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... As an important petroliferous basin in East China, the Bohai Bay Basin had over 50 sags, and each a sag had relatively independent hydrocarbon generation and accumulation units (Lu and Qi, 1997;Zhao and Chi, 2000;Jiang et al., 2014Jiang et al., , 2015a. Most petroliferous sags in the basin were of overpressure, predominantly caused by undercompaction and hydrocarbon generation. ...
... Most of the large-scale overpressure sections were distributed in the Member 4, Member 3 and Member 1 of Shahejie Formation and Member 3 of Dongying Formation. Since the Paleogene sedimentary center migrated from the basin periphery to the basin center (the area around the central Bohai Bay), spatial and time distribution of Paleogene source rocks gradually became new and more from the basin periphery to the basin center (Zhao and Chi, 2000), so did overpressure horizons. ...
... It could be seen that the main formation period of overpressure coincided with the major hydrocarbon-generation period, thus the formation of overpressure was mainly related to hydrocarbon generation. Vertical distribution of major hydrocarbon source rocks and difference in the major hydrocarbon-generation period were the main causes for differences in current pressure structures of the three types, and the deep reason caused such differences was the different filling evolution history of sags (Lu and Qi, 1997;Zhao and Chi, 2000;Jiang et al., 2014Jiang et al., , 2015a. ...
Pressure fields of different sags in a petroliferous basin were significantly different, and it was related to hydrocarbon enrichment. Based on pressure data of different sags in Bohai Bay Basin, types of pressure field and its distribution characteristics were well discussed, and relationships between overpressure and hydrocarbon generation, pressure field and hydrocarbon enrichment were also investigated. Results showed that the Paleogene pressure fields of different sags in Bohai Bay Basin could be divided into three types: normal pressure type, single overpressure type and double overpressure type. These three types of pressure fields had the zoned features in their distributions. The normal pressure fields were mostly distributed in peripheral sags of the basin, while the single overpressure fields were widespread in the basin, and the double overpressure fields were concentrated in the areas around the Bohai Sea. Hydrocarbon generation had a significant effect on overpressure formation, thus overpressure horizons generally were corresponded to major source rock horizons, and differences of filling evolution history and main hydrocarbon-generation strata of different sags might be important factors for the formation of these three types of pressure fields. Overpressure in source rock horizons was closely related to hydrocarbon enrichment. Horizontally, hydrocarbons were mainly distributed around the overpressure center, and the secondary migration distance of hydrocarbons was affected by the degree of overpressure. Vertically, hydrocarbon distribution was controlled by the type of pressure fields. Hydrocarbons in the normal pressure sags were mainly enriched in the major hydrocarbon-generation horizons and their adjacent horizons, hydrocarbons in the single overpressure sags were mainly distributed in the hydrocarbon-generation horizons and its upper and lower horizons, while in the double overpressure sags, the hydrocarbon enrichment degree in Neogene was high. Hydrocarbon enrichment in sags was controlled by the degree of overpressure in hydrocarbon-generation horizons, and these oil-rich sags had relatively large overpressure degree.
... There have been a lot of studies focusing on the Neogene hydrocarbon accumulation in the Bohai Bay Basin (Deng, 1999;Zhao and Chi, 2000;Zhu et al., 2009;Zhao et al., 2015). Their findings especially advanced in the fundamental petroleum geology (Jiang et al., 2014;Deng et al., 2017), the fault-controlled reservoirs Cai et al., 2001;Gong and Wang, 2001;Mi, 2001), the oil and gas distribution pattern (Xue et al., 2001;Zhang et al., 2003Zhang et al., , 2008, and the hydrocarbon accumulation mechanism and its features (Hao et al., 2004;Deng, 2012;Zhou et al., 2016). ...
