Late-stage Hydrocarbon Accumulation in the Bozhong Depression of the Bohai Bay Basin as Controlled by Neotectonism

Neotectonism occurred intensively in the Bozhong depression in the Bohai Bay Basin, which was reflected vertically by dramatic subsidence and a number of uplifts and laterally by notable fault movements. This particularity has resulted in the special petroleum geological conditions of the Bozhong depression which are different from those of adjacent lands. For example, the source rocks of the Shahejie Formation were overpressured and hydrocarbon generation occurred in the late stage; the Dongying Formation was deeply buried below the hydrocarbon-generating threshold, therefore there were sufficient oil sources. The rapid subsidence led to starved sedimentation of the Guantao Formation fine sandstone and the regional Minghuazhen Formation lacustrine shale, which formed the Neogene regional reservoir-caprock association. The active faults formed in the neotectonism became passages for oil to migrate from the Paleogene to Neogene. The traps formed by late fault activity and accompanied anticlines provided spaces for the formation of reservoirs. All the above factors match well with one another in the Bozhong depression, providing favorable conditions for the formation of a series of large oilfields in the region

A method of identifying effective source rocks and its application in the Bozhong Depression,Bohai Sea,China

Research on effective source rocks directly affects the accuracy of identifying hydrocarbon resources, and indirectly affects the exploration decisions in petroliferous basins. Although the previous evaluation methods of effective source rocks vary relatively widely, a complete quantitative evaluation approach has not yet been developed. For that reason, we redefined the concept of effective source rocks based on the existing research results. Surrounding this definition, and guided by the hydrocarbon expulsion theory, the quantitative model called ＂two stages and three steps＂ method is established to predict effective source rocks. Its application in the Bozhong Depression indicates that among the four sets source rocks in the Bozhong Depression, the Member 3 of the Shahejie Formation （Es 3 ） has the largest effective source rock thickness, and the Member 1 Member 2 of the Shahejie Formation （Es 1＋2 ） is the second largest .The effective part of dark mudstone is only 30%-80% of the total volume and with the increase of buried depth and improvement of quality, the effective part increases. Comprehensive analysis indicates that the ＂two stages and three steps＂ method is a practical technique for effective source rock prediction.

Original Sedimentary Pattern of an Inverted Basin:A Case Study from the Bozhong Depression,Offshore Bohai Bay Basin

The third member of Shahejie Formation （Sha-3 member; 42–38Ma of Eocene） in the Bozhong Depression, offshore Bohai Bay Basin was subject to multiple post-depositional modifications. The present structural framework of the Bozhong Depression, which is characterized by sags alternating with uplifts, does not reflect its original sedimentary pattern. Previous studies have not discussed the post-depositional modification of this succession, including the sedimentary pattern variations and the depositional geodynamic setting. This work determined the characteristics of the post-depositional modification and original sedimentary pattern of the Bozhong Depression through analysis of seismic data, well-log data and fission-track ages. The results demonstrate that the Shijiutuo rise, a major structural feature of the current basin, did not exist during the major depositional stage of the Sha-3 member, when the Qinnan sag was largely connected to the Bozhong sag to form a single contiguous deposition area within the basin. By contrast, the Shaleitian and Chengbei rises, located in the western part of the Bozhong Depression, have existed before the depositional period of the Eocene Sha-3 member; these features were manifested as syn-depositional tilted fault blocks, the uplifted footwall blocks of which provided sediments for the neighboring Shanan and Chengbei sags. The western part of the Bonan low rise, located in the southern part of the Bozhong Depression, did not experience uplifting during the depositional phase of the Eocene Sha-3 member. The Huanghekou sag was connected with the Bozhong sag in the western part of the Bozhong Depression. The original sedimentary boundary of the southern Miaoxi sag possibly extended eastward about 10 km and connected with the Bozhong sag at its northern part. The present-day Bodong low rise, which is bounded by the Tan–Lu fault zone, also formed after the depositional period of Eocene Sha-3 member. It is thus concluded that the Bozhong Depression formed a connected large-scale sub-basin during the depositional stage of the Eocene Sha-3 member. Several neighboring sags that are now separated by rises, including the Qinnan, Shanan, Chengbei, Huanghekou, Miaoxi and Bodong sags, formed a single contiguous depositional area during the Eocene. The significant differences between the present and original basin patter and framework provide valuable information for better understanding the history of basin inversion and its impact on related hydrocarbon-