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By using thin section identification, cathodoluminescence, major and trace elements and fluid inclusion tests and authigenic illite dating, based on observation of core cracks, combined with the microscopic characteristics and imaging logging characteristics of fractures, the stages of the fractures in the Huagang Formation of the central reversal tectonic belt of the Xihu Sag in the East China Sea, and the matching relationship between the fracture development stages and the oil and gas charging stages are clarified. There are diagenetic fractures and tectonic fractures in the reservoirs of the Huagang Formation in the study area. The diagenetic fractures developed during the diagenetic stage of the reservoirs and have less effect on oil and gas migration and transport. The tectonic fractures are divided into three stages based on tectonic movements controlling the fractures and their relationships with hydrocarbon charging: The first stage of fractures was generated in the early stage of the Himalayan Movement–Longjing Movement (12–13 Ma ago), when the tectonic stress caused the sutures and shale strips to twist, deform, and break. Tectonic microfractures generated in this period had short extension, narrow width, and poor effectiveness, and had little effect on oil and gas migration and transport. The second stage of fractures came up during the middle-late period of Himalayan Movement–Longjing Movement (9–12 Ma ago), when tectonic movements caused the development of tectonic fractures in the central reversal tectonic belt, these fractures are of large scale, long extension, and good effectiveness, and matched with the first stage of large scale oil and gas charging (9–12 Ma ago), so they play an important role in oil and gas migration, transportation, and accumulation. The third stage of fractures were created from Himalayan Movement–Okinawa Trough movement to the present day (0–3 Ma ago), the fractures are tectonic ones developing successively; matching with the second stage (0–3 Ma ago) of large-scale oil and gas charging, they created conditions for continuous natural gas migration and transportation. All these prove that the development of reservoir fractures in the Huagang Formation of Xihu Sag can provide seepage space and continuous and effective channels for efficient migration and accumulation of oil and gas.

Based on integrated investigation of seismic multi-attribute, well logs, cores and geological data, the sedimentary facies and lithofacies analysis are conducted in the Paleogene Pinghu Formation of the Xihu Depression, East China Sea. The Pinghu Formation is the main hydrocarbon-bearing strata and source rock interval in the Xihu Depression, and represents a second-order sequence. It can be subdivided into three third-order sequences, (from base to top: SQ1, SQ2, and SQ3), from SQ1 to SQ3 Sequences displayed almost similar depositional patterns, displaying different characteristics of lithology and facies distribution. The fan shaped and lobate proximal tidal-delta facies are deposited at the western edges with relatively strong hydrodynamics, while the lobate and belt shaped scattered tidal-flat facies are deposited at the center of the area with relatively weak hydrodynamics. From SQ1 to SQ3, the distribution of the sand-rich tidal-delta facies evolves from a large range to a small, then larger again, are predominantly influenced by the hydrodynamics, tectonic and topography. This study demonstrates facies analysis using sequence stratigraphy and seismic sedimentology provide useful insight to Paleogene oil and gas exploration in East China Sea Shelf Basin and other similar basins and can also be effectively used in areas which lack well or outcrop data.

The system of sediment dispersal of the continental margin has always been a hot and difficult study topic because of its diversity and complexity, and its dispersal processes and influence on the exploration of reservoirs have puzzled most researchers. Based on the comprehensive analysis of the seismic geomorphology and fault combinations, the seismic facies, and the sum of positive amplitude attributes, we describe the characteristics of the sediment dispersal pathways, and discussing its influence on the exploration of sand-rich reservoirs of the middle part of Pinghu Formation in the Pingbei Slope Belt of Xihu Depression (East China Sea Shelf Basin). The results showed that three modes of sediment dispersal system were established by comparing the geological background and the sediment dispersal pathways of the Pingbei Slope Belt. 1) The “straight mode” developed in the Tuanjieting area is characterized by a group of effective pathways directly connected to the channels developed in the land, then go through the marginal fault and ended in the gentle slope. The sand-rich lobes consisted of rapid accumulation of sediments are the most important target for the exploration of reservoirs, the scale of reservoirs could be easily recorded by the measurement of the area and the thickness of the chaotic reflectors developed in this area. 2) The “restricted mode” developed in the Baoyunting area is largely limited by the depression bounded by the uplift, the sediment dispersal pathways are genetically related to the scale and shape of this depression. The depocenters could be a guidance for the development of sand-rich reservoirs, large-scale sand-rich reservoirs could be formed as long as the depression is large enough. 3) The “migration mode” developed in the Kongqueting area is controlled by the slope and normal fault combinations, the long-distance and short-distance sediment dispersal pathways are developed. The area that developed the long-distance sand-rich sediments in the background of relatively low fault growth ratio shall be the most important target for reservoir exploration and easy to form large-scale sand-rich reservoirs.