The PL 19–3 Oilfield is the only super-large monolithic oilfield with oil and gas reserves up to 1 × 109 t in the Bohai Bay Basin, and it has been successfully developed. Exploration and development practices have provided abundant data for analyzing formation conditions of this super-large oilfield. On the basis of the exploration and development history, fundamental reservoir features, and with available geological, geophysical and test data, the hydrocarbon accumulation conditions and key exploration & development technologies of the PL 19–3 Oilfield were discussed. The key conditions for forming the super-large Neogene oilfield include four aspects. Firstly, the oilfield is located at the high position of the uplift that contacts the brachy-axis of the multi-ridge slope in the biggest hydrocarbon-rich sag in the Bohai Bay Basin, thus it has sufficient hydrocarbon source and extremely superior hydrocarbon migration condition. Secondly, the large-scale torsional anticlines which formed in the Neogene under the control of the Tanlu strike-slipping movement provide sufficient storage spaces for oil and gas preservation. Thirdly, the “multiple sets of composite reservoir-caprock assemblages” developing in the special shallow-water delta further contributes greatly to the effective storage space for oil and gas preservation. Fourthly, due to the coupling of the uplift and strike slip in the neotectonic period, extensive faulting activities constantly released the pressure while the late period massive hydrocarbon expulsion of the Bozhong took place at the same time, which assures the constant and intense charging of oil and gas. The super-large PL 19–3 Oilfield was controlled by the coupling effects of all those special geologic factors. In view of this oilfield's features (e.g. violently reformation caused by strike slip, and the special sedimentary environment of shallow-water delta), some key practical technologies for exploration and development have been developed. Such technologies include: the special prestack depth migration processing for gas cloud zones, the prediction of thin interbed reservoirs based on high-precision inversion of geologic model, the reservoir description for the shallow-water braided river delta, the quantitative description for remaining oil in the commingled oil reservoirs with wide well spacing and long well interval, and the well pattern adjustment for formations during high water cut period in the complex fluvial-facies oilfields. Keywords: PL 19–3, Hydrocarbon accumulation conditions, Multiple composite reservoir-caprock assemblages, Constant and intense charging of oil and gas during later period, Gas cloud zone, Exploration and development technology
... It transects the eastern margin of the Bohaiwan basin, and has significantly influenced the formation and evolution of Cenozoic subbasins in the eastern Bohaiwan basin and its hydrocarbon accumulation and reservoir formation. However, different researchers have different understandings on the Tan-Lu fault zone offshore the Bohai Sea  . This paper will describe and discuss the structural characteristics of the Tan-Lu fault zone offshore the Bohai Sea based on the interpretation of seismic data from the Cenozoic basins. ...
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.
Apatite/zircon fission track thermochrono1gy from 16 samples in the Taishan Mountain, Luxi uplifting reveals the uplifting time and speed for the region after late Proterozoic time. The data show that the Taishan Mountain was characterized by slow uplifting speed from late Proterozoic to late Cretaceous and rapid uplifting after Cenozoic by at least three rapid uplifting events, during 48Ma (early Eocene), 44 - 37Ma (mid-late Eocene) and 23 - 20Ma (early Miocene). These relatively rapid uplifting events in Cenozoic coupled well with the tectonic evolution of the Jiyang Depression. The uplifting events imply that there was a union basin of the Jiyang depression Lu xi uplifting before the time of 44Ma, and the union basin was separated by the 44 - 37Ma's uplifting events of the Taishan Mountain. Moreover, it controlled the forming of the hydrocarbon rocks of the Jiyang depression. These Cenozoic uplifting events are also corresponded with the tectonic evolution of the Bohai Bay Basin and its peripheral mountains.
In the prolific Zhanhua Depression of the Bohai Bay Basin, eastern China, over 76% of the proven petroleum reserves occur in the Neogene Guantao and Minhuazhen formations. Detailed geological and geochemical mapping of the Phanerozoic strata in this area reveals that oil shales and dark mudstones in the Es4, Es3 and Es1 members of the Eocene–Oligocene Shahejie Formation have excellent potential as petroleum source rocks. Regional distributions of total organic carbon content, kerogen type and thermal maturation indicate that abundant mature source rocks are present in several discrete sag areas bounded by extensive basement-related fault networks. In contrast, source rocks along the paleo-topographic highs are currently immature with respect to the generation of conventional oils. This study examined the key molecular characteristics for the common isoprenoid, hopanoid and steroid biomarkers, in relation to source facies variations in lacustrine sediments. The hydrocarbon source potential and expulsion efficiency for each of the source rock units were evaluated, which integrated the source rock characteristics with regional thermal subsidence and sediment burial. An important observation made during this study is the striking contrast between the source potential and proven oil reserves within each stratigraphic section, illustrating how significant the migration of hydrocarbon fluids derived from deep mature source kitchens through over 2000 m of fine-grained formations must have been to form several giant oil accumulations in the shallow strata. These results support the concept of the “Neogene fault-fracture mesh petroleum plays” proposed by Zhang et al. (2004) [Marine & Petroleum Geology, 21, 651–668] and the vital role that the faults and fracture systems have in controlling hydrocarbon migration conduits and accumulation habitat in this rift basin.
