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Neotectonics and petroleum accumulation in offshore Chinese basins
Abstract
The Tertiary offshore Chinese basins have been undergoing the most rapid post-rift thermal subsidence since the end of the Miocene (about 5.2 Ma). The thermal subsidence is accompanied by tectonic re-activation. Tectonic movements, i.e. the neotectonics as defined in this paper, modify and control the ultimate petroleum accumulation and distribution in these offshore petroleum-bearing basins. The concept, distinguishing characteristics, genetic mechanism of the neotectonics as well as their controlling role on petroleum accumulation are presented and discussed in this paper. The main manifestations of the neotectonics in the offshore Chinese basins include the formation of unconformities on the end of the Miocene and the Pliocene within the sequences, the shift of sedimentation centers, the strong late-stage fault activities and the active natural earthquakes. The intensity, development mechanism of the neotectonics, and their controls on petroleum accumulation and distribution are quite different in different basins. Some case studies from the Bozhong depression in the Bohai Bay basin, the Yinggehai basin in the South China Sea, and the Xihu depression in the East China Sea basin indicate that the neotectonics plays an important role on late petroleum accumulation in the offshore Chinese basins.
... In the Bohai Bay Basin, tectonic motion since 5.1Ma is called "neotectonism" (Gong and Wang, 2001;Gong, 2004). The faults formed during neotectonism are termed as "neotectonic faults" (Gong, 2004;Gong and Wang, 2001;Hao et al., 2006;Gong et al., 2010). ...
... In the Bohai Bay Basin, tectonic motion since 5.1Ma is called "neotectonism" (Gong and Wang, 2001;Gong, 2004). The faults formed during neotectonism are termed as "neotectonic faults" (Gong, 2004;Gong and Wang, 2001;Hao et al., 2006;Gong et al., 2010). Though neotectonic faults display small displacement, these late-stage neotectonic faults have acted as significant vertical conduits for petroleum migration from deep to shallow Neogene traps (Cai and Luo, 2001;Mi, 2001;Hao et al., 2007;Gong et al., 2010). ...
... The results of homogenization temperature of fluid inclusions are similar to those of the previous studies of petroleum charging periods in Shijiutuo uplifts . These data suggest that the petroleum accumulation periods are featured by late-stage rapid migration and accumulation since the onset of neotectonism (Gong and Wang, 2001;Gong, 2004;Hao et al., 2006Hao et al., , 2007Gong et al., 2010;. ...
An important exploration breakthrough has been achieved in the western subsag of the Bozhong subbasin, but the hydrocarbon accumulation mechanism was not clear. The hydrocarbon migration and accumulation mechanism of the western subsag of the Bozhong subbasin is discussed based on the modeling of hydrocarbon migration pathways and the study of late-stage reactivation of neotectonic faults. Three processes for lateral petroleum migration and accumulation can be recognized in the studied area: 1) migration and accumulation along the T8 unconformity; 2) migration and accumulation within the Guantao Formaiton (N1g); and 3) interaction of migration and accumulation along the T8 unconformity and Guantao Formation (N1g). The reactivation of faults in the studied area began 5.1 Ma. Following hydrocarbon lateral migration, late-stage reactivated neotectonic faults serve as effective vertical conduits for hydrocarbon migration and accumulation into the shallow Minghuazhen Formation (N1m) reservoirs. The modeling results of preferential petroleum migration pathways (PPMPs) and favorable accumulation areas are consistent with the actual exploration results. Two kinds of potential exploration targets can be predicted in the northwestern Bozhong subbasin: the first kind is hydrocarbon accumulation areas near or within generative kitchens (e.g. Target 1); the other kind is hydrocarbon accumulation areas removed from the generative kitchens, but with hydrocarbon sourcing from multiple generative kitchen and numerous PPMPs (e.g. Target 2). Studying preferential petroleum migration pathways will help reduce exploration risk.
... Nearly 33% of the total oil production of China comes from the Bohai Bay basin (Hao et al., 2009(Hao et al., , 2011. The offshore area of the Bohai Bay basin (Figure 1) has been explored since the early 1960s (Gong et al., 2000;Gong, 2004). So far, more than 30 · 10 8 tons (19.65 · 10 9 bbl) of oil have been found, of which 85% have undergone biodegradation to varying extents (Guo et al., 2010). ...
... The postrift sediments consist of the Guantao, Minghuazhen, and Pingyuan Formations ( Figure 3). These formations are widespread and are dominated by fluvial deposits (Gong, 1997(Gong, , 2004Zuo et al., 2011). The offshore area of the Bohai Bay basin is distinctly different from other areas in the Bohai Bay basin in two ways. ...
... The offshore area of the Bohai Bay basin is distinctly different from other areas in the Bohai Bay basin in two ways. First, it has been the depositional center of the Bohai Bay basin since deposition of the Oligocene E 3 d began (Gong, 2004;Hao et al., 2007) and is filled with up to 12 km (7.5 mi) of Paleogene and Neogene sediments (Gong et al., 2000;Hao et al., 2009). Second, strong, late-stage faulting in the Bohai offshore area reactivated basement faults and created new faults in the Neogene and Quaternary sediments (Gong, 2004;Hao et al., 2009). ...
The vast majorityofdiscovered oilsin the Bohai offshore area have undergone biodegradation ranging from 1 to 9 on the PM scale (a scale to rank the level of biodegradation, proposed by Peters and Moldowan, 1993). The extent of distribution and biodegradation of all discovered oils in the Bohai offshore area was investigated systematically using geologic and geochemical data to reveal controlling factors of varying levels of biodegraded oils. Based on the analysis of the environment and material significances and the resistance to degradation of biomarkers, the biomarker parameter assemblage that is suitable for the oil-source correlation of severely biodegraded oils (higher than PM 6) in the Bohai offshore area was determined. The spatial distribution and biodegradation extent are mainly controlled by the current burial depth, the duration of biodegradation, the area of the oil-water contact (OWC), and a late strike-slip movement of the Tanlu fault. Almost all biodegraded oils are found in shallow reservoirs above 2000 m (6562 ft). The longer the oils are present in these reservoirs or the larger the area of the OWC the reservoirs show, the greater the extent of biodegradation will be. The late, strong strike-slip movement of the Tanlu fault may have significantly enhanced the biodegradation extent of several oils in fields located in the Tanlu fault zone by introducing oxygenated freshwater from the surface or near surface and creating a more suitable environment for biodegradation. The C19 tricyclic terpane/C23 tricyclic terpane, C24 tetracyclic/C26 tricyclic terpane, and gammacerane/C24 Tetracyclic do not seem to be influenced by biodegradation and show obvious differences between the three different source-rock intervals. Such a biomarker parameter assemblage can be used successfully to determine the origin of severely biodegraded oils (higher than PM 6) by correlating with extracts of possible source rocks in the Bohai offshore area. Copyright © 2017. The American Association of Petroleum Geologists. All rights reserved.
... The Bozhong subbasin is located in the offshore area of the Bohai Bay Basin. Deposition started in the early Oligocene (Gong, 2004), with thick Oligocene synrift sediments lying beneath the Miocene to Quaternary post-rift sediments (Figure 2). The Bozhong sag is located in the center of the Bozhong subbasin, and is also the center of sedimentation and hydrocarbon generation for the subbasin. ...
... During the synrift stage, sedimentation was mainly restricted to the grabens and half grabens with primarily lacustrine sediments of the Eocene Kongdian (Ek), Shahejie (Es), and Dongying (Ed) formations being deposited (Figure 2) (Wu et al., 2006). During the postrift stage, the Guantao (Ng), Minghuazhen (Nm), and Pingyuan formations (Qp) were deposited throughout the entire basin (Figure 2) (Xiao and Chen, 2003; Gong, 2004). ...
... Generalized stratigraphic column and tectonic evolution of the Paleogene sequence in the western slope of the Bozhong sag (Zhao and Zhao, 2002; Gong, 2004; Zhao et al., 2005; Feng and Xu, 2006; Xu et al., 2008 ...
By using recently acquired three-dimensional seismic data, a seismic-based sediment provenance analysis was conducted in the late Paleogene sequence of the western slope of the Bozhong sag, Bohai Bay Basin, where the main depositional center was between the Shaleitian uplift and the Shijiutuo uplift. Three styles of sediment-transport pathways were identified in the study area, including sediment transport via (1) faulted troughs, (2) incised valleys, and (3) structural transfer zones. The Paleogene deposits in the study area were primarily controlled by the faulted-trough pathways, which are northeast-southwest oriented in between different northeast-southwest-trending faults with sediments derived primarily from the Shaleitian uplift. The sediments to the east of the Shaleitian uplift were interpreted to have sourced via relatively long-distance transportation and deposited along the northeast-southwest-trending faulted troughs, forming a deltaic sediment belt. In contrast, sediments derived from the Shijiutuo uplift, which were transported by the incised-valley pathways and deposited in the southern margin of the uplift, formed proximal fan-deltas. The depositional systems in the study area are characterized by the coupling of source-faulted-trough pathway-deltaic-lacustrine deposits in the eastern margin of the Shaleitian uplift and that of source-incised-valley-pathway-fan-deltaic-lacustrine deposits near the southern margin of the Shijiutuo uplift. The proposed spatial distribution of the sand bodies extends the distribution range for potential reservoir sand bodies beyond the currently exploration area. This work may serve as a useful reference for sedimentary provenance analysis in other types of sedimentary basins. Copyright © 2014. The American Association of Petroleum Geologists. All rights reserved.
... Thicknesses are taken from representative profiles of Liaoning Bureau of Geology and Mineral Resources, 1989;Wang, 1989). Chen, 2003;Gong, 2004;Fig. 6b, c). ...
... Ar/ 39 Ar mineral ages of the Liaonan MCC are from Liu et al. (2008), that of the Yiwulüshan MCC from Zhang et al. (2002a), that of the Kalaqin MCC from Wang and Zheng, 2005, and that of the X-T ductile shear zone from this paper. (b) The Cenozoic sedimentary column of the Liaohe basin with a summary of the tectonic evolution (modified after Hu et al., 2001;Gong, 2004;Li et al., 2014). (c) Two selected cross-sections based on reflection seismic profiles show half-graben and graben structural framework of the Liaohe basin, which is the NNE extension of the Bohai Bay Basin. ...