Eocene coal-bearing source rocks of the Pinghu Formation from the W-3 well in the western margin of the Xihu Sag, East China Sea Shelf Basin were analyzed using Rock-Eval pyrolysis and gas chromatography–mass spectrometry to investigate the samples’ source of organic matter, depositional environment, thermal maturity, and hydrocarbon generative potential. The distribution patterns of n-alkanes, isoprenoids and steranes, high Pr/Ph ratios, abundant diterpanes, and the presence of non-hopanoid triterpanes indicate predominant source input from higher land plants. The contribution of aquatic organic matter was occasionally slightly elevated probably due to a raised water table. High hopane/sterane ratios and the occurrence of bicyclic sesquiterpanes and A-ring degraded triterpanes suggest microbial activity and the input of microbial organisms. Overwhelming predominance of gymnosperm-derived diterpanes over angiosperm-derived triterpanes suggest a domination of gymnosperms over angiosperms in local palaeovegetation during the period of deposition. The high Pr/Ph ratios, the plot of Pr/n-C17 versus Ph/n-C18, the almost complete absence of gammacerane, and the distribution pattern of hopanes suggest that the samples were deposited in a relatively oxic environment. Generally, fluctuation of redox potential is coupled with source input, i.e., less oxic conditions were associated with more aquatic organic matter, suggesting an occasionally raised water table. Comprehensive maturity evaluation based on Ro, Tmax, and biomarker parameters shows that the samples constitute a natural maturation profile ranging from marginally mature to a near peak oil window. Hydrogen index and atomic H/C and O/C ratios of kerogens suggest that the samples mainly contain type II/III organic matter and could generate mixed oil and gas.

This research study presents the results of an integrated study employing core samples, wireline logs, and seismic dataset collected in the study area for analysing the syn-depositional fault's control of the evolution of the axial meandering river delta-sublacustrine fan deposits during the Ed2x depositional period. The results indicate that the delta deposits that had developed during a retrogressive period, with the characteristics of retrogressive stacking patterns, had resulted from a high subsidence rate caused by the syn-depositional faults with large fault throw. These appeared to be characterized by strong amplitude, low frequency, and continuous seismic reflections on the seismic profiles, and high-amplitude serrated cylinder-shaped stacking patterns in the wireline logs, with no obvious sedimentary structures. In contrast, the delta deposits developed during a progressive period and characterized by progressive stacking patterns were determined to have resulted from a low subsidence rate caused by the syn-depositional faults with small fault throw. They were found to be characterized by weak-to-medium amplitude vermicule-shaped seismic reflections, as well as low-amplitude bell-shaped or funnel-shaped stacking patterns, with soft-sediment deformation structures.

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 (E 2 s 4 , 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 E 2 s 4 source rock interval in the Liaodong Bay sub-basin were studied using geological and geochemical data. The E 2 s 4 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), C 19 /C 23 tricyclic terpane (C 19 /C 23 TT), C 24 tetracyclic terpane/C 26 tricyclic terpane (C 24 Tet/C 26 TT), low to medium hopane/sterane (H/S), medium 4-methyl steranes/ΣC 29 steranes ratio (4-MS/ΣC 29 ST), C 35 22S/C 34 22S hopane (C 35 /C 34 SH) and high gammacerane/αβC 30 hopane (G/H), extended tricyclic terpane ratio [ETR=(C 28 TT + C 29 TT)/(C 28 TT + C 29 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 E 2 s 4 source rocks in the Liaozhong and north Liaoxi sags is too high to provide oils for fields. The E 2 s 4 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 E 2 s 4 -derived oils should be taken seriously in the future exploration in the south and middle Liaoxi sag and on adjacent uplifts.

The vast majority of discovered oils in the Bohai offshore area have undergone biodegradation varying from 1 to 9 on the PM scale (a scale to rank the level of biodegradation proposed by Peters and Moldowan, 1993). The distribution and biodegradation extent of all discovered oils in the Bohai offshore area were investigated systematically using geologic and geochemical data to reveal controlling factors of varying levels of biodegraded oils. Based on the analysis on environment and maternal 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. Spatial distribution and biodegradation extent are mainly controlled by 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. The longer the oils are present in these reservoirs or the larger the area of 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. C19 tricyclic terpane/C23 tricyclic terpane (C19/C23 TT), C24 tetracyclic/C26 tricyclic terpane (C24 Tet/C26 TT) and Gammacerane/C24 Tetracyclic (Gam/C24 Tet) 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 determined the origin of severely biodegraded oils (higher than PM 6) by correlating with extracts of possible source rocks in the Bohai offshore area.

The Northeastern Laizhouwan Sag (NLS) is a marginal sag in the Bohai Bay Basin, China. Paleogene source rocks from 3 wells (K-a, K-b and K-c) in this region, i.e., the third to the first member of the Shahejie Formation (Es3, Es2, and Es1), and the third member of the Dongying Formation (Ed3) were investigated for their petroleum potentials and depositional environments using geochemical and palynological methods. Among these members, the Es3 and Es1 members hold considerable hydrocarbon potential, while the rest are relatively fair. Although there are variations between the wells, the organic matter (OM) in the Es3 member is primarily composed of low mature or mature type I-II1 kerogen, which were primarily contributed by lower aquatic organisms and prokaryotes. During the Es3 stage, saline, stratified, and anoxic bottom water prevailed in the K-a well, while freshwater and weak reducing conditions dominated the K-b well. The OM in the Es1 member was contributed more by lower aquatic organisms than that in the Es2 member, but both are primarily composed of types II1-II2 kerogen; further, OM in the Es2 member is low mature, while that for the Es1 member ranges from immature to low mature, and higher maturity is expected in deeper buried places for the two members. Saline and anoxic bottom water prevailed during the both Es2 and Es1 members. Compared with other members, noticeable terrigenous OM was deposited during the Ed3 stage, which is dominated by immature type II2 kerogen. Lower salinity and weak reducing depositional conditions occurred in the Ed3 member. OM enrichment in the Es3 member was mainly determined by primary productivity, while that of the K-a well was also enhanced by the anoxic bottom water condition. The accumulations of OM in other members are speculated to have been controlled by the deposition rate.