The concept and division method for petroleum accumulation system, based on layer structure, are presented according to characteristics
of layer structure and reservoirs. The petroleum accumulation system of Jiyang depression can be divided into four levels,
namely petroleum accumulation system combination, petroleum accumulation system, oil-gas reservoir combination and reservoir.
The petroleum accumulation system combination includes three types of genetic pattern: other-source buried-ridge accumulation
system combination in pre-Paleogene, medium-deep layer self-source accumulation system combination in Paleogene, and shallow
layer other-source accumulation system combination in Neogene. This paper also describes the reservoir types and their distribution
characteristics of different petroleum accumulation system combinations and their interior units.
Based on the results of laboratory simulation experiments and field observations, the concept of ‘fault-fracture mesh petroleum play’ is proposed to explain the secondary petroleum accumulations in the Neogene reservoirs of the Jiyang Superdepression, Bohai Bay Basin, Eastern China. A well-defined fault-fracture mesh petroleum play usually contains three basic elements, an oil source network at the base, a transient storage for migrating hydrocarbons in the middle, and a petroleum accumulation network at the top. The oil source network provides fairways for hydrocarbons generated in a deep source to migrate vertically upward to shallow strata, often consisting of primary faults and fracture zones and/or regional unconformities in direct contact with mature petroleum source beds, and secondary faults intercepting lenticular fluvial sandstones. The transient storages for migrating hydrocarbons include the thick, porous, mat or sheet-like sandstones deposited under the fluvial, braided stream setting and occurring over relatively large geographic areas. These sandstones serve as temporary storage spaces, through which deep-sourced petroleum fluids migrate further, either laterally or vertically into shallower reservoirs. The upper petroleum accumulation network provides suitable habitats for oil and gas accumulation, comprising structural, lithologic, stratigraphic and combination traps. The Jiyang Superdepression of the Bohai Bay Basin hosts several excellent examples of the fault-fracture mesh petroleum play, where hydrocarbons in the Neogene reservoirs were derived either directly from Eocene-Oligocene source rocks, or from the redistribution of pre-existing oil accumulations in the deeper reservoirs. Reactivation of basement-involved faults and regional unconformities provided conduits for channeling the pre-Neogene hydrocarbons into the shallow strata. The magnitudes and frequencies of the faulting activities and their relative timings to those of peak oil generation and expulsion were critical factors for controlling the quantities of hydrocarbons transmitted to the Neogene strata. The thick sandstones and conglomerates in the lower Guantao Formation acted as excellent transient storages for the deep sourced oils, which were then trapped to form accumulations in the shallow structural, lithologic, stratigraphic and combination traps of the Guantao (Ng)–Minhuazhen (Nm) formations. With the recent discovery of several giant oilfields in the shallow Neogene reservoirs both onshore and offshore Bohai Bay Basin, the proposed ‘fault-fracture mesh petroleum play’ concept helps the exploration geologists to modify and clarify long held traditional concepts of petroleum play types in this basin, and expand upon the new exploration horizons of the shallow subtle petroleum traps.