The aim of the study to investigate the cooling and exhumation history of deep crustal rocks along the Taili-Yiwulüshan metamorphic core complex (MCC) corridor and correlate these with volcanic-clastic deposition in the Late Jurassic to Early Cretaceous supra-detachment Fuxin-Yixian basin in the west and the mostly Eocene Liaohe basin in the east. This corridor exposes low- to medium-grade metamorphic Neoarchean rocks and Late Triassic, Late Jurassic and Early Cretaceous granitic rocks, which exhumed from the middle to shallow crustal levels. New ⁴⁰Ar/³⁹Ar ages of muscovite, biotite, K-feldspar and apatite (U-Th-Sm)/He ages in combination with compiled published geochronological data from the granitic rocks constrain three major stages of cooling exhumation and deformation from Late Jurassic through Early Cretaceous to Eocene interrupted by Late Cretaceous tectonic quiescence. The onset of activity and exhumation and cooling is diachronous along the MCC corridor. At Taili in the south, the main activity of the sinistral shear zone is constrained between 160 and 140 Ma with major cooling from crystallization temperatures of granitoids from 700–750 °C to 160 °C (first stage). Along the Waziyu ductile detachment at the western border of the Yiwulüshan MCC major cooling and exhumation is bracketed between 130 Ma and 100 Ma (second stage) and thus indicates a northward progradation of deformation and exhumation activity. However, cooling ages as old as 159 Ma along the eastern margin of the Yiwulüshan MCC indicate an earlier initiation of cooling and westward rolling-hinge type propagation (first stage). Late Cretaceous and Paleogene times are characterized by a tectonically quiet period with only minor cooling and exhumation (stage three). The fourth and final stage of exhumation and cooling to very low temperatures occurred during Late Paleocene to earliest Eocene times and is associated with the transtensional or extensional opening of the Liaohe basin.
... The Liaodong Bay sub-basin, with proven oil reserves greater than 10 Â 10 8 tons (Hao et al., 2009a;Gong et al., 2010;Jiang et al., 2010), is one of the most petroliferous sub-basins in the Bohai Bay basin ( Fig. 1). This sub-basin has been explored since the early 1960s (Gong, 1997(Gong, , 2004Gong et al., 2000). Several large and medium oil fields such as the SZ36-1 and JX1-1 oil fields were discovered (Fig. 1B). ...
... These Formations were mainly deposited in fluvial-lacustrine environments (Lu and Qi, 1997;Gong, 1997;Wu et al., 2006). The postrift sediments including Guantao (Ng, 24.6e12Ma), Minghuazhen (Nm, 12-2Ma) and Pingyuan (Qp, 2-0Ma) Formations from bottom to top (see Fig. 3 in Liu et al., 2016) are widespread, and are dominated by fluvial deposits (Li et al., 2003;Gong, 1997Gong, , 2004Xiao and Chen, 2003;Yang and Xu, 2004). ...
The Liaodong Bay sub-basin is one of the most petroliferous sub-basins in the Bohai Bay basin. The fourth member of the Eocene Shahejie Formation (E2s4, 50.5–42 Ma) which is an important source rock interval in other sub-basins was rarely studied in the Liaodong Bay sub-basin. The hydrocarbon generating potential, biomarker assemblage and accumulation contribution for discovered oil fields of the E2s4 source rock interval in the Liaodong Bay sub-basin were studied using geological and geochemical data. The E2s4 source rocks in the Liaodong Bay sub-basin display relatively high total organic carbon contents (TOC) and Rock-Eval hydrogen indices (HI) and can be considered as a set of excellent oil-prone tendency source rocks. The source rocks interval is characterized mainly by relatively low Pristane/Phytane (Pr/Ph), C19/C23 tricyclic terpane (C19/C23 TT), C24 tetracyclic terpane/C26 tricyclic terpane (C24 Tet/C26 TT), low to medium hopane/sterane (H/S), medium 4-methyl steranes/ΣC29 steranes ratio (4-MS/ΣC29 ST), C35 22S/C34 22S hopane (C35/C34 SH) and high gammacerane/αβC30 hopane (G/H), extended tricyclic terpane ratio [ETR=(C28 TT + C29 TT)/(C28 TT + C29 TT + Ts)] and were deposited in anoxic to sub-oxic saline-water environment with no or minor terrigenous organic matter and medium dinoflagellate input. The maturity of E2s4 source rocks in the Liaozhong and north Liaoxi sags is too high to provide oils for fields. The E2s4 source rocks in the south and middle Liaoxi sag displayed opportune maturity within the oil window and were proved by oil-source correlation having important accumulation contribution for discovered oil fields in this area. The E2s4-derived oils should be taken seriously in the future exploration in the south and middle Liaoxi sag and on adjacent uplifts.
... Three important Paleogene source-rock intervals (Ed 3 , Es 1 , and Es 3 ) exist in the Bohai Sea, the offshore area of the Bohai Bay Basin (Figures 1A, 3). Because of rapid postrift thermal subsidence (Gong, 2004), the Bohai Sea experienced strong late-stage faulting starting at the end of the Miocene (ca. 5.2 Ma), with tectonic motions generally defined as neotectonic (Gong and Wang, 2001;Gong, 2004;Li, 2004). ...
... Because of rapid postrift thermal subsidence (Gong, 2004), the Bohai Sea experienced strong late-stage faulting starting at the end of the Miocene (ca. 5.2 Ma), with tectonic motions generally defined as neotectonic (Gong and Wang, 2001;Gong, 2004;Li, 2004). The Shijiutuo uplift is north of the Bozhong sag, the largest subbasin in the Bohai Sea (Figure 1). ...
In the Qinhuangdao 29 (QHD29) oil field, oil generated from the first member of the Shahejie source rocks is mainly contained in Paleogene reservoirs, whereas deeper oil sourced from the third member of the Shahejie (Es-3) source rocks is generally accumulated in Neogene reservoirs. The present study was undertaken to better understand the differences in petroleum accumulation in the QHD29 oil field and to provide suggestions for future petroleum exploration on the Shijiutuo uplift. Laser Raman spectroscopy reveals that carbon dioxide (CO2) exists with hydrocarbon gas in the same fluid inclusions. Measured homogenization temperatures of aqueous inclusions range from 80 degrees C to 160 degrees C (176 degrees F to 320 degrees F), indicating late-stage rapid petroleum accumulation with a charging time of no earlier than 5.1 Ma. The results of grains containing oil inclusions measurements reveal the presence of paleo-oil accumulation in the current natural gas column. In terms of the boundary fault activity rate (FAR), CO2 distribution is quite relevant to the late stage activity of boundary faults, with high content of CO2 corresponding to the section of the F1 fault with high-intensity activity (higher FAR values). Both Neogene and Paleogene reservoirs in well Al contained predominantly Es-3-derived oil and were accompanied by abundant mantle CO2. This reveals the segmented vertical transport of petroleum in the fault: both the mantle-derived CO2 and hydrocarbons vertically migrated and accumulated in shallower reservoirs in the high-activity intensity section of the boundary fault. This may account for the occurrence of predominantly Es-3-derived oil in the reservoirs near the section of the fault with high activity intensity. In the eastern part of the QHD29 oil field, vertical migration may have been limited because of the relatively low intensity of fault activity, and the distribution of sandstones seems to dominate the petroleum accumulation. Our research reveals that lithologic traps in the Es-3 stratum may still have great potential for exploration along the slope of the Shijiutuo uplift.
... Petroleum migration is a key factor to form large oilfields in the Bozhong subbasin. (3) Neotectonism is very distinctive in the Bozhong subbasin (Gong and Wang, 2001; Gong, 2004). The intense neotectonic motion is reflected by the densely distributed faults in the Neogene and Quaternary formations (Hao et al., 2007; Gong et al., 2010). ...
... In the Bohai Bay Basin, " neotectonism " is defined as tectonic motions since 5.1Ma, and the related faults are called as " neotectonic faults " (Gong and Wang, 2001; Gong, 2004; Hao et al., 2006 Hao et al., , 2007 Gong et al., 2010). Neotectonic faults developed mainly in the Neogene and Quaternary sediments. ...
... The 10, 000 km 2 Liaodong Bay subbasin, one of the six major sub-basins of the Bohai Bay basin (Allen et al., 1997), is located on the northeastern Bohai Bay basin (Fig. 1). This sub-basin has been explored since the early 1960s (Gong, 1997(Gong, , 2004Gong et al., 2000). So far more than 10 Â 10 8 tons of oil has been found (Hao et al., 2009a;Jiang et al., 2010). ...
... The postrift sediments contain Guantao (Ng, 24.6e12 Ma), Minghuazhen (Nm, 12e2 Ma) and Pingyuan (Qp, 2e0 Ma) Formations from bottom to top. These Formations are widespread, and are dominated by fluvial deposits (Li et al., 2003;Gong, 1997Gong, , 2004Xiao and Chen, 2003;Yang and Xu, 2004). ...
The Liaoxi uplift, one of the most petroliferous uplifts in the Bohai Bay basin, is sandwiched by two sags containing multiple mature source rocks. The origins of oil located on the northern end of this uplift, both in terms of source rock intervals and in terms of generative kitchens, were studied using biomarker distributions from 75 source rock extracts and 15 oils. Two oil families have been identified using 4 biomarker parameters and could be linked to the source rocks of the first and second members of the Shehejie formation (E2s1+2) and the third member of the Dongying formation (E3d3) respectively. E2s1+2- derived oils were found in the Liaoxi sag and on the Liaoxi uplift and were interpreted to originate primarily from the Liaoxi sag. E3d3-derived oils were found in the Liaozhong sag and originated from the Liaozhong sag. The Liaoxi uplift have not been charged by E2s3-drived oils from the Liaoxi sag mainly because it departed from the dominant migration direction of E2s3-drived oils. The Liaoxi uplift have not been charged by E2s3, E2s1+2 and E3d3-drived oils from the Liaozhong sag mainly because there were no delta sandstones developed on the slope between the Liaoxi uplift and the Liaozhong sag to serve as migration channels from source rocks to uplift in all three formations. The sandstone up-dip pinch out traps in E2s3 formation in the Liaoxi sag, traps within the Liaozhong sag and traps on the slope between the Liaozhong sag and the Liaodong uplift have a better chance to accumulating petroleum.
... The BZ19-6 structural belt on the deep structural ridge in the southwestern part of the Bozhong sag, namely the study area, is a tectonic belt surrounded by the southwest and south sub-sag of Bozhong Sag (Gong, 2004;Xue et al., 2015). The southeastern part of the BZ19-6 structural belt is adjacent to the Bonan low uplift and the southwestern part is connected to Chengbei low uplift. ...