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 (Es3) 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°C to 160°C (176°F to 320°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 A1 contained predominantly Es3-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 Es3-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 Es3 stratum may still have great potential for exploration along the slope of the Shijiutuo uplift.

The Tan-Lu fault system in the central Liaodong Bay subbasin, Bohai Bay Basin, has complex structural characteristics, and its tectonism during the Cenozoic is an important factor in oil accumulation. Three-dimensional seismic data were used to document the structural features and evolution of the system. Variations in source rock occurrence, oil catchment area, and faulting intensity were comprehensively evaluated to discuss the heterogeneity of oil occurrence along the system. Detailed analysis of the seismic data indicates a right-lateral slip for the Tan-Lu fault system. Transpression and transtension occur on two branches of the Tan-Lu fault with different orientations, indicating a likely slip azimuth that is between the two orientations, i.e., 30°–35°. The strike-slip began at the middle stage of deposition of the Shahejie Formation and reached its climax during deposition of the first and second members of the Dongying Formation. To date, the slip is still continuing. In the transpressional system, the lower strata on opposite sides of the strike-slip fault were shortened and folded, whereby the upper strata were stretched and normal faulted. As a result, a negative flower structure developed immediately above a positive flower structure. Strain distribution in this system is similar to that in a classic fold. Moderate tectonic deformation could enlarge the oil catchment area, benefiting oil accumulation. Weak or intense deformation is unfavorable for commercial oil accumulation because of a small oil catchment area and poor oil preservation, respectively.

A comparative study on organic geochemistry and kinetics was performed on two representative coal measure mudstone and coal samples of the Eocene Pinghu Formation in the Xihu Sag, East China Sea Shelf Basin, China. Gas chromatography–mass spectrometry analysis of the aliphatic fractions of the source rock extracts was performed to characterize the maturity, depositional conditions, and source materials in the samples. The kerogens isolated from the mudstone and coal were pyrolyzed in a closed-system at two heating rates to determine the kinetics for natural gas generation, and the results were extrapolated to geological conditions. Biomarker assemblages show that the mudstone (type III kerogen) and coal (type II/III kerogen) contain predominantly terrigenous source materials with the effect of microbial reworking, and were deposited under oxic conditions within an unstratified water column. The mudstone has slightly more algal material input and less oxic conditions during deposition than the coal. The rocks are at the early-mature stage but the coal is slightly more mature than the mudstone. The activation energies for natural gas generation from the mudstone and coal distribute mainly in the range of 47–62 kcal/mol and 50–69 kcal/mol with a frequency factor of 2.0 × 1011 s−1 and 1.0 × 1013 s−1, respectively. The lower activation energies for the mudstone than the coal suggests an earlier onset of natural generation. Assuming a constant burial heating rate of 3 °C/Ma for the calculated activation energy distributions, the calculated temperature for a given transformation ratio can differ by > 60 °C, indicating varying kinetic stability due to different kerogen composition and structure. This significant and unexpected difference suggests that more attentions should be paid to the kinetic heterogeneity of coal bearing source rocks, and we must be particularly cautious about selecting appropriate kinetic models in petroleum system modeling.

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 9-1 (PL9-1) oil field, which contains China's third largest offshore oil accumulation (in-place reserves greater than 2.28 × 10 8 ton or 1.49 × 10 9 bbl), was found in shallow reservoirs (700–1700 m, 2297–5577 ft) within the most active fault zone in east China. The PL9-1 field contains two oil-bearing series, the granite intrusions in Mesozoic (Mz) and both the sandstone reservoirs in Neogene Guantao (Ng) and Neogene Minghuazhen (Nm) Formation. The origins of the PL9-1 field, both in terms of source rock intervals and generative kitchens, were determined by analyzing biomarker distributions for 61 source rock samples and 33 oil samples. The Mesozoic granite intrusions, which hold more than 80% of the oil reserves in the field, were charged in the west by oil generated from the third member (Es 3 ) of the Shahejie Formation in the Bodong depression. The Neogene reservoirs of the PL9-1 field were charged in the west by oil generated from the third member (Es 3 ) of the Shahejie Formation in the Bodong depression and in the south by oil generated from the first member (Es 1 ) of the Shahejie Formation in the Miaoxibei depression. Interactive contact between the large fan delta and the mature source rocks residing in the Es 3 Formation of the Bodong depression resulted in a high expulsion efficiency from the source rocks and rapid oil accumulation in the PL9-1 field, which probably explains how can this large oil field accumulate and preserve within the largest and most active fault zone in east China.