In order to understand the controlling of intensively faulted zones on the hydrocarbon migration and accumulation, the Wangfu depression was selected as a case study. Based on the lateral and vertical characteristics of intensively faulted zones, the intensively faulted zones were classified into four types in the Fuyang oil layer of Wangfu depression, including graben-antithetic fault terrace, graben-antithetic-consequent fault terrace, graben-horst-antithetic fault terrace and graben-horst-consequent fault terrace. Through analyzing evidences of Fuyang oil layer in intensively faulted zones, the effect of intensively faulted zones on hydrocarbon migration and accumulation were discussed. The results show that the intensively faulted zones have transverse sealing and lateral fluid conduction function in secondary migration, and influence oil-gas accumulation. While the strike of the intensively faulted zone nearly parallels the dip direction of formation, the horst and antithetic on both sides of it are favorable part to collect oil. While the strike of the intensively faulted zone nearly plumbs the dip direction of formation, the horst and antithetic on the side that is near the depression center are propitious to catch oil. The crossovers of the intensively faulted zones are avail for oil accumulation.
The Liaodong dome is a region of localized uplift and deformation within the Liaodong Bay, Bohai, offshore China. 3-D seismic dataset, vitrinite reflectance and apatite fission track data provide an exceptional opportunity to document the evolution of the Liaodong dome which developed coeval with the Tan-Lu strike-slip fault zone with a rift system. The 3-D seismic data demonstrates that the dome formed before the deposition stage of EsM sequence (40Ma), and it uplifted again during or after the depositional stage of Ed sequence (32-24Ma). Apatite fission track and vitrinite reflectance data indicate that the dome experienced two stages of cooling episodes, Paleocene to Early/Middle Eocene (65-40Ma) and Late Oligocene to Late Miocene (30-11.5Ma), usually indicating uplifting. Both the seismic stratigraphy and thermal history analysis show that: 1) the Liaodong dome is part of the Jiaoliao terrane; 2) regional continental rifting climaxed during 65-40Ma, as it did the rift-shoulder uplift of the dome; 3) the reactivation of the Tan-Lu fault caused a second uplift processes of the Liaodong dome during 30-11.5Ma; 4) the Liaodong dome uplifted independently and separated from the Jiaoliao terrane. Our results also suggest that it is important to take uplifting evolution into consideration to target a potential petroleum reservoir.
Neogene-Quaternay faulting activities and their influence upon hydrocarbon accumulations in Bohai Bay Basin were analyzed based on hydrocarbon distribution characteristics and geological settings. It shows that the late faulting activity influenced the macroscopic distribution of the hydrocarbon accumulation assemblage and controlled the Neogene oil and gas enriched types and enriched belts of different depressions. Effect of faults different activity on hydrocarbon accumulation can be characterizedby fault activity difference coefficient (FDC). When the early and mid-generation and expulsion of hydrocarbon, the fault activity is strong and when late generation and expulsion of hydrocarbon the fault activity is weak, that is most favorable for hydrocarbon accumulation. Based on fault activity rate and FDC, we categorized the faults into I, II and III kinds in terms of their controlling ability on oil and gas accumulation. The first two types with faster fault activity rate and larger FDCs, were distributed in places where oil and gas are highly enriched in the Neogene and the last type with slower fault activity rate and small FDC was in places where oil and gas enrichment is poor in the Neogene. Based on the understanding, we propose that differences in faulting activities controlled oil and gas accumulations and the types of faults determined the level of oil and gas enrichment.
Uplift is a large-scale positive structural element of sedimentary basin and has strong influences on the structural evolution, depositional system and fluid flow in the basin. Uplift has the poly-mechanism in its generation. The extension, compression and strike-slipping environments can give rise to the formation of uplift. With the transition of geodynamic settings, the varied primary-generated uplifts may show the distinct characteristic of superposition owing to the combination of the above-mentioned mechanisms and entitle a kind of much complicated structure and configuration. The evolution of uplift has undergone a process from the formation-period (or embryo-period), growing-period to mature-period and then succeeding, decreasing gradually or disappearing in the end. This process can be called as uplift cycle. Uplift can thus show a shape of any accordingly period of such a developing process. The existing uplift today is in some periods of growing. Uplift is an important area for oil and gas accumulated in petroliferous basin and has great influences upon pool-formation. The uplift during deposition period has restricted the development of the source rock-reservoir rock-cap rock assemblages. The uplift during reservoir-forming period controlled oil-gas accumulation. The uplift during adjusting period or modifying period governed the re-distribution of oil and gas, and the uplift during perching period or burying period controlled the position of oil-gas occurrence. The sedimentary basins are largely the superimposition of proto-type basins in different periods. The poly-history superposition has resulted in the favorable functions on uplift, such as intervals of the multiple oil and gas-bearing in paleo-highs, additional hydrocarbon plays exactly above or below superimposition contacts, large-scale oil-gas accumulation zones resulted from the multi-period charging and accumulation of hydrocarbon and increase of pool-forming probabilities in the uplift, or the generation of the fault-dominated oil-gas accumulation zones owing to the linkage among faults of different periods and the multiple independent petroliferous zones separated by the sealing beds of a later petrogenic origin. The axial and slope zones of the uplifts are favorable for generating a large-scale oil-gas accumulation dominated by the anticline and the lithologic-stratigraphic reservoirs respectively.