The BZ19‐6 structural belt is the largest buried hill oil‐gas‐bearing area recently discovered in Bohai Bay Basin. The formation and evolution of the buried hill reservoirs are complex. Presently, the development law and controlling factors of this reservoir is unclear. Based on 3D seismic, drilling data, thin section, and regional tectonic background, we explored the controlling factors and development law of the high‐quality buried hill reservoir. The results show that the buried hill reservoir can be vertically divided into a weathered glutenite zone, weathered fracture zone, internal fracture zone, and tight zone. The development of reservoir is mainly controlled by lithotypes, tectonic movement, and weathering. The rocks in the study area with low compressive strength, provided the material basis for the formation of fractures. The strongly reformed multi‐stage tectonic movements provides the external dynamics for the formation of fractures. Weathering further improved the physical properties of the reservoir. The buried hill reservoir is mainly distributed in the weathered fracture zone and internal fracture zone. Currently, natural gas mainly found in nearly E‐W‐trending and N‐S‐trending fractures. The nearly E‐W‐trending fractures are developed in the thrust nappe structure formed by N‐S‐trending extrusion in the Indosinian. The hinge zone of the structure is the main development area of fractures. The nearly N‐S‐trending fractures are associated with strike‐slip faults formed by sinistral strike‐slip stress in the Yanshanian. Under the background of NNE–SSW direction extensional stress during the Himalayan, early fractures were reactivated, opened and expanded, forming the reservoir space for later natural gas charging.
... The Paleogene basin shows obvious "basin-ridge" structure during the rifting stage, and primarily developed halfgraben and graben internal depressions with pervasive vulcanism (Wang, 1998). In the Neogene, the basin mainly experienced post-rifting thermal subsidence (Gong, 2004;Wang et al., 2012). From the perspective of the whole basin, the strata distribution in sags were stable during the depositional periods of the Kongdian and Shahejie Formations. ...
The hydrocarbon source of Bohai Bay Basin is dominated by oil-prone kerogens of type II2-II1 within semi-deep and deep lacustrine facies. In the Neogene period, faults were well-developed via significant structural activity. The Bohai Sea is generally considered to have no geological basis for the formation of large natural gas fields. Through analogous analysis of domestic and international gas fields, the key geological factors that restrict formation in continental rift lacustrine basins were studied, including gas source, preservation conditions, and reservoirs. A natural gas enrichment and accumulation model within a petroliferous basin is presented. The model indicates that rapid subsidence and high-intensity gas generation within petroliferous sags during the late stages were main contributors to natural gas field formation. Archean metamorphic buried hill reservoirs and thick, overpressure mudstone with strong vertical sealing ability provided favorable storage space and preservation, respectively. Using the model, an integrated Archaean metamorphic buried hill condensate gas reservoir, Bozhong 19–6, was discovered in the Bohai Bay Basin. The natural gas reserves are about 450×10⁹ m³, equivalent to oil production of 800 ×10⁶ m³, and signify a breakthrough in natural gas exploration of continental rift lacustrine basins in China.
... Based on the assessments of Wang et al. (2001) and Sun et al. (2003), the YBHB was in the initial rifting stage. As Gong (2004) argued, the YGHB was in a convergent setting. Meanwhile, the triangle diagram in Fig. 14 short catchment infilled by proximal sediments. ...
The northwestern South China Sea (NW-SCS) presents a spatial-temporal evolution of its source-to-sink (S2S) system, that recorded the movement of the Indochina block and the collision between the Indian and Eurasian plates. However, complex tectonic movements and topographic evolution have led to controversial studies about the spatial-temporal distribution of allogenic materials and autogenic sediments. In this study, we integrated a number of multiscale data to review the S2S system evolution of the northwestern area of the South China Sea from the Eocene to the Miocene. The series include the initial synrift, postrift and thermal subsidence stages. First, we identified the major sediment pathways using seismic profile interpretation. We reconstructed the sediment distribution and lithofacies association within the dispersal sink area based on seismic data, core samples and grain-size analysis. We quantitatively studied the sediment flux and catchment area based on mass balance calculation modeling. Following this, we acquired rare-earth element (REE) data and zircon U–Pb age data from the bedrocks of the continental blocks and the South China Sea Cenozoic basement and rivers in the NW-SCS area to restore the provenance area and evaluate the migration distance by inspecting the heavy mineral assemblages. According to the above studies, three phases of S2S dynamics were identified. 1) The initial rifting stage occurred during the Eocene, and active rifting in the South China Sea induced a major topographic change. A large amount of autogenic slump sediments and a smaller amount of paleo-Red River sediments were injected into the Beibu Gulf Basin (BBGB) and the Yinggehai-Song Hong Basin (YGHB). 2) The postrift stage occurred during the Oligocene. Regional uplift and tectonic inversion probably linked to the clockwise rotation of the Indochina Block, broke the connection between the Beibu Gulf Basin and the Red River catchments. The paleo-Qin River and the paleo-Lian (Nanliu) River brought a large amount of clastic material into the Beibu Gulf Basin from the Cathaysia Block. 3) The thermal subsidence stage occurred during the Miocene: the eastward topographic tilt was enhanced, and the drainage basin entered postrift stage. The development of the Paleo-Red River and its branches, the Lam River and the Ma River, brought distal sediment into the YGHB, while the rivers in the west Cathaysia Block transported sediment into the BBGB. Statistical analysis of the S2S system parameters suggests that the average terrain altitude played a dominant role in determining the S2S system volume during the Eocene, while the dominant role transferred to the sediment supply during the Oligocene and Miocene.
... The syn-rifting stage is composed of four sub-stages: the initial (stage I), the expansion (stage II), the expansion and deep subsiding (stage III) and the contraction (stage IV) [37]. The syn-rifting sediments were deposited in lacustrine environments and restricted to the grabens and half grabens [40,41], while the post-rifting sediments are dominated by fluvial deposits [42,43]. ...
The fluid inclusion technique was utilized to reveal the petroleum charging events in the lithologic reservoirs embraced in the Late Eocene Shahejie Formation of the Minfeng sag, Bohai Bay Basin, East China. Petrography, fluorescence microspectrometry, and microthermometry were systematically carried out on 15 double-polished thin sections handled from reservoir core samples of the third Member of the Shahejie Formation. The results show that three generations of petroleum inclusions with fluorescence colors of yellow, yellowish green and bright blue were entrapped along the healed fractures in detrital quartz grains of these samples. The fluorescence features of petroleum inclusions illustrate that inclusion oils have different maturities and were products of source rocks at different stages. In addition, the trapping time of petroleum inclusions was determined by combining the homogenization temperatures of their coeval aqueous inclusions with thermal-burial histories. By integrating the petrographic occurrence, characteristics of petroleum inclusions, and the maturity and the trapping time of the studied inclusion oils, it is jointly constrained that the lithologic reservoirs of the Late Eocene Shahejie Formation in the Minfeng sag underwent three petroleum chargings, which occurred during 37.8~25 Ma, 11.7~3.5 Ma and 1.4~0.1 Ma, respectively. The petroleum from each charging period migrated from the center of the sag to the edge, and the lower the maturity of the migrating petroleum, the longer the migration duration.
... Following Hu et al. (2001), we refer the offshore portion of the BBB to the Bohai Basin. The study area experienced the same tectonic evolution with the onshore part of the BBB until the Late Pliocene when Neotectonism occurred (Gong, 2004). The term Neotectonism in the Bohai Basin refers to tectonic motions since 5.1 Ma driven by the far-field effect of the India-Eurasia collision (Xu et al., 2011), accounting for numerous nearly EW-trending faults and traps in the Neogene-Quaternary sequences Gong, Zhu, et al., 2010;Zou et al., 2011). ...
Thorough understanding of seabed fluid flow system is of great significance to geohazard identification and hydrocarbon exploration as it can reshape the seabed and act as an indicator of subsurface hydrocarbon resources. For the first time, an integrated study of side‐scan sonar, single‐ and multi‐channel seismic data and magnetic data reveals a complex fluid flow system composed of various seafloor expressions (i.e., pockmarks and mounds) and shallow fluid migration pathways in the central‐west Bohai Sea off northeast China. Gas chimneys, mud diapirs and a dense network of Quaternary faults are the main fluid migration pathways in the shallow subsurface. The gas chimneys can be classified into three categories (Type‐A formed by relatively rapid gas escape, Type‐B formed by episodic fluid expulsion and Type‐C formed by fluid escape from mud diapirs), based on their distribution and seismic character, implying variability in formation processes. Sediment remobilization and basement‐involved faults contribute to deep fluid migration into shallow depths. As a seal for up‐moving fluids, the nature and thickness of Holocene marine sediments generally decide the permeability and overburden pressure that may control the distribution of pockmarks and mounds since they are almost distributed above relatively thin Holocene deposits (thickness < 20 m) and localized coarse surface sediments. The results of the interpretation gain an improved understanding of the geological processes controlling the genesis and spatial distribution of gas chimney formation, and show the significance of gas chimney classification. The distribution pattern of different types of gas chimneys may signify the difference of geological background and fluid flow process, like fluid migration through faults or flow of mobilized sediments, that is crucial for the evaluation of global petroleum systems and Carbon Capture and Storage studies.
... Many newly developed faults in the Cenozoic are distributed in the middle segment, whereas few or no Cenozoic faults have developed in the northern and southern segments. The tectonic movement that occurred in the middle segment of the Bohai Bay Basin during the post-rift subsidence stage of the Neogene-Quaternary is called "Neotectonism" (Gong, 2004;Li, 2004). The characteristics of the tectonic movement in this period included the intensive development of shallow faults and high deposition rate. ...
Clay smear is an important formation mechanism of fault lateral sealing. The Shale Gouge Ratio (SGR) is often used to quantitatively predict clay smear, but the SGR mechanism only considers the clay smear of the hanging wall, ignoring the clay smear of the footwall and the influence of the mudstone distribution. Thus, the application effect of SGR is not ideal in certain cases. In this paper, based on a dynamic evolution model of clay smear in shallow siliciclastic rocks, the problems of SGR application are analyzed and ameliorated, and the Advanced Shale Gouge Ratio (ASGR) index for evaluating clay smears is proposed. We add two factors, the clay smear of the footwall and the mudstone distribution, to the clay smear evaluation and propose two ASGR algorithms, which differ according to whether each interior point in an individual mudstone is regarded as an independent calculation unit or all interior points are taken as a whole. The ASGR index is applied to an exploration case study of the Kendong uplift of the Bohai Bay Basin. The ASGR results of fault F1 show that, the clay smear effect of the footwall is superior to that of the hanging wall in most cases, and sometimes only the footwall plays a key role in clay smear, which illustrates the importance of the clay smear of the footwall. This study reveals the ASGR as an alternative method for the fault sealing evaluation of shallow siliciclastic rocks and can serve as a helpful reference for further research in this area.