The exploration history of Bohai Sea reveals that Liaodong Bay is relatively rich in natural gas, which is mainly gas condensate; however, the source rocks in Liaodong Bay share a medium maturity level with values of %Ro being generally Three immature Paleogene source rocks were selected to complete gold tube pyrolysis experiments. The experiment results reveal that the secondary gas from oil cracking can be neglected until the cumulative yields of C6–C14 compounds reach the maximum (corresponding to %Ro ∼ 1.3). Above %Ro ∼ 1.3%, oil starts to crack to wet gases substantially. Based on comparative analysis of pyrolysis results and source rock geochemical parameters, we constructed a hydrocarbon generation model, especially for hydrocarbon gas, and validated the calculated %Ro values for pyrolysis conditions. The C1–C5 yields for source rocks at ∼1.3 %Ro in gold tube pyrolysis approximate the maximum gas yield for source rock in open system pyrolysis, indicating the majority of primary gas from kerogen cracking has been generated at ∼1.3 %Ro, with its yields being 156 ml/g TOC, 100 ml/g TOC and 117 ml/g TOC for Ed3, Es1 and Es3 source rocks, respectively. This suggests that source rocks with %Ro

The Bohai Bay Basin is a classic non-marine rift basin in eastern China. The Paleogene Dongying sequences are the main hydrocarbon-bearing stratigraphic unit in the basin. Using three-dimensional (3-D) seismic data and one well control in the BZ3-1 Block in the western slope of the Bozhong Sag, we analyzed 3-D facies architectures of the Dongying sequences. The Dongying Formation, a second-order sequence, can be subdivided into four third-order sequences (from base to top: SQ1, SQ2, SQ3, and SQ4). The facies architecture was analyzed by using the seismic sedimentology approach based on 3-D seismic data. Sediment of the Dongying sequences was derived from the northern Shijiutuo Uplift via four major configurations of incised valleys, namely “V”, “U”, “W”, and composite shaped incised valleys. Seismic stratal slices reveal branching and converging characteristics of the channels from upstream to downstream. On the basis of an integrated analysis of well log, core data, seismic facies based on multi-seismic attributes, three sedimentary facies (e.g., “delta”, “fan-delta”, and “shore” or “shallow lacustrine” facies) have been recognized. The four types of incised valleys and their evolution control the sedimentary systems in the sedimentation area. The numbers and sizes of the fans are controlled by the sedimentary systems at various scales. Incised valley-fill and deltaic sand bodies are excellent hydrocarbon reservoirs and potentially good exploration targets for the study area. The reservoir quality of sequences SQ1, SQ2, and SQ3 become better gradually from base to top. The proposed sediment dispersal patterns may aid in the prediction of potential reservoir distribution. This study also demonstrates that facies architecture analysis using sequence stratigraphy and seismic sedimentology may serve as an effective approach for constructing 3D facies models for petroleum exploration in areas lacking of well or outcrop data.

The Bodong Sag, located in the Bohai Sea, offshore China, is one of the most petroliferous basins in China. Based on three dimensional seismic reflection data and time slice data, we analyze the fault system of the Bodong area in detail, establish the fault structure pattern of different types and summarize the distribution of the fault system. It is concluded that the development characteristics of the Cenozoic fault system are in accordance with the dextral stress field of the Tanlu Fault, which displayed a brush structure with NNE strike-slip faults as its principal faults, NE-trending extensional faults as secondary faults and EW-trending faults as minor faults. Faults can be divided into (1) strike-slip type, (2) extensional type, (3) strike-slip extensional type and (4) extensional strike-slip type. The spatial structures of different faults have obvious differences because of the fault properties and activity intensity. The fault system at different stages shows tremendous differences because of the transition of the Tanlu Fault from sinistral strike-slip to dextral strike-slip, the transition between extension and strike-slip, and the transition from mantle upwelling to thermal subsidence. According to the controlling effect of faults on basin structure, the Cenozoic basin experienced four evolutionary stages, (a) transition stage from sinistral strike-slip to dextral strike-slip, (b) strike-slip extensional faulted stage, (c) extensional strike-slip faulted stage and (d) strike-slip depression stage. The identification of temporal and spatial differences of faults could be used as a significant guideline for oil and gas exploration in the Bodong area.