Paleogene-Neogene strata in the area of North China platform is very thick in Bohai Bay basin, but relatively thin in southern North China. In the Ordos basin, however, Paleogene-Neogene strata are only developed in northwest area. During Paleogene, the main sedimentary type in Bohai Bay basin was rift-type deposition and the main sedimentary facies were shore-shallow lake, whose source came from multiple areas, such as Yanshan fold belt in the north, Taihang Mountain uplift in the west, Luxi uplift in the south, Liaodong uplift in the east and Cangxian uplift and Chengning uplift in the interior of the basin. The southern North China was divided into many depressions with different sizes during Paleogene, with the development of sediments of fluvial facies. During Paleogene, the deposition only occurred in the northwest of Ordos basin, where fluvial sediments were developed. During Neogene, sedimentary zones were mainly located in the Bohai Bay basin, southern North China and the peripheral graben of Erdos basin, mainly with the development of fluvial facies and shore-shallow lacustrine facies. Large areas of Bohai Bay basin were uplifted during early Neogene, leading to the shrinkage of the lake basin and the denudation of parts of the areas. During the mid-late Neogene the basin formed a unified depression, the sedimentary facies were mainly fluvial facies and locally shore-shallow lacustrine facies. Southern North China entered into the depression development stage after rifting during Neogene, when large areas accepted deposition and developed fluvial facies. The western region along the periphery of the basin of Ordos formed a set of fluvial facies and shore shallow lake facies foreland basin deposits. Favorable exploration region of oil and gas was Bohai Bay basin during Paleogene-Neogene.
Qikou Sag is one of the most important hydrocarbon rich sags in the Bohai Bay Basin with landscape of more depression and few uplift, and various kinds of slopes. With the constant improvement of exploration, the slope area is an important object of exploration. Taking Qibei slope as an example, the controlling mechanisms of regional tectonic framework, distribution of sandbodies and reservoir have been discussed intensively by the study of Qibei slope. Three points can be generalized as following: (1)The difference on basement subsidence controls the formation of the slope and the fractured bedrock controls the development style of the slope. (2)The within and outside sources supply sand, regulation of the groove conveys sand and the steep or gentle slope controls sand. (3)The duplicate migration, two recharging, oil higher than gas, the differential accumulation and concentration of favorable facies control the formation of oil and gas reservoir. Then we innovate and develop the hydrocarbon accumulation theory and establish the exploration techniques of the lithostratigraphy reservoir in the sag enriched in oil and gas: Choosing slope-establishing model-seeking breakthrough-tracing sand body-optimizing schemas. The result has effectively directed the pre-exploration of the lithostratigraphy reservoir in the slope area of the Dagang Oilfield and discovered sizable reserves.
Based on more than 30 years' research in reservoir forming mechanism and exploration practice in Neogene system in Jiyang Depression, theory of fault-fracture mesh petroleum plays was put forward. This kind of oil-gas migration and accumulation pattern determined that those inherited draping anticlinal structural belt were favorable for oil and gas to accumulate and easy to form reservoir. At basin margin, if caprock stably developed, stratigraphic overlap trap would form in storage layer. On condition of well developed source fault and badly developed secondary fault, oil and gas would migrate laterally and form stratigraphic overlap reservoir. As for area inside the basin, if secondary fault belt was connected with source fault, or structural belt with source fault connected storage layer and secondary fault network connected storage layer as well as favorable reservoir-caprock assemblage, lithologic reservoir would come into being. All elements composing the theory were analyzed. With application of this theory, breakthrough progresses were made in exploration of stratigraphic overlap reservoir at basin margin and lithologic reservoir inside the basin in Neogene system of Jiyang Depression.