... The buried hill stratum of the Archean is the main petroleum-bearing structure in this area and is the target of this paper. The Archean stratum consists of buried metamorphic rock hill reservoirs that has experienced multiple stages of tectonic movements, including the Caledonian, Hercynian, Indosinian, Yanshan, etc. [28], [29]. Early faults have considerable effects on the transformation of the buried hill metamorphic rock. ...
Reservoir classification is an important component of reservoir geological modelling and reservoir evaluation and identification. Using a single conventional logging curve to identify complex heterogeneous reservoir types has always been a difficult task in logging interpretation. For the first time, this study reveals the advantages of recurrent neural networks in the identification of heterogeneous reservoirs and proposes an optimal parameter bidirectional long short-term memory (Bi-LSTM) recurrent neural network reservoir classification model with optimal parameters that can make full use of logging sequence information. The data used in this work originate from 3 wells in the BZ gas field in China. First, the rationality of the data set and the generation of sequence data were studied in detail, and the logging curve response sequence data, which can fully characterize the reservoir characteristics, were obtained. Then, through multiple simulation experiments, the optimal network structure and hyperparameters were determined, and a Bi-LSTM network model with 5 hidden layers and the optimal network parameters was established. The model was used to predict fractured, pore-fracture and fracture-pore reservoirs in the buried hill metamorphic rock buried beneath the BZ gas field. A comparison with the prediction results of 5 classic machine learning methods and baseline models shows that the Bi-LSTM model with the optimal parameters is superior to the other machine learning methods, especially regarding the prediction accuracy of pore-fracture reservoirs, and the overall accuracy is 92.69%. The method proposed in this paper can accurately identify the strata developed in different types of storage space and significantly improves the reservoir identification accuracy.
... The Bozhong Depression has been the depositional centre of the Bohai Bay Basin since the Oligocene Dongying Formation was deposited (Gong, 2004;Hao et al., 2007) and is filled with up to 12 km of sedimentary rocks. The sedimentary provenance of the Bozhong Depression during the E 1-2 k to the E 3 d 2 L was predominantly derived from intra-basinal uplifts (e.g., the Shijiutuo Uplift and Shaleitian Uplift, Fig. 1B) until a provenance switch at the boundary between the E 3 d 2 L and the E 3 d 2 U ; subsequently, the dominant provenance was transformed to extra-basinal orogens . ...
The porosity-enhancing capability of secondary porosity in a closed sandstone geochemical system has been suggested to be small in past studies. However, the mechanisms responsible for the formation of abnormally high porosity/permeability remain unclear. The Oligocene Dongying Formation in the Bozhong Depression, offshore Bohai Bay Basin, preserves abnormally high porosity, i.e., 36.4% (average) or 42.1% (median) higher than background values. This work selects samples with abnormally high porosity (AHP), samples with normal porosity (NP) and samples with abnormally low porosity (ALP) and compares their differences in physical properties (porosities, permeabilities and thin section porosities), diagenetic minerals, pore structures, compaction and cementation, detrital compositions and grain texture in an attempt to find the real factors controlling this heterogeneity. An effective export of dissolution by-products (Al³⁺ and Si⁴⁺) from AHP to NP and ALP leads to 2.01% (average) or 2.11% (median) newly created porosity in AHP, which is still significantly lower than the porosity difference between AHP and NP. In comparison, compaction reduction and mechanisms beneficial for compaction reduction should be the major factors controlling AHP formation. The content and distribution pattern of early carbonate cements, compared to the sedimentary factors (grain size and rigid grains), have significant influences on compaction reduction. Only samples with moderate contents (0.5–3.0%) of uniformly point-distributed early carbonate cements and with effective export of dissolution by-products can form AHP. Three formation models for ALP and two formation models for NP are also proposed. In summary, the evolution of porosity/permeability is controlled by early carbonate cements and induced compaction reduction (major causes) and by feldspar dissolution and redistribution of by-products (minor causes). This work provides new constraints on the formation of AHP and heterogeneity and is therefore important for sandstone reservoir quality prediction.
... The Bozhong depression is located in the central part of offshore Bohai Bay Basin and covers an area of 16,000 km 2 . It has been the depositional center since the beginning of the Oligocene (Gong, 2004), and the Cenozoic sediments can be up to 7.3 km thick. ...
Studying the fundamental influence of basin-floor topography on the growth and morphology of sedimentary systems has largely focused on submarine fans; however, in lacustrine settings, only a limited number of studies are primarily concerned with confined sublacustrine fans, although such fans can form high-quality hydrocarbon reservoirs. By using recently acquired three-dimensional seismic data, a series of unusual confined and semiconfined sedimentary units with seismic reflection patterns different from those of previous studies have been identified. The identified sedimentary units are developed in the Paleogene succession on the northern slope of the Bonan uplift, Bohai Bay Basin, China, and are characterized by either vermicular-like chaotic progradational reflections or coupled vermicular-like and progradational seismic reflections in the dip direction and channel-shaped bidirectional onlap reflections along strike directions. Through restored paleogeomorphology, six elongate incised valleys (V-1 to V-6) with diverse scales on the uplift slope are clearly documented. The identified sedimentary units were deposited within these incised valleys. The units identified in this study are considered “confined” or “semiconfined” based on the following several aspects, they: 1) were deposited within elongate depocenters along dip directions; 2) are clearly separated from each other by topographic highs along strike directions; 3) have belt-shaped sediment dispersal patterns in planar view; and 4) have the thickest deposits primarily distributed in the middle and distal parts. Through comparing sedimentary landforms, feeder systems, seismic reflection characteristics and spatial distribution patterns between sedimentary systems, the confined and semiconfined systems identified in this study display both similarities to and profound differences from previously studied confined sublacustrine fans and are here considered a new unusual type. We anticipate this work may serve as a useful reference for confined sedimentary system research in other types of sedimentary basins.
... In addition to long-term active faults cutting through both the Paleogene and Neogene intervals (Yang et al., 2017), there are also late-stage neotectonic faults in the Neogene since 5.1 Ma . These faults serve as main pathways for vertical hydrocarbon migration from the Paleogene source rocks to the reservoirs above (Gong, 2004;Gong and Wang, 2001;Xu et al., 2019;Zhu et al., 2009). ...
The Chengzikou Uplift (CZKU) is a second-order structure surrounded by several hydrocarbon generative kitchens in the Bohai Bay Basin. However, six recently drilled wells on the CZKU failed to prove hydrocarbon occurrences, which causes confusion for the understanding of hydrocarbon migration and accumulation on the CZKU. Mainly based on the seismic database, this paper attempted to determine the hydrocarbon migration pathways and enrichment mechanisms on the CZKU. The results show that the lateral and vertical hydrocarbon migration seems to take place through a stepped pathway system consisting of permeable layers within unconformities, permeable carrier beds, and faults. Through a simple 3D modeling considering topography changes only, the hydrocarbons tend to migrate from the northern sags to the southern slope and uplift with parallel mainstreams and accumulate with convergent mainstreams. The potential hydrocarbon traps mainly include stratigraphic, structural, and structural-lithologic traps, most of which have been confirmed by previous explorations. Subsequently, two preliminary interpretations for the dry well on the CZKU are made: the first is hydrocarbons mainly accumulated in the Paleogene stratigraphic (onlapping) traps within the sag and slope area, conducting no further migration to the uplift; the other is hydrocarbons may migrate to the slope and uplift but were trapped in structural-lithologic traps associated with faults beyond the area of the dry wells. Moreover, favorable hydrocarbon accumulation targets are predicted to reduce exploration risk.
... The BZ19-6 structural belt on the deep structural ridge in the southwestern part of the Bozhong sag, the Bohai Bay Basin, is an anticlinal structural belt outlined by the southwest sub-sag and south sub-sag of the Bozhong sag, in the form of an uplift surrounded by sub-sags ( Fig. 1) [25,26]. It is bordered to the Bonan low-bulge in the southeast, the Chengbei low-bulge in the west, the Huanghekou sag in the south, and the main sub-sag of the Bozhong sag in the north [22e24]. ...
The BZ19-6 deep buried-hill structural belt in the southwest of Bozhong Sag, Bohai Bay Basin, is a newly discovered super-giant oil and gas bearing area. The study on its reservoirs is still in the early stage, and the characteristics and control factors of reservoir development are not understood deeply. In this paper, cores, sidewall cores, rock sections were analyzed and described. Then, based on regional structural setting, mud logging and logging data, the buried-hill reservoirs in this area were analyzed from the aspects of petrological characteristics, reservoir space types and physical properties, the inherent factors influencing the development of the reservoirs were discussed, and distribution laws of the reservoirs were investigated. And the following research results were obtained. First, the deep buried-hill reservoirs of this belt are a pan-buried hill reservoir system composed of the Palaeocene–Eocene Kongdian Fm glutenite in the upper part and the Archean buried-hill metamorphic granite in the lower part. A multi-layer reservoir structure of glutenite pore zone, weathering crust dissolution fracture zone and inner fracture zone is formed. These reservoirs are complex in genesis and diverse in type. Second, the Archean buried-hill metamorphic granite reservoir can be vertically divided into weathering crust, inner fracture zone and tight zone, and it presents the dual characteristics of porous and fractured media. Third, the buried-hill weathering crust is mainly affected by strong dissolution and leaching superimposed with fracturing, forming fractured-porous reservoir space. The reservoir of inner fracture zone is mainly controlled by the superimposition of three-phrase fractures, which forms the main development period of buried-hill fractures since the Yanshanian. Fourth, the glutenite of Kongdian Fm is a typical sieve deposit and it is mainly controlled by the late dissolution. Fifth, migmatization and supercritical fluid cryptoexplosion play a constructive role in the development of the reservoirs. In conclusion, the understanding of buried-hill glutenite and metamorphic reservoir system developed in this belt is conductive to determining the target and direction of next oil and gas exploration in this area. Keywords: Bohai bay basin, Bohai sea area, BZ19-6 deep buried-hill structural belt, Archean, Deep metamorphic granite reservoir, Paleogene, Glutenite reservoir, Dissolution, Structural fracture
... The ooid deposits indicate a high nearshore carbonate productivity corresponding to a regional carbonate production that took place at onshore setting during Member 1 in the Bohai Bay Basin (Du, 1990;Sun et al., 2008;Yang, Qiu, Gregg, Puckette, & Liu, 2017). The reasons for high nearshore carbonate productivity are inferred by (a) a low tectonic activity and subsidence rate during this studied intervals throughout the whole basin (Gong, 2004;Hao et al., 2010); (b) a relatively warm water temperature due to an arid climate (Zhang, Liu, & Zhang, 2005); (c) relatively shallow depth (shoreface setting) for carbonate sediments, especially allochems, to be deposited. ...