The Liaozhong Sag is one of the major oil-bearing areas in offshore Bohai Bay Basin and is obviously characterized by late-stage hydrocarbon accumulation. In this paper our study was focused on strong activities of tectonic movement in the Liaozhong sag. In terms of source rock thermal history analysis, generation, migration and accumulation of hydrocarbon, late-stage hydrocarbon accumulation and enrichment pattern were summarized. Apatite fission track analysis and evolutionary history of the Easy% Ro model simulation of source rock showed that major source rocks became matured rapidly at the time 5.1Ma from the present. Fault analysis, showed that hydrocarbon migration in the study area was mainly the result of fault transportation. Fluid inclusions indicated that the accumulation period of oil and gas reservoirs was 5.1Ma from the present. Our study showed that late-stage rapid hydrocarbon generation, late-stage vertical fault transportation, hydrocarbon accumulation near source rocks and late-stage rapid in-filling are the main features of hydrocarbon accumulation in the Liaozhong Sag. The abnormal high pressure and fluid inclusion homogenization temperature anomaly are indications of late-stage episodic reservoiring and in-filling. Regional reservoir-cap rock combination of Eocene Shahejie Formation and Oligocene Dongying Formation, predominant transport channels and relevant traps formed by the complex tectonic movement of the Tan-Lu Fault are major factors controlling hydrocarbon accumulation. Episodic reservoiring of formation overpressure, near-source accumulation and fault control constitute the oil and gas enrichment pattern of the Liaozhong Sag.

This paper presents revision of the Neogene chronostratigraphic framework of the Huanghekou area in the Bohai Bay Basin, northern China, on the basis of integration of palaeomagnetic data and Neogene seismic stratigraphy. The modern Lake Dongting was selected for analyses of bottom sediments, water depth and elemental composition. Four elements, namely Al, V, Ni and Ga with R 2 value larger than 0.7, were chosen to establish a quantitative relationship (F P) between elemental composition and water depth of bottom sediments. A water depth of ~2.4 m was identified as an accumulation depth of Candoniella albicans, which is used to derive environmental variable (Φ) of formula F P. Candoniella albicans is also found within the Neogene well W10 in the Huanghekou area. Considering 2.4 m as D value in formula F p and measured element values at different depths of W10, Φ values were obtained at different depths. The quantitative palaeowater depths of each drilling well were estimated for the first time in this area by considering elemental composition of chip samples of four drilling wells in the Huanghekou area, which are directly iterated to the F F. Palaeowater depth reconstruction of the Neogene in the Huanghekou area indicates an initial deepening trend and a later shallowing trend during the evolution, with average and deepest bathymetry at ~2.3 and ~4.8 m, respectively. Frequent fluctuations of palaeowater depth control the sand dispersal pattern in a shallow delta during the lower part of the Lower Member of Minghuazhen Formation.

For lack of 3D seismic data and logging data,the previous research of sedimentary facies in the Bohai sea sag were not detailed enough for fine reservoir description.Based on the latest high-resolution three-dimensional seismic data,we carried out the high-resolution well-seismic sequence analysis and seismic sedimentology research for the Lower Ed2 of Dongying Formation in the BZ3-8 Block,western slope of Bozhong Sag.Results show that on the north and the south sides of the study area,there occurred different sediment provenance systems and facies.During LST and HST periods,fan-shaped and flower-shaped seismic multi-attributes and amplitude anomalies dominated in the northern part of the study area,indicating fan-delta deposits,while northeast banded seismic multi-attributes and amplitude anomalies were found in the southern part of the study area,that was interpreted as the braided-river delta deposits.During the TST period with the rapid rise in lake level,small-scale fan deltas remained in the north.The sedimentary facies evolution of the system tracts of the sequence of Lower Ed2,from LST(rich sand),TST(rich mud) to HST(rich sand),reflects a classical distribution pattern of stratigraphic lithofacies in a sequence stratigraphic framework,which is significant to reservoir and source rock prediction.

This paper discusses origin and charging histories of the Bozhong (渤中) 25-1 field (BZ25-1) in the Bozhong sub-basin, Bohai (渤海) Bay basin. Petroleum is contained in both Paleogene and Neogene reservoirs in the BZ25-1 field. The origin of the field was studied using biomarker distributions for 61 source rock samples and 34 oil samples. Oil in the Paleogene reservoirs was derived from the third member (Es3, 43.0–38.0 Ma in age) of the Eocene Shahejie (沙河街) Formation, whereas oil in the Neogene reservoirs was a mixtures of oil generated from the third member and the first member (Es1, 35.8–32.8 Ma) of the Eocene Shahejie Formation. Charging of the shallow Neogene reservoirs was dynamic, probably ongoing, which was a combined result of the existence of active source rocks, the development of overpressure and the fault reactivation since about 5.1 Ma. On the contrary, the deep Paleogene reservoirs experienced intensive oil leakage as indicated by the high GOI (grains-containing-oil-inclusions) values (up to 30%) for samples below the present-day oil-water contact. Part of the oil that leaked from the Paleogene reservoirs re-accumulated in the shallow Neogene reservoirs. The variation of oil origins within the BZ25-1 field, and the dynamic petroleum charging into the shallow Neogene reservoirs and leakage from the deep Paleogene reservoirs have important implications for petroleum exploration.