This paper investigates the characteristics of sedimentary faces, source rocks, and hydrocarbon accumulation in detail within the Paleogene and Upper Cretaceous in the northern Kaikang trough, Muglad Basin. Analysis shows that the fluvial floodplain, delta, and shore-shallow lake facies are well developed in the main formations and several reservoir-cap assemblages are formed. Research on geochemical indicators, hydrocarbon generation, and expulsion potential of source rocks indicated that the thickness of good source rocks in Paleogene ranges from 50 to 200 m and TOC value can reach 0.5–1.3%, but it is immature and has no hydrocarbon generation potential. Nevertheless, the Upper Cretaceous source rocks are mostly matured, but its hydrocarbon generation is limited because of lower TOC (only 0.5–0.8%) and thinner thickness (only 10 m). The main effective source rock is the AG Group in the Lower Cretaceous, which is distributed throughout the area. The evolutionary history shows that most of structures in the central troughs lack hydrocarbon potential, because they were formed since the Paleogene, which are later than the main accumulation period of the AG source rock. The fault terrace zones on both sides of the Kaikang trough have obtained petroleum discovery, but the distribution of oil layers is much complicated. Hydrocarbon accumulation is controlled by formation dips, fault activity intensity, and fault lateral docking characteristics. The weaker active fault block in late period and more effective trap are the key factors to hydrocarbon enrichment at the fault terrace zones. There are two typical reservoir-forming modes; one is the small fault distance reservoir-forming mode of the lower primary reservoir, and the other is the secondary reservoir-forming mode of the upper and middle combination.
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.
Based on the study of distributional rules and sources of Neogene hydrocarbons, characters of fault activities and fault sealing properties, we recognized two kinds of migration and accumulation modes for Neogene hydrocarbons in Jizhong Depression, Bohai Bay Basin, namely a far-source stepped lateral migration pattern in slope zones and a near-source vertical migration pattern in fault terrace belts. The vertical migration pattern in fault terrace belts occurs typically in eastern step-fault zones in Raoyang Sag, where Neogene reservoirs are distributed adjacent to kitchen subsags and nearby strongly active faults of source rocks, and the accumulation degree and layer system of Neogene hydrocarbons are determined by the intensity and duration of fault activities during the main hydrocarbon accumulation period. While the far-source stepped lateral migration pattern dominates the Neogene hydrocarbon occurrence in the Wen'an slope belt, Baxian Sag, where Neogene reservoirs are far from kitchen subsags, the vertical migration of hydrocarbons is not strong due to a weak tectonic movement of the late stage, and the migration pattern and accumulation direction of Neogene hydrocarbons are determined by the lateral sealing ability of faults.
Huanghekou depression of Bohai Bay basin is in a special tectonic environment. Dueto the existing of the Tan-Lu fault zone the tectonic mechanism of extension and strike slip affected the whole strata profoundly, especially the sequence and depositional models of Palaeogene. This phenomenon led to huge differences of the sequence statigraphy and depositional architecture. Thus this study aims in using seismic datas, complemented by well logs and cores to analyze thoroughly the sequence stratigraphy and depositional architecture of the Huanghekou depression and their models in response to the background of extension and strike-slip tectonic mechanisms in this rift lacustrine basin. Palaeogene is divided into 9 composite sequences on the basis of unconformities on depression margins and correlative conformities in the depression center. Every sequence iscomposed of a regional depositional cycle from transgression with an onlapping lacustrine expanding systems tract (EST) to regression with a prograding highstand systems tract (HST). There are five depositional systems in the Huanghekou depression including fan delta, braid river delta, meandering river delta, the lacustrine depositional system and sublacustrine fan. One of two depositional models is that fan delta and sublacustrine fan were developed in north steep slope. The middle was similar with the north. In south gentle slope depositional systems were braided river deltas. The other is that the depositional system was mainly developed by meandering river delta in the north. In the middle the large-area beach bar can be found and the large scales of braided river deltas were developed continuously in south gentle slope. Finally through the analysis of the physical characters and the sandstone reservoir of the different micro-facies, we know that under the extensional model the best sandstone reservoir distributed in mouth bar and far bar of braided river delta in south gentle slope belt. Under the duplex tectonic mechanisms, the best sandstone reservoir can be seen in beach bar of shore and shallow lacustrine depositional system.