Mixed siliciclastic–carbonate sediments were well developed in the BZ‐X block as a subunit in the Huanghekou Sag in the southeastern part of the Bohai Bay Basin (China) during the Member 1 of the Shahejie Formation. An integrated approach based on detailed microscopic observation and core descriptions was applied to characterize the lithofacies types, facies associations, and depositional environment of the mixed sediments system. A detailed sedimentary facies analysis suggests that various lithofacies types can be grouped into four facies associations: mixed bioclastic sandy beaches (FA1), mixed clastic bioclastic shoal (FA2), mixed sandy oolitic shoals (FA3), and mixed distal bioclastic sandy beaches (FA4). These facies associations are interpreted to be deposited in varying depositional environments ranging from foreshore to shoreface setting in a shallow lake setting. An interplay of the climate regime, nearshore carbonate productivity, and current activity was proved to control the lithofacies types, dispersion and depositional environment change of mixed sediments in different depositional phases. Meter‐scale mixed cycles were found in this shallow lake setting and their features seem to differ from the classical mixed sediment models. It is suggested that these particular cycles in the stratigraphic record might indicate a typical mark of mixed sediment system in shallow areas of the lacustrine basin.
... During the Cenozoic Era, the area was influenced by the Himalayan orogeny (55-5.1 Ma). The Cenozoic structural evolution of the Bohai Sea area is complex but can be divided into a Paleogene rifting stage and a Neogene-Quaternary postrifting thermal subsidence stage ( Figure 2) (Hou et al., 2000;Gong and Wang, 2001;Gong, 2004;Sun et al., 2008).The rifting stage includes four substages: (1) early synrift substage during 65~42 Ma, (2) late synrift substage during 42~38 Ma, (3) the first thermal subsidence substage during 38~32.8 Ma, and (4) renewed rifting substage during 32.8~24.6 ...
The Bohai Sea area, offshore of the Bohai Bay Basin, is one of the most petroliferous regions in China, with proven original oil in place of approximately 2.4 x 10(9) m(3) (150.94 x 10(8) bbl) and proven original gas in place of over 5 x 10(12) m(3) (1.76 x 10(13) ft(3)). Cumulative oil production is over 50 million tons (3.5 x 10(8) bbl). In this study, using the limited data on source rock thickness, core samples, and Rock-Eval pyrolysis along with sedimentary facies analysis, source rock characteristics of different depositional settings were identified, and the thickness, richness, organic matter type, and thermal evolution of four sets of source rocks in the Bohai Sea area- the second member of Dongying Formation (E(3)d(2)), the third member of Dongying Formation (E(3)d(3)), the first and second members of Shahejie Formation (E(2)s(1-2)), and the third member of Shahejie Formation (E(2)s(3))-were predicted and evaluated. Subsequently, the intensity and history of hydrocarbon expulsion for different sags was systematically compared and analyzed. The greatest thickness of the four sets of source rocks in the Bohai Sea area is 400-800 m (1300-2600 ft). The average richness of the organic matter of these source rocks is 1.74%-2.87%. The E(2)s(3) set has the highest organic matter abundance; E(2)s(1-2) has the lowest. The organic matter of these source rocks is mainly type I and type II, but their evolutions differ. The vitrinite reflectance of E(3)d(2) is 0.5%-1.0%, that of E(3)d(3) is 0.7%-1.25%, that of E(2)s(1-2) is 0.75%-1.75%, and that of E(2)s(3) is 0.75%-2.0%. The cumulative hydrocarbon expulsion of the four sets of rocks is 4.14 x 10(10) t (2.90 x 10(11) bbl). The E(2)s(1-2) set has the greatest expulsion amount: 1.75 x 10(10) t (1.22 x 10(11) bbl). The peak stages of hydrocarbon expulsion of the four sets of source rocks were during Neogene Minghuazhen Formation (12.2-2.0 Ma) and Neogene Guantao Formation (16.6-12.0 Ma). The Bozhong sag expelled the most hydrocarbons, followed by the Liaozhong, Qikou, and Huanghekou sags.
... The Qiongdongnan Basin underwent two tectonic evolutionary stages, rifting (Eocene and Oligocene) and post-rifting (Miocene, Pliocene and Quaternary), separated by the breakup unconformity corresponding to 21.0 Ma (T60) (Kang et al., 2014). The post-rifting stage was subdivided into two sub-stages of thermal subsidence and rapid subsidence separated by 11.6 Ma (T40) (Gong, 2004;Su et al., 2014). Previous studies considered that deepwater slope system in the northern South China Sea was first formed at the Changchang Depression (Xie et al., 2008) and then spread westward. ...
Based on the interpretation of high resolution 2D/3D seismic data, sedimentary filling characteristics and fullfilled time of the Central Canyon in different segments in the Qiongdongnan Basin of northwestern South China Sea have been studied. The research results indicate that the initial formation age of the Central Canyon is traced back to 11.6 Ma (T40), at which the canyon began to develop due to the scouring of turbidity currents from west to east. During the period of 11.6–8.2 Ma (T40–T31), strong downcutting by gravity flow occurred, which led to the formation of the canyon. The canyon fillings began to form since 8.2 Ma (T31) and were dominated by turbidite deposits, which constituted of lateral migration and vertical superposition of turbidity channels during the time of 8.2–5.5 Ma. The interbeds of turbidity currents deposits and mass transport deposits (MTDs) were developed in the period of 5.5–3.8 Ma (T30–T28). After then, the canyon fillings were primarily made up of large scale MTDs, interrupted by small scale turbidity channels and thin pelagic mudstones. The Central Canyon can be divided into three types according to the main controlling factors, geomorphology-controlled, fault-controlled and intrusionmodified canyons. Among them, the geomorphology-controlled canyon is developed at the Ledong, Lingshui, Songnan and western Baodao Depressions, situated in a confined basin center between the northern slope and the South Uplift Belt along the Central Depression Belt. The fault-controlled canyon is developed mainly along the deep-seated faults in the Changchang Depression and eastern Baodao Depression. Intrusion-modified canyon is only occurred in the Songnan Low Uplift, which is still mainly controlled by geomorphology, the intrusion just modified seabed morphology. The full-filled time of the Central Canyon differs from west to east, displaying a tendency of being successively late eastward. The geomorphology-controlled canyon was completely filled before 3.8 Ma (T28), but that in intrusion-modified canyon was delayed to 2.4 Ma (T27) because of the uplifted southern canyon wall. To the Changchang Depression, the complete filling time was successively late eastward, and the canyon in eastern Changchang Depression is still not fully filled up to today. Difference in full-filled time in the Central Canyon is mainly governed by multiple sediment supplies and regional tectonic activities. Due to sufficient supply of turbidity currents and MTDs from west and north respectively, western segment of the Central Canyon is entirely filled up earlier. Owing to slower sediment supply rate, together with differential subsidence by deep-seated faults, the full-filled time of the canyon is put off eastwards gradually. © 2015, The Chinese Society of Oceanography and Springer-Verlag Berlin Heidelberg.
... The sediments deposited at the synrifting stage were only restricted to the grabens or half grabens and were deposited primarily under lacustrine setting (Chen et al., 1998;Wu et al., 2006). The facies deposited during the post-rifting stage as represented by the deposition of the Guantao, Minghuazhen and Pingyuan formations, were predominantly of fluvial origin (Xiao and Chen, 2003;Gong, 2004). ...
The growth of trap-boundary faults and changes in fault sealing capacity during activation may significantly affect the evolution of hydrocarbon migration and accumulation. Previous studies have often overlooked the interaction between changes in sealing capacity and the dynamic processes of hydrocarbon migration and accumulation during fault growth/activation. This study focuses on the trap-boundary fault within the Longkou 7 − 6 (LK7-6) structure, an ideal site for examining the impact of fault activity on hydrocarbon migration and accumulation. Seismic and borehole data were used to quantitatively investigate the history of fault growth and activation, as well as changes in fault-sealing capacity during reactivation. This was achieved by calculating fault throw and shale gouge ratio (SGR). Additionally, basin and petroleum system modeling (BPSM) was employed to reconstruct the dynamic evolution of hydrocarbon migration and accumulation during reactivation. The geological and geophysical studies indicate that the trap-boundary fault consists of at least three segments connected through lateral propagation. The fault experienced growth during the Paleogene, followed by a decrease in activity, and later entered a phase of neotectonic reactivation around 5.1 Ma. This reactivation of the trap-boundary fault played a dual role in hydrocarbon migration and accumulation, providing vertical pathways for migration while also causing vertical leakage. The dynamic evaluation of fault sealing capacity demonstrates that fault reactivation could alter lithological juxtapositions, thereby compromising fault seal integrity and facilitating cross-fault hydrocarbon leakage. The study enhances the understanding of the interaction between fault dynamic evaluation and hydrocarbon migration and accumulation, particularly through the integration of fault activity history and fault-sealing capacity evaluation with basin and petroleum system modeling (BPSM). This approach offers new insights into exploration strategies for fault-bound traps, representing a significant advancement over traditional static analysis.
The Bozhong19-6 (BZ19-6) condensate gas reservoirs, located in the southwestern Bozhong sub-basin, Bohai Bay Basin, China, were paleo-oil reservoirs in the geological past and have undergone at least three successive hydrocarbon charging events. The hydrocarbon migration and accumulation process of “early oil and late gas” has occurred in the current reservoirs. At the end of the sedimentation of the Guantao Formation (N1g, ∼12 Ma), the reservoirs began to fill with first stage low-moderate mature crude oil. At the late stage of the Lower Minghuazhen Formation (N1ml) (∼6.7 Ma), the reservoirs were largely charged with second stage high mature crude oil. Since the deposition of the upper Minghuazhen Formation (N2mu, ∼5.1 Ma), the paleo-oil reservoirs were transformed into shallow Neogene reservoirs due to the reactivation of basement faults. From the late stage of the N2mu to the present day (∼2.8–0 Ma), the reservoirs were rapidly filled by natural gas within a short period. In addition, analysis of the formation of the reservoir bitumen and the conspicuous loss of the lower molecular weight n-alkanes in the crude oil reveal that the injection of a large amount of gas in the late stage caused gas flushing of the early charged oil.