In this paper, the origin and accumulation time of CO2, and the transformation of clastic reservoir bed by CO2 in the A2 structure at the actic area of Qin Nan Sag were studied. The elevated δ13CCO2, R/Ra values and 40Ar/36Ar values showed that CO2 was mantle derived. CO2-bearing inclusions were associated with hydrocarbon inclusions, and CO2-bearing inclusions included hydrocarbon components, showing migration and filling of CO2 and hydrocarbon at the same stage. Basin modeling and homogenization temperature of inclusions indicated that the time of hydrocarbon and CO2 accumulation was mainly less than 3 Ma. CO2 late filling resulted in strong dissolution of feldspar and carbonate cement, with a high content of intragranular dissolution pores and a high proportion of intergranular primary pores, mostly from carbonate cement dissolution. Acidic environment inhibited illite growth, and dissolution of feldspar and acidic environment promoted formation of kaolinite. CO2 late filling brought out obvious thermal fluctuation, which was in favor of redistribution of dissolved matter, and top strong cementation was found at rock cores. Because of late accumulation of CO2, acidic environment of reservoir bed lasting for a long time, reservoir bed at a high temperature, undergrown dawsonite showed weak influence on reservoir bed quality. Dissolution of feldspar and carbonate cement, inhibition of illite growth, a high content of authigenic kaolinite, a low content of dawsonite, and redistribution of dissolved matter played a decisive role in improving reservoir quality, with rarely seen high permeability.

The Cenozoic basin offshore the Bohai Sea underwent a multicycle-rifting during its evolutionary process, which resulted in the multiple unconformities in the strata. The tectonic activities shown by these unconformities have different manifestations and influences on the basin evolution. The authors systematically analyze the tectonic evolution characteristics of the sags offshore the Bohai Sea with a large set of hydrocarbon exploration data. The analysis reveals that two phases of tectonic activities during the late depositional stage of the third member of Shahejie Formation (about 38 Ma) and the late depositional stage of Dongying Formation (about 24 Ma) reflect the significant changes in the basin’s features and structural framework before and after these tectonic activities. As a result, the two phases of tectonic activities are recognized as important turning points (i.e., tectonic transitions) of Cenozoic basin evolution. The regional dynamic backgrounds of the two phases of tectonic transitions are also discussed. It is suggested that the early tectonic transition occurred at about 38 Ma under such regional dynamic background that a huge kinematics adjustment happened between Eurasian Plate and its neighboring plates, i.e., Pacific Plate and Indian Plate. Meanwhile, the Tan-Lu Fault’s slip reversed from left lateral to right lateral. The late tectonic transition occurred in late Paleogene (about 24 Ma) and reflected the Himalayan orogeny’s influence on the Chinese continent and even the Asian continent; at the same time, the stress field produced by the escape tectonics was related to the Himalayan orogeny, superposed on the pre-existing stress field, and then enhanced the right lateral slip activity of the Tan-Lu Fault.

The sequence-stratigraphic framework and depositional systems of an Oligocene lacustrine rift succession in the Liaozhong depression of Bohai Bay Basin in northeastern China were investigated using seismic profiles, complemented by well logs and cores. Seven sequences are identified on the basis of unconformities on basin margins and correlative conformities in the basin center. Depositional systems are associated with systems tracts within sequences. Lowstand systems tracts are dominated by sublacustrine fans; transgressive systems tracts are mainly composed of beach-bar deposits; and highstand systems tracts are characterized by deep lacustrine fan or braid-delta deposits. Episodic tectonic movement was the principal factor that controlled the development of the sequences. Lake-level changes resulting from climatic oscillations may have controlled the development of these sequences. The balance between tectonic subsidence and sediment supply controlled the type of lake basin. The Bohai Bay Basin was a balanced-fill basin in the early and middle stages of the Dongying Formation and evolved into an overfilled basin at the end of deposition of the Dongying Formation. This study enhances the understanding of depositional system configuration and systems tracts in a typical rift basin and proposes that the most favorable sandstone reservoirs are developed in sublacustrine fan deposits. Relative lowstand sublacustrine fan deposits capped by relative highstand prodelta or deep lacustrine mudstones form the highest potential lithostratigraphic traps.

Three Paleogene syn-rift intervals from the Bohai Bay Basin, the most petroliferous basin in China, were analyzed with sedimentological and geochemical techniques to characterize the lateral source rock heterogeneities, to reveal the environmental and ecological changes through geologic time and to construct depositional models for lacustrine source rocks under different tectonic and climatic conditions. The third (Es3) and first (Es1) members of the Eocene Shahejie Formation and the Oligocene Dongying Formation (Ed) display widely variable total organic carbon contents, hydrogen indices and visual kerogen compositions, suggesting changes in organic facies from deep to marginal sediments. Carefully selected deep-lake facies samples from any interval, however, display fairly uniform biomarker composition. These three intervals have distinctly different biomarker assemblages, which indicate weakly alkaline, freshwater lakes with a moderately deep thermocline during Es3 deposition, alkaline-saline lakes with shallow chemocline during Es1 deposition and acidic, freshwater lakes with deep, unstable thermocline during the deposition of the Dongying Formation. Such environmental changes corresponded to changes in subsidence rate and paleoclimate, from rapid subsidence and wet climate during Es3 deposition, through slow subsidence and arid climate during Es1 deposition to rapid subsidence and wet climate during Ed deposition and resulted in synchronous changes in terrigenous organic matter input, phytoplankton community and primary productivity. The co-evolution of environments and organisms controlled by tectonic subsidence and climate accounted for the deposition and distribution of high quality lacustrine source rocks with distinctly different geochemical characteristics. Most rift basins experienced changes in subsidence rates and possibly changes in climates during their syn-rift evolutions. The models constructed in this paper may have important implications for source rock prediction in other lacustrine rift basins.