The Jiyang Depression is a Cenozoic rifted basin full of oil and gas. In different zones, there are apparent differences on the degree of hydrocarbon enrichment. There might be three conditions of enrichment for lithological reservoirs in the depression. 1) Rich oil resources were substantial conditions of hydrocarbon enrichment, because there were many sets of source rocks with huge thickness, widespread distribution, large resource extent and favorable contact relation between sandbodies. 2) High quality sandbody lithological traps were accumulating conditions of hydrocarbon enrichment. These traps should have characteristics of large-scale, favorable types of reservoir faciès and favorable reservoirs. Large-sized sandbody lithological trap or many overlaying small-sized lithological traps were forms of expression for large-scale traps. Favorable reservoir facies involved fan delta, sublacustrine fan and river facies. In these facies, the middle of fan was the best place for hydrocarbon enrichment. Favorable reservoirs characterized with good types of sandbody and matching induced porosity. 3) Confluence pathway was carrier condition of hydrocarbon enrichment. Abundant hydrocarbon enriched in high quality sandbody traps by fault confluence pathway and porous (or crack) confluence pathway which were important factors for hydrocarbon enrichment. So the conditions of enrichment in the Jiyang Depression displayed the characteristics of major factors. Three basic conditions controlled the formation of enrichment. To the distribution of hydrocarbon in space, only few districts could form the enrichment.
Shigang Fault is located within the centre of Jinhu Sag, which is the major fault zone of this sag with complex structure and evolution process. Based on the 3D seismic data and quantitative analysis, the characteristics of geometry and kinematics of Shigang Fault was analyzed during the late Cretaceous-Cenozoic period, meanwhile the property transferred from extension to strike-slip of the fault and its segmentation character were also discussed. In the late Cretaceous-Cenozoic, Shigang Fault has experienced three processes, including the secondary normal fault stage, boundary normal fault stage and right-lateral strike-slip normal fault stage. On the plane, Shigang Fault has obvious segmentation character, from north to south, the fault property varieties from strike-slip feature to shear-tensional feature, from tensional-shear feature to normal fault feature. It shows a strike-slip component in the northern part, normal fault in the southern part. The differential deformation is controlled by the pre-existing fault in basement, which reveals the change of the extension direction. The extension and strike-slip resulted in the abundant hydrocarbon accumulation in Shigang Fault Zone.
The controlling effect of fault dense belts on the preferred direction of oil-gas migration is discussed and favorable parts of fault dense belts for oil accumulation inside and outside oil source area are discussed based on the characteristics of fault dense belts of Fuyang Formation in Sanzhao Depression, combining with the oil and gas distribution of in Fuyang Formation of study area in this paper. The results show that there are four types of fault dense belts in Fuyang Formation of Sanzhao Depression, namely antithetic-graben-antithetic fault terrace, horst-graben-antithetic fault terrace, antithetic-graben-consequent fault terrace, and horst-graben-consequent fault terrace. The strike of fault dense belt is the preferred direction of oil-gas migration and horsts and antithetic inside oil source area of fault dense belts are favorable parts for oil accumulation where the strikes of fault dense belt and the layer are parallel or small-angle intersection. Inside oil source area horst and antithetic of fault dense belts are favorable parts for oil accumulation. And horsts and antithetic outside oil source area are favorable parts for oil accumulation and the grabens are secondary favorable ones where the angle of the strikes of fault dense belt and layer ranges from 0 to 45 degrees. The antithetic and horsts are preferential for oil accumulation where the angle ranges from 45 to 90 degrees.
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.