The Yinggehai Basin is a unique NNW-trending petroliferous basin in the northwestern South China Sea. This paper mainly utilized stratigraphic, tectonic and seismic data by characterizing the geological structures and conducting the geo-mechanical analysis to study the formation, evolution and dynamics of the Yinggehai Basin. The study indicates that the Ailaoshan-Truong Son extruded terrane is composed of multiple secondary extruded bodies. The Red River fault zone, located within the Qiangtang-Simao-Yinggehai mantle flow channel and basin zone, experienced transform-type sinistral strike-slip motion before the basin forming stage and formed a NW-trending extruded mantle uplift, which activated the Yinggehai basin. After experiencing the rift depression, fault depression, and fault subsidence, the basin eventually formed large-scale, thick sedimentation features with ideal hydrocarbon-forming conditions at the end of the Miocene. Later, the basin dynamically transformed and entered a period of tectonic superposition, reworking, and thermal subsidence. Superposition of the NNW thrust sinistral strike-slip fault zone on the northern Hanoi sub-basin complicated the basin structure. Since the Pliocene, the southern Yinggehai main basin has been transformed into an extensional dextral strike-slip environment that hosted numerous mud diapirs. The thin crust and high geothermal gradient provide favorable conditions for the large-scale accumulation of natural gas.
Latest exploration practices revealed appreciable petroleum resources in the deep pre-Cenozoic reservoirs in the southwest area of the Bozhong depression; however, the origins and accumulation mechanisms of deep hydrocarbons remain not fully understood. Here, an integrated study of the geochemical characteristics of overlying source rocks, maturity evaluation of petroleum, and origin of natural gas was conducted to characterize the dynamic petroleum accumulation and illustrate the main factors influencing the differential phases of petroleum distribution in the research area. The third member of the Dongying Formation (Ed3) mudstone in the research area displays high organic richness and is characterized by oil-prone organic matter, indicating high petroleum potential. The oils contained in the CFD18 and BZ13 buried-hill traps were mainly derived from the Ed3 and Shahejie Formation (Es) source rocks, respectively, with natural gas of kerogen cracking origin. Buried-hill oil from the BZ13 field displays a high thermal maturity than that of the CFD18 oils, which was evidenced by maturity parameters of saturated and aromatic hydrocarbons and concentrations of stigmastane. Nevertheless, maturity index of diamondoids and carbon isotopic composition of natural gas indicate the possible presence of mixing of higher mature Es fluids for the CFD18 field. This research underlines the contribution of the overlapped effective source rocks to the buried-hill traps. The causes of multiple petroleum phases in the deep reservoirs in the research area are mainly linked to dynamic petroleum accumulation rather than source rock type and oil secondary cracking. Although faults are present to act as vertical migration conduits due to late-stage activation with high intensity, abundant petroleum supply due to near-source charge and favorable constitution relationship of source rock and reservoir rock provide crucial conditions for deep petroleum accumulation in a tectonic activation zone.
The Bonan sag in the Zhanhua Depression is a prolific oil-producing Cenozoic basin located in the Bohai Bay Basin of China. Three overpressure systems have been identified in the Paleogene Dongying Formation to the first interval of the Paleogene Shahejie Formation, the third interval, and fourth interval of the Paleogene Shahejie Formation, respectively. A combination of drill stem test data, well-log responses, geochemical data of source rocks, basin modeling, and vertical effective stress-density and density-sonic velocity cross plots, were used to investigate the distribution and mechanisms of different overpressure systems in the Bonan sag. The sonic log responds to overpressure development with anomalously high acoustic travel time in the Bonan sag. The Eaton method combined with sonic log data was applied to accurately predict the overpressure magnitude of the three overpressure systems. The overpressure mechanisms differ for each overpressure system. The overpressure in the Dongying Formation to the first interval of the Shahejie Formation, with a top depth from 2000 to 3000 m, is attributed to disequilibrium compaction. The combined mechanisms of disequilibrium compaction and oil generation lead to overpressure in the third interval of the Shahejie Formation with a top depth from 2200 to 3700 m. The overpressure in the fourth interval of the Shahejie Formation, with a top depth from 2500 to 4300 m, is mainly caused by disequilibrium compaction, supplemented by the minor influence of oil generation. Mudstones are the sealing rocks for the overpressure systems in the Dongying Formation to the first interval of the Shahejie Formation and the third interval of the Shahejie Formation. Sealing from the overlying gypsum-rich layer is crucial for overpressure preservation in the fourth interval of the Shahejie Formation. The difference in the oil generation capacity of source rocks and sealing condition controls the variation of overpressure mechanisms in the three overpressure systems in the Bonan sag.
Mixed siliciclastic-carbonate sediments have been widely studied, especially those of marine origins. However, it is rarely investigated in continental settings. Boreholes from the Qinhuangdao-A (QHD-A) block in central Bohai Bay Basin sampled lacustrine mixed sediments during the Eocene Es1-2s period (32-38Ma), providing an opportunity to facilitate this aspect of research. Here, an integrated study of seismic, core data, and microscopic analysis revealed the formation and evolution of the mixed sediment system. Core observations and microfossil identifications show that mixed sediments characterizing volcaniclastic sand and gravel mixed with bioclastic debris are composed of shallow-depth biota assemblages. The relatively warm climate, low tectonic activity and low siliciclastic influx may result in prevailing occurrences of nearshore biota. Preexisting siliciclastics featuring a well-rounding shape experienced secondary transportation to mix with the nearshore biota, forming the major mixing type: “compositional mixing”. Seismic data, well-logs, and lithofacies logs divided mixed sediments into three evolutional units (U1-U3). U1 and U2 were dominated by “compositional mixing”. Within the U1, it occurred locally due to high sediment input during the low-level period. Peak took place in the U2 because lake-level rise led to increasing accommodation and decreasing siliciclastic influx. With further deepening of water depth in the U3, shallow-depended biota was not well developed and carbonates increased. Lake-level change results in a distinct depositional sequence of mixed sediments that bioclastic particles mixed with siliciclastics in shallow period and centimeter-scale carbonates interbedded with siliciclastic beds in the highstand. These findings bring new insights into mixed deposits, especially in continental settings.
Fourteen Paleogene rock samples from the Bohai Bay Basin, including samples from the third member of the Oligocene Dongying Formation (Ed3) and the third member of the Eocene Shahejie Formation (Es3), underwent total organic carbon (TOC) analysis, Rock-Eval pyrolysis, stable carbon (δ¹³Ccarb) and oxygen isotope (δ¹⁸Ocarb) analysis of carbonates, stable carbon isotope analysis of organic matter (δ¹³Com), biomarker and trace element compositional analysis to reconstruct paleolimnological environments and investigate the factors that control the occurrence of high quality source rocks. The Rock-Eval and TOC results revealed that the source rocks meet the standard for good to excellent source rocks. The biomarker parameters and trace elements suggest that warm and humid paleolimnological conditions prevailed during the deposition of Es3. In addition, heavier values of δ¹³Com and light values of δ¹⁸Ocarb indicate the existence of a hydrologically open paleolake with high primary productivity, which is supported by relatively high 4-methyl sterane index values and high barium/calcium (Ba/Ca) ratios. Ed3 rocks formed under less reducing and more acidic conditions, which is supported by high pristane/phytane, C27 diasteranes/C27 regular steranes and kaolinite/illite ratios and low strontium/barium (Sr/Ba) and vanadium/chromium (V/Cr) ratios. Moreover, Ed3 is characterized by a relatively high input of terrestrial organic matter under humid but cooler settings. A long-term positive shift of δ¹⁸Ocarb revealed by the Ed3 samples is associated with changes in oxygen isotopic composition of inflow water (higher precipitation) and temperature (colder). The conditions of both high productivity and enhanced preservation controlled the distribution of good source rocks in the Bohai Bay Basin (BBB). Es3 is more likely linked with high primary productivity, which resulted in an overall positive shift in δ¹³Com, whereas Ed3 may be more related to favourable preservation conditions associated with relatively high sedimentation rates.
Fault growth and reactivation activities have been interpreted as potential controlling factors in vertical hydrocarbon migration, but the coupling and direct comparison of fault growth/reactivation history and hydrocarbon distribution patterns remain poorly understood. The spatial distribution of oil fields in the Western subsag of the Bozhong subbasin in the Bohai Bay Basin is closely related to fault geometry and presents a unique opportunity to investigate the coupling mechanism of the evolution of normal faults and petroleum migration and accumulation. The evolution history of two major normal faults, Faults F1 and F2, was investigated via three-dimensional seismic reflection data, and Fault F2 was associated with the spatial distribution of oil. Quantitative throw analyses, including throw versus distance plots, throw versus depth (T-z) plots, throw contour projections, and expansion indexes (EI) were employed. Fault F2 exhibited a large displacement between the foot- and hanging walls along most of the fault plane and thickened hanging walls of the upper fault surface. Laterally, the fault comprised three segments that linked along the strike at local throw minima. Vertically, the fault formed through the coalescence of two parts via dip linkage. Both the isolated- and coherent-fault models may have contributed to the evolution of Fault F2 during different periods and can be summarized in several steps, including blind fault propagation as individual segments, free surface breaching, lateral and dip linkage, and subsequent frequent reactivation as a coherent structure. A key result of this study was that petroleum migration and accumulation could be directly controlled by multiple growth and reactivation activities of faults.
Based on studies on the contents, origin and filling history of CO2, and the seismic interpretation of the Bohai Sea, the sea area of the Bohai Bay basin, the distribution of mantle-derived CO2 and roles of fault structures have been examined in this study. Mantle-derived CO2 is mainly distributed in the structures within the Bozhong area, the center of the Bohai Sea. Laser Raman spectrum shows that CO2 inclusions are symbiotic with hydrocarbon inclusions with a charging time no earlier than 5.1 Ma. Late-stage (5.1–0 Ma) active basement faults are the major pathways for the migration of mantle-derived CO2. The intensity of the activity influences the CO2 content. Abundant CO2 occurs in the structures with basement faults that have relatively high fault activity rate (FAR) values. Along the basement fault, high CO2 content appears near the segments with high FAR values. Fault combination patterns affect the accumulation layers of mantle-derived CO2. Structures with fault combination patterns that resemble a “flower” or “Y” commonly accumulate CO2 in Cenozoic shallow layers as well. The branch faults that intersect with basement faults facilitate the adjustment of CO2 from deep to shallow layers. The uplifted mantle is the deep tectonic context, and the upwelling mantle magma is the source of the development of mantle-derived CO2. Below the middle crust, mantle-derived CO2 may be vertically transported along the lithosphere-scale strike–slip faults. Within the middle and upper crust, the late-stage active basement faults are the major conduits for mantle-derived CO2.
