throughout the study area. These parameters are compatible with the recorded pressure history and tracer movement between wells, and will enable key reservoir management decisions to be tested and optimized.