The distribution and genetic mechanisms of abnormal pressures in the Bohai Bay Basin were systematically analyzed. Abnormal pressures are widely developed in the Bohai Bay Basin, primarily in the Paleogene E2s4, E2s3, Es1, and Ed formations. From the onshore area of the Bohai Bay Basin to the center of the Bozhong area, the top depth of the overpressured zone in each depression increases gradually, the overpressured strata in each depression gradually move to younger formations, and the pressure structure successively alters from single-bottom- overpressure to double-bottom-overpressure and finally to double-top-overpressure. The distribution of overpressured area is consistent with the sedimentary migration controlled by the tectonic evolution of the Bohai Bay Basin, which is closely related to the hydrocarbon-generation capability of active source rocks. The overpressured strata are consistent with the source-rock intervals in each depression; the top of the overpressured zone is synchronous with the hydrocarbon generation threshold in each depression; the hydrocarbon generation capability is positively correlated with the overpressure magnitude in each formation. Undercompaction was the main mechanism of overpressure for depressions with fluid pressure coefficients less than 1.2, whereas hydrocarbon generation was the main mechanism for depressions with fluid pressure coefficients greater than 1.5.
Significant differential hydrocarbon enrichment occurs in depressions in a petroliferous basin. There are multiple depressions in the Bohai Bay Basin, and each depression as a relatively independent unit of hydrocarbon generation, migration and accumulation, contains significantly different hydrocarbon generation conditions and enrichment degree. On the basis of previous documents and a large number of statistical data, this work comparatively analyzed the differential hydrocarbon enrichment and its major controlling factors in depressions of the Bohai Bay Basin. The results show that depressions in the Bohai Bay Basin have various hydrocarbon enrichment degrees, and can be categorized into four types, namely enormously oil-rich, oil-rich, oily and oil-poor depressions. In general, the enormously oil-rich and oil-rich depressions are distributed in the eastern part of the basin along the Tan–Lu and Lan–Liao faults, whereas depressions in the western part of the basin are poor in hydrocarbons. Moreover, the vertical distribution of hydrocarbons is also highly heterogeneous, with Pre-Paleogene strata rich in hydrocarbons in the northern and western depressions, Paleogene strata rich in hydrocarbons in the entire basin, and Neogene strata rich in hydrocarbons in the off-shore areas of the Bohai Bay Basin. From early depressions in onshore areas to the late depressions in offshore areas of the Bohai Bay Basin, the source rocks and source-reservoir-cap rock assemblages gradually become younger and shallower, and the hydrocarbon resource abundance gradually increases. Hydrocarbon supplying condition is the key factor constraining the hydrocarbon enrichment for different depressions, while the main source-reservoir-cap rock assemblage, sufficient hydrocarbons and the transportation capacity of faults control the vertical distribution of hydrocarbons. The main factors controlling hydrocarbon enrichment are different for different layers. The hydrocarbon supplying condition of source rocks is the key controlling factor, whereas the source-reservoir configuration, the main source-reservoir-cap rock assemblages, and the fault transportation are the main factors of hydrocarbon enrichment in the Paleogene, Paleogene and Neogene, respectively.
The petroleum systems of Chinese nonmarine rifted and depression basins, dominated by lacustrine strata, have distinctive source rocks, reservoir types and trap characteristics. The rifted basins are characterized by numerous faults and multiple structural salients and sags (half grabens). Sags include many subdivisions and smaller isolated sags. Most sags constitute relatively independent petroleum systems that have efficiently generated and expelled hydrocarbons, have excellent reservoir properties in a variety of sand-body types, and have multiple traps. These attributes impart a petroliferous character to the entire sag. Depression basins (intracratonic flexural basins) developed on large cratons and hosted large lacustrine systems. They feature very gentle structure, large deltaic sand-bodies, source rocks in extensive contact with sand-bodies, heterogeneous low-porosity-low-permeability reservoirs and large, widespread lithology-controlled pools. In recent years, large oil and gas reserves have been discovered in these two types of lacustrine-dominated basins, contributing significantly to the growth of reserves in onshore China, and stratigraphic oil and gas pools have become the major type of accumulation in nonmarine lacustrine basins.
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