The Cenozoic offshore Bohai Bay basin has similar graben-horst configurations as in many rift basins, however, different areas of it have different stratigraphic fill, resulted from jointly effects of basement fabrics, rifting and strike-slipping. Within the Liaodongwan subbasin, the Liaodong rise is a transpressional structural belt bounded by the Tanlu strike-slip fault. The Bodong rise is similar to the Liaodong rise. The eastern Bozhong subbasin has a graben-horst configuration caused by extensional stress. The southern and northwestern parts of the Bozhong subbasin are characterized by large-scale horsts that are limited by south-dipping listric normal faults. The Bozhong subbasin shows many similarities with the faulted sag basin. The southern Bohai Bay basin is characterized by EW-oriented half-grabens and horsts resulted from S-N extensional stress. The Cenozoic evolution of the basin began with Paleogene rifting, followed by the Neogene–Quaternary thermal subsidence stage. The main rifting episode, caused by the upwelling of the mantle, is characterized by strong differential block-faulting during the deposition of the Eocene Sha-3 member. During the deposition of the Oligocene Dongying Formation, previously isolated and scattered sags connected with each other and became a broad and unified lake basin. In addition, the dextral movement of the Tanlu fault zone initiated the development of several transpressional structural belts in fault bends. In the eastern basin, extensional systems were also superimposed by strike-slip systems. During thermal subsidence, neotectonic movement produced many secondary faults in shallower strata, and the dextral movement of the Tanlu fault zone also altered the eastern basin architecture.
The Penglai 7-6 (PL7-6) structure is located on the Tan-Lu fault zone, a huge fault zone in east Asia. Tectonism of the Tan-Lu fault zone produced complex structural deformation and was an important factor in petroleum accumulation. Three-dimensional seismic data were employed to document the structural geometry and evolutionary process of the PL7-6 structure, and implications for petroleum exploration in a releasing bend of a strike-slip fault system were discussed. Systematic analysis of the seismic data indicates that the main fault (the BD2 fault) of the Tan-Lu fault system slipped right-laterally during the Cenozoic. Strike-slip movement of the BD2 fault peaked during deposition of E1-2k to E3d3, and decreased largely in intensity during E3d1-2 deposition. In the Neogene and Quaternary, the BD2 fault was reactivated. Formation of the Bodongnan low uplift is related to the connection between the Bodongnan low uplift (N) and Bodongnan low uplift (S). The Bodongnan low uplift (N) is represented by an anticline produced by strike-slip movement of the BD2 fault, and the Bodongnan low uplift (S) is represented by the relatively uplifted strata in the vicinity of an extensional fault (the BD3 fault). The PL7-6 structure developed above a releasing bend formed by the subsequent strike-slip movement of the curved BD3 fault. Orientations of the shallow faults in the PL7-6 structure are not only associated with the lower BD3 fault, but also controlled by the basement high. Unlike traditional releasing bends that are generally disadvantage to petroleum accumulation due to the lower structural elevation in the center, the releasing bend in the PL7-6 structure is favorable for petroleum accumulation because of the three reasons: (1) adjacent to generative kitchens, (2) located on a regional structural high, and (3) served by the releasing-bend-related faults acting as conduits for vertical petroleum migration.
The study area is located in the Langgu sag of Northern Jizhong depression, Bohai Bay Basin, East China. In order to achieve exploration breakthrough in deep buried hill, key engineering technologies are developed and used to accurately demonstrate important target identification by recognizing new hydrocarbon accumulation patterns resulting from the analysis of multi-stage structure-controlled trap mechanism and the detailed study of controlling factors over high-quality Ordovician reservoirs based on new high-accuracy 3D seismic data. This study reveals a new evolution mechanism of buried hill controlled by structural superposition, experiencing "the uplift from thrusting in Indo-Chinese to early Yanshan epoch, uplifted block faulting into horsts in middle Yanshan epoch, horsts tilting into belt in Eocene, and belt reversion into trap", and thus puts forward a new mechanism for reservoir forming controlled by a superposition of "dolomite, karsting, and faulting". Three types of reservoir development are identified, including "regional layered pore, local block micropore-fracture, and fracture hole pore layer-block composite", and an accumulation pattern in deep buried hill is constructed, characterized by "efficient hydrocarbon supply from gas-type source rock, predominant migration through fractured surface-nonconformity surface, and stratum- and mass-controlled accumulation", which has guided the 40 years' exploration of Ordovician Yangshuiwu buried hill zone and made a great breakthroughs. Novel relevant exploration technologies have been developed, involving high-accuracy imaging, high-precision well logging identification of hydrocarbon reservoir, ultra-high temperature deep drilling and completion, ultra-high temperature carbonate reservoir stimulation, etc, which solve a worldwide problem that has restricted the exploration of the ultra-high temperature buried hill for many years. These technologies make possible the highest daily production of over 100 m3 oil and 0.5 million m3 gas respectively and sustain a high and stable production for a long term, which guarantee the clean energy supply for Beijing-Tianjin-Hebei region.
Based on core data, thin section and logging, this study investigates the basic characteristics and occurrence regularities of the Ordovician buried-hill carbonate reservoir in the Bozhong 21-2 tectonic belt. Results show that the reservoir lithology is primarily limestone, followed by dolomite, dolomitic limestone or limey dolomite, and silty mudstone. The reservoir spaces are primarily composed of secondary porosity. These reservoirs can be divided vertically into weathered-crust karst reservoirs and inner dissolution-type reservoirs. The distribution of reservoirs is controlled chiefly by karstification. The high karst units in the study area are favorable for the development of the two reservoir types.
Neotectonics has changed the coupled process of endogenic and exogenic geological dynamics, which mold the modern landform. Geomorphologic analysis is essential for identifying and understanding the tectonic activity and indicates the responsive mechanism of the landform to tectonic activity. At first, this research reconstructed the twisted Shanpen period planation surface, computed the valley floor width-to-height ratio of Sancha river and extracted the cross sections marking the river terraces to analyze the characteristics of the neotectonics. And then, the relation between neotectonic movement and landform development was analyzed by dividing the landform types. At last, the spatial variation of landform evolution was analyzed by extracting the Hypsometric Integral of sub-catchments. The Sancha river catchment’s neotectonic movement presents the tilt-lift of earth’s crust from NW to SE, which is characterized by the posthumous activity of Yanshan tectonic deformation. The spatial distribution of river terraces indicates that Sancha river catchment has experienced at least four intermittent uplifts and the fault blocks at both the sides of Liuzhi–Zhijin basement fault have differentially uplifted since the late Pleistocene. As the resurgence of Liuzhi–Zhijin basement fault, the Sancha river catchment was broken into two relative independent landform units. The spatial variations of the landform types near the Sancha river and the sub-catchments’ landform evolution are characterized by periodic replacement. The styles of geological structure have controlled the development of landform far away from the Sancha River and influenced the landform evolution. The posthumous activities of the secondary structure have resulted in the spatial variation of sub-catchments’ landform evolution, which presents periodic replacement with local exceptions. The present study suggests that spatial variations of the development and evolution of modern landform of Sancha River catchment owe their genesis to the interplay between the hydrodynamic force and tectonic activity in the neotectonic period. Likewise, the application of geomorphic indicators also provides a new way to assess the regional crustal stability.
The distribution characteristics of the oil-water contact are the basis for the reservoir exploration and development and reserves evaluation. The reservoir with a tilted oil-water contact has a unique formation mechanism, and the understanding of its distribution and formation mechanism will directly affect the evaluations for the reservoir type, well deployment, selection of well pattern and type, determination of test section, and reserves evaluation. Based on the analysis of reservoir characteristics, petrophysical properties and geological structure in 40 reservoirs worldwide with tilted oil-water contacts, the progress of the research on the formation mechanisms of titled oil-water contacts is summarized in terms of the hydrodynamic conditions, reservoir heterogeneity, neotectonic movement and oil-gas exploitation. According to the formation mechanism of tilted oil-water contacts and the needs of exploration research, different aspects of research methods are summarized and classified, such as the calculation of equipotential surfaces for oil and water in the formation, analysis of formation pressure and analysis of reservoir physical properties and so on. Based upon statistical analysis, it is suggested that the degree of the inclination of the oil-water contact be divided based on the dip of oil-water contact (DipTOWC). The tilted oil-water contact is divided into three categories: large dip (DipTOWC≥55 m/km), medium dip (4 m/km≤DipTOWC<55 m/km), and small dip (DipTOWC<4 m/km). The classification and evaluation method can be combined with structure amplitude and reservoir property. The formation mechanism of domestic and international reservoirs with tilted oil-water contacts are summarized in this paper, which have important significance in guiding the exploration and development of the oilfield with tilted oil-water contacts, reserves evaluation, and well deployment.
The Liaodong Bay sub-basin is a classic non-marine rift sub-basin in the Bohai Bay, northeastern China. The study area is located on the east side of Liaoxi uplift in the west slope of Liaodong Bay sub-basin. It sits on a draped anticline zone above the paleo-uplift and contains the second biggest offshore hydrocarbon field found in China to date. The sub-basin is bound to the west by the TanLu fault zone, the most active and largest fault active zone in eastern of China, and has been active from the Mesozoic to present. The spatial distribution and temporal evolution of the depositional systems in the lacustrine rift basin were significantly controlled by topography of paleo-uplift and the distribution of sediment transport pathways. Using 3D seismic and densely spaced well data, we systematically analysed the spatial distribution and temporal evolution of sediment transport pathway of the deltaic deposits in the SZ36-1 oilfield in the eastern slope of the Liaozhong sag. Two types of sediment transport pathway, including the fault relay ramps and erosional valleys, were recognised: (i) the fault relay ramps between two sub-parallel faults; and (ii) a series erosional valleys on the uplift, with ‘V-’, ‘U-’ or ‘W-'shaped cross-sectional patterns. Seismic stratal slices reveal that the erosional valleys branch and converge from upstream to downstream. The paleomorphology of the Liaoxi uplift (e.g., erosional valleys and sub-uplifts) and their evolution control the depositional systems and the pattern of sediment dispersal in the rift lacustrine basin area. The research indicates that paleogeomorphology controlled the direction of sediment transport, the capacity and position of sediment accommodation, influenced the type of sedimentary micro-facies and the spatial distribution pattern of the sediments. Seismic stratal slices and paleogeography maps reveal the erosional valleys shrunk progressively with sedimentary fills, resulting in decreasing gradients of the depositional slope, and provide a gentle geomorphology for a large-scale fluvial-delta depositional system to develop.