The Bohai Offshore Area is located in the northeastern marine area of the Bohai Bay Basin in northern China. It is one of the most productive petroleum basins in China. In this study, thermal gradient and heat flow distribution results for the Bohai Offshore Area (Bohai Subbasin) were obtained based on new oil-testing temperatures, drill-stem test temperatures (DSTs), bottom-hole temperatures (BHTs) and documented thermal conductivity data. The average geothermal gradient is 32.0±4.6°C/km for the entire offshore area. Heat flow in the Bohai Subbasin ranges from 33.5 mW/m2 to 84.0 mW/m2, with an average value of 60.8±8.7mW/m2. Thermal gradient evolutions of several sags in the Bohai Subbasin were obtained using constraints for single well modeling from vitrinite reflectance ( Ro) and apatite fissin track (AFT) data. Modeling results show that there were different thermal histories for different parts of the subbasin. Generally, the thermal gradient gradually decreased with geological evolution. The thermal gradient was 57∼53°C/km in the Paleocene epoch and 53∼43°C/km during the deposition period of the Shahejie Formation (e.g., Eocene epoch). This value slowly decreased after the Oligocene (38∼43°C/km). It was 38∼34°C/km in the Miocene, and it is 32°C/km at present-day. This thermal gradient evolution relates well to activities in the Tanlu Fault Zone and to tectonic subsidence. The first episode of the syn-rift initial subsidence of the basin shows a higher thermal gradient during the Paleocene∼Eocene epochs. The second episode of thermal subsidence corresponds to a lower thermal gradient after the Oligocene epoch. The geothermal regime of the Bohai Offshore region will provide some useful data to study regional tectonic characteristics of the basin and the evolution of hydrocarbon source rocks.

This paper discusses origin and charging directions of oil fields on the Shaleitian Uplift, Bohai Bay basin. The Shaleitian Uplift is a footwall uplift surrounded by three sags containing mature source rocks. The origins of the four oil fields on the Shaleitian Uplift, both in terms of source rock intervals and in terms of generative kitchens, were studied using biomarker distributions for 61 source rock samples and 27 oil samples. Hierarchical cluster analysis using 12 parameters known to be effective indicators of organic matter input and/or depositional conditions allowed the identification of six oil types or classes. These six oil classes could then be linked to three distinct source rock intervals ranging in age from 43.0 Ma to 30.3 Ma. The third member (43.0–38.0 Ma in age) and first member (35.8–32.8 Ma) of the Eocene Shahejie Formation, and the third member of the Oligocene Dongying Formation (32.8–30.3 Ma) each sourced one class of oil. The other three classes represent mixtures of oil generated from multiple source rock intervals. Traps on the Shaleitian Uplift were charged in the east by oil generated from the Eocene Shahejie Formation in the Bozhong Sag, in the southeast by oil generated from the Eocene Shahejie and then Oligocene Dongying formations in the southwestern part of the Bozhong Sag and/or in the eastern part of the Shanan Sag, and in the southwest by oil generated from the Eocene Shahejie Formation in the western part of the Shanan Sag. The estimated migration distances range from less than 5 km to about 20 km. The compositional heterogeneity within fields and multiple-parameter comparisons between oils from nearby wells in different fields have proven to be a powerful tool to determine the in-filling histories of oil fields in cases where multiple source rock intervals and multiple generative kitchens exist.

Wrench motion is relatively common in basins but often goes undetected because of its subtle structural imprint. Even so wrench-related folding is of significance in petroleum exploration and development. Based on the Riedel shear nomenclature, we apply the model of wrench-related folding to deformation in the Bohai sea basin. The folding related to R-shears, T-tensional structural systems (T-systems) and C-compressive structural systems (C-systems) in a Riedel shear model are discussed. The R-system related folding results from differential movements between the two sides of fault blocks, in map view the fold is oriented (90°-φ/2) (in the initial deformation stage) or more than (90°-φ/2) from the principle displacement zone (PDZ), where φ is the angle of internal friction. The T-system related folding is related to the rollover of the hanging wall, and in map view the fold is initiated at 45° from the PDZ in the initial deformation stage. The C-system related folding is resulted from the compression induced by a wrench action, in map view the fold was oriented 45° from initial deformation in the PDZ. Bohai Sea basin fold orientations fit our models, and the wrench-related folds play important roles in oil and gas accumulation.

Using the new high-quality 3D seismic data, this paper addresses the salt structures in the KL11 area of the Laizhouwan depression in the southern offshore Bohai Bay basin. In the study area, the salt in the Sha-4 Member of the Paleogene Shahejie Formation thickened, and then formed an S–N trending salt wall, which changes shape regularly along its trend from salt diapir to salt pillow. The change in thickness of the suprasalt layers record five growth phases of the salt wall from the Eocene to the Quaternary: (1) early diapirism, (2) active diapirism, (3) passive diapirism, (4) relative structural quiescence, and (5) arching. The evolution of the salt structures was mostly governed by the multi-phase compression induced by the dextral strike-slip of the Tan–Lu fault, which formed a restraining bend in the study area. There was an original passive stock in the south, which was later tectonically squeezed by E–W compression and became a diapir. As the shortening propagated to the north from the original stock, the salt pillow was created in the north. Relative structural quiescence then followed until the next phase of compression, which arched the thick roof of the salt wall.

The origin of the fourteen major oil fields in the Bozhong sub-basin, Bohai Bay basin was studied based on the results of Rock-Eval pyrolysis on more than 700 samples and biomarker analysis on 61 source rock samples and 87 oil samples. The three possible source rock intervals have different biomarker assemblages and were deposited in different environments. The third member of the Oligocene Dongying Formation (E3d3, 32.8–30.3 Ma in age) is characterized mainly by high C19/C23 tricyclic terpane (>0.75), high C24 tetracyclic terpane/C26 tricyclic terpane (>2.5), low gammacerane/αβ C30 hopane (

The Shijiutuo uplift is a major uplift to the north of the Bozhong depression, the largest generative kitchen in the Bozhong subbasin, Bohai Bay Basin. Although the N35-2 trap on this uplift contains a medium-size oil accumulation and the Q32-6 trap contains China's third largest offshore oil accumulation, the Q31-1 trap between the N35-2 and Q32-6 traps with very similar evolution history was confirmed to be dry. Biomarker associations of crude oil and source rock samples were analyzed, and three-dimensional migration pathway modeling was conducted to investigate the origin of oils and mechanisms for oil enrichment and depletion on the uplift. Multiple-parameter oil-source correlation and hierarchical cluster analysis using 10 selected biomarker parameters allowed the identification of four source-related oil classes. Almost all oils from the Shijiutuo uplift are derived from the Eocene Shahejie Formation, whereas oils found between the Shijiutuo uplift and the Bozhong depression either are derived from or have important contributions from the Oligocene Dongying Formation. Variations in oil classes and biomarker parameters suggest sequential migration of oil generated from the Shahejie and then Dongying formations in the Bozhong depression, which is reasonably supported by petroleum migration pathway modeling. Oil charge from two oil-prone source rock intervals and, more importantly, focusing of oil originating from a large area of the Bozhong generative kitchen into the same trap accounted for oil enrichment and formation of China's third largest offshore oil field in the Q32-6 structure. The complexity and primary control of the sealing surface (top surface of the carrier bed) morphology on the positions of migration pathways caused the Q31-1 trap to be shielded from migration of oil originating from the Bozhong depression, resulting in oil depletion in this trap. Shadows to petroleum migration may occur because of the three-dimensional behavior of petroleum migration, and two-dimensional migration modeling may be misleading in predicting petroleum occurrences.

The Penglai 19-3 (PL19-3) oil field, the largest offshore oil field in China, was found in shallow reservoirs (700–1700 m, 2297–5577 ft) within the most active fault zone in east China. The PL19-3 anticline was not finally formed until about 2.0 Ma and is cut by densely distributed faults. Source rock and crude oil samples from the PL19-3 field were analyzed to determine the origin and formation mechanisms of this large oil field. Three organic-rich, oil-prone source rock intervals exist in the Bozhong subbasin, each of which has a distinct biomarker assemblage. Oil samples from different wells have different biomarker associations, and three source-related oil classes were identified within the PL19-3 field based on biomarker compositions and multivariate analysis of the data. The PL19-3 field displays considerable compositional heterogeneity. The compositional heterogeneity within the field and comparison between oil samples from the PL19-3 field and those from nearby structures suggest three field-filling directions, which is consistent with the results of migration pathway modeling. The PL19-3 field was charged in the north by oil generated from Dongying Formation source rocks in the eastern Bozhong depression and Bodong depression, in the southeast by oil generated from Shahejie Formation source rocks in the Miaoxi depression, and in the northwest by oil generated from Shahejie Formation source rocks in the central Bozhong depression. Oil charge from multiple source rock intervals and multiple generative kitchens and focusing of oil originating from a large area of the Bozhong depression into the same trap resulted in rapid oil accumulation in the PL19-3 structure and the formation of this large oil field in a very young trap within an active fault zone.

The Tan-Lu fault zone across the eastern margin of the Cenozoic basins offshore the Bohai Sea is a NNE-trending right-lateral strike-slip fault system developed in the Cenozoic basin cover. It cuts through NE-to NNE-striking major extensional faults that controlled the formation of Paleogene basins. Recent petroleum exploration indicates that Cenozoic structural activities of the Tan-Lu fault system have directly or indirectly affected oil and gas distribution offshore the Bohai Sea. As part of a deep fault zone the Tan-Lu fault zone has been activated since the Oligocene, and obviously affected the tectonic evolution of offshore Bohai basins since then. The formation of Paleogene rift basins offshore the Bohai Sea has utilized the pre-existing structural elements of the Tan-Lu fault zone that developed in the late Mesozoic.