Chengdao is an offshore area in the Bohai Bay Basin that contains approximately 25.7 × 108 bbl of oil and gas reserves within the sandstone reservoirs in Neogene strata. However, previous predictions of hydrocarbon accumulation in Neogene traps are inaccurate, resulting in a current failure rate of 50% when drilling for hydrocarbons in this area. To build an improved exploration model for Neogene traps, we select 92 traps from Neogene strata in the Chengdao area to quantify the filling degree, which is an indicator of hydrocarbon accumulation efficiency. The quantified filling degree is based on actual geological and exploration data and differs significantly among various trap types. The filling degree of traps also varies significantly with their structural locations and decreases generally from the northwest to the southeast along the Chengbei Fault zone. Vertically, the filling degree is highly heterogeneous, initially increasing from the bottom to the middle of Neogene strata and then decreasing towards the top of the strata. These Neogene hydrocarbon reservoirs are sourced from the Paleogene, and as they lay vertically away from the source rocks, their hydrocarbon enrichment is constrained largely by hydrocarbon migration distance and vertical migration pathways. The sealing capacity of faults and cap rocks, sandbody orientation and reservoir sedimentary facies determine the maximum column height, which in turn affects the amount of hydrocarbon accumulation within these traps. A scatter plot analysis of individual controls and volumetric filling for each trap type is compiled using multivariate linear regression analysis to quantify controls and the dominant control of hydrocarbon accumulation is determined.
The Bohai Bay Basin contains many depressions with varying degrees of hydrocarbon enrichment associated with the geological structures of different depressions. This study discussed the relationship between the geological structures and hydrocarbon enrichment of the depressions in the Bohai Bay Basin. Based on the Paleogene strata distribution and the length to width ratio of different depressions, their geological structures are divided into three types in plan-view: open (length/width < 2), narrow (length/width > 4) and transitional types (length/width 2–4). In cross section, the geological structures can be divided into dustpan I, dustpan II and double-faulted types. Based on tectonic evolution and sedimentary characteristics, the depressions are classified into early-formed, inherited and late-formed categories. Generally, narrow depressions are mainly located in the northeast and southwest of the Bohai Bay Basin, while open depressions are dominantly distributed in the central area of the basin; late-formed depressions are mainly around the Bohai sea area, and early-formed depressions are mostly located in the periphery of the basin. Geological structures of the depressions control the formation of the source, reservoir and cap rocks as well as hydrocarbon accumulation setting, and further influence the pay zones and oil-bearing sequence. In detail, dustpan II and double-faulted depressions mainly have A-type sags, which often possess better hydrocarbon generation conditions than dustpan I ones; hydrocarbons in open dustpan II depressions tend to accumulate in the central uplift areas or buried hill, while those in narrow dustpan I depressions always accumulate in gentle slope belts. The oil-bearing sequence for different evolutional depressions corresponds well with the sedimentary strata of the main development stages of depressions. In early-formed depressions, hydrocarbons are mainly enriched in deeply buried reservoirs, while in late-formed depressions hydrocarbons are abundant in the relatively shallow traps. In summary, most inherited and late-formed dustpan II depressions are enriched in hydrocarbons due to their extensive source rocks and good source–reservoir–seal assemblages, whereas dustpan I and early-formed depressions are relatively poor in hydrocarbons.
Many Quaternary folds and faults are identified on the seismic reflection profiles acquired in the adjacent areas of Qianliyan Island of southern Yellow Sea. The faults are mainly of reverse fault. The folds are mainly gentle and tilted, caused by fault activities. Most folds and faults occurred below a plane of unconformity between middle Pleistocene and its overlying upper Pleistocene. Some faults, such as Qianliyan fault, are late Pleistocene active fault. Many faults and folds were synsedimentarily developed in the lower Pleistocene and underlying strata, revealling rather strong tectonic activity, especially at the basin boundary where the faults even cut into the late Pleistocene stratum. The formation of folds and faults was attributed to the last tectogenesis of the Yellow Sea in middle Pleistocene, which maybe correspond to the Kunlun-Huanghe movement of Qinghai-Tibet plateau. At that time, the N-S striking horizontal stress drove the early-developed basin boundary faults reactive and further induced the NEE or E-W striking tectonic deformation in the overlying sedimentary layer.
Based on 3D high-resolution seismic data, a series of the high-amplitude reflections (HARs) were discovered in Meishan Formation in deepwater areas of the southern Qiongdongnan Basin using an isometric variance slice technique. The detailed observation of these HARs revealed that the generally E-W oriented HARs have variable scales, being 2-15 km in length and 500-5000 m in width. Small-scale faults are well developed within the HARs. Furthermore, the HARs show various features in different parts of the study area, for instance, they come in an irregular spotted shape in the east but stripped shape in the west. In sections perpendicular to the strike, the HARs are generally V-shaped or U-shaped by appearances and develop mainly on the uplifted Meishan Formation, beneath which there often exist buried faults formed during the rift-subsidence stage and having a strike consistent with the extension of terminal Oligocene faults. Therefore, the origin of HARs was attributed to the fault activation after the rifting of the basin, which resulted in an upward migration of fluids from underlying Paleogene strata, a water-rock interaction of deep fluids along the fault system developed within Meishan Formation, and probably a fluid charge thereafter. The discovery of the HARs in the study area is of directive importance in hydrocarbon exploration in deepwater areas of Qiongdongnan Basin.
Neotectonism controlled and adjusted the accumulation and distribution hydrocarbon in Bozhong area. Combining the qualitative appraisement with semi-quantitative description method, this paper discusses the controlling action on hydrocarbon forming conditions and its accumulation effect in late-stage fault activity based on the analyses of the fault system characteristics and the fault activity rate (FAR) difference between 62 I and II faults. The results show that strong active faults (FAR>25 m/Ma) have a transportation role, however, weak active faults (FAR<10 m/Ma) mainly restrict hydrocarbon migration. Medium active faults (10 m/Ma<FAR<25 m/Ma) have hydrocarbon sealing and transportation dynamic equilibrium role. Faults having transportation or sealing role along the strike show "segmentation" feature, and the multi-layer enrichment pools form under the controlled "segmented" active faults. The traps located at the dominant migration path of the gentle slope of the uplift are far from hydrocarbon kitchen. The paleocene strata of the medium-weak active fault and "active" source rocks at the end of uplift area, or regional combination of medium-strong active fault and "active" source rocks are the most interesting hydrocarbon exploration areas.
Deepwater reservoir has become one of the major issues in submarine hydrocarbon exploration in China recently. Based on high resolution 2D and 3D seismic data, depositional patterns and architectures of deepwater depositional systems are identified on the northwestern continental margins of the South China Sea. Apart from hemipelagic and pelagic mudstones, a number of gravity flow deposits are extensively developed, including large scale mass transport deposits (MTDs), submarine canyons and sediment waves. Four slope types are identified, including progradational type, slumping type, channelized type, wide and gentle type slopes. Each slope type has different morphological features and depositional architectures. Giant MTDs occur on the slumping and channelized slopes, whereas the sediment waves are only developed on the lower part of wide and gentle slope and the two sides of the Changchang segment of the Central Canyon. Due to special morphological features with an eastward flared shape and increasing water depth in the deepwater area since the Late Miocene, the unique Central Canyon with a trend paralleling to shelf break of northern margins is formed along the axial deepest water-depth line. The fillings in the Central Canyon are composed by not only turbidity channels from the head segment of the canyon in the west, but also the MTDs from the continental slope in the north, particularly from the slumping and channelized slopes. The Central Canyon system could be considered as multiple-sources to sink system for sediment transportation, and also a system containing important reservoirs in the deepwater area on the northwestern continental margin of the South China Sea.
In this paper, we discuss the influence of structural figuration and structural dynamics on the transporting and reservoiring system in the Qikou sag. The results show that the Paleogene graben faults generated complex half graben-like framework on NW-SE orientation and complex duplex fault graben on N-S orientation in the Qikou sag, constrained by the Cangdong extensional fault system and Qikou extensional fault system. On a plane, the structural palaeogeomorphology pattern with the multi-convex and multi-concave, convex and concave separation formed a favorable space for lithologic reservoirs. The tectonic style of Qikou sag controlled the distribution of hydrocarbon accumulation. The main faults control the oil and gas migration and accumulation and secondary faults rework the oil and gas distribution and form the advantages conduit system. Both favorable reservoir facie and structural slope are prone to the formation of favorable trap. In the Qikou sag, we conclude four types of oil-gas accumulation, i.e. steep slope, gentle slope, broken joint associated with near-source, intrabasinal knick zone. The selected exploration targets have been verified by drilling instance.
Bohai Bay Basin, located in eastern China, is considered a Cenozoic rifted basin. The basin is atypical in terms of its Neogene-Quaternary postrift subsidence history in that it experienced intensive tectonic reactivation, rather than the relative tectonic quiescence experienced during this stage by most rift basins. This Neogene-Quaternary tectonic reactivation arose principally in response to two tectonic events: (1) activity on a dense array of shallow faults and (2) accelerated tectonic subsidence that occurred during the postrift stage. These two events were neither strictly temporally nor spatially equivalent. The dense array of shallow faults form a northwest-southeast-trending belt in the central part of the basin, with displacement induced by the reactivation of older northeast- and northwest-trending basement faults and an associated substantial component of strike-slip displacement occurring after 5.3 Ma. The intensive reactivation of these faults contributed to the atypically accelerated rate of postrift tectonic subsidence of the basin that commenced ca. 12 Ma. However, this was not the sole cause of this accelerated tectonic subsidence: A combination of geological activity deep within the crust led to the buildup of intraplate stresses, and this, combined with ongoing thermal subsidence, acted as an additional contributory factor that drove unusually high rates of subsidence for this basin. This episode of accelerated postrift tectonic reactivation resulted in conditions favorable for hydrocarbon accumulation.
The field size distribution method, such as the Pareto model, is commonly used in hydrocarbon resource assessment in China.
It has the advantage of predicting not only the total resources but also the individual field sizes. However, the method has
a large uncertainty range in the model parameter estimates. A multi-parameter-constrained Pareto model is proposed to improve
parameter estimations. The constraints introduced include: a) total resource estimate from other methods (e.g. genetic method);
b) the largest size of undiscovered fields from an empirical relationship between play resources and the largest field sizes
in well explored basins; and c) the number of undiscovered fields constrained by the number of untested traps (including subtle
traps). By introducing the three constraints, the method provides realistic results with less uncertainty and is consistent
with petroleum system models derived from hydrocarbon exploration. The application of this proposed method to Bozhong Sag
in the Bohai Bay Basin suggests a total resource of 4130 × 106 m3 in 48 fields, of which 1165×106 m3 are in 27 discovered fields and 2965 × 106 m3 are in the remaining 21 undiscovered fields. The largest discovered field is 369 × 106 m3 and the largest remaining field expected is 853 × 106 m3. The results of the application appear to be consistent with the current exploration status and geological understanding
of the study area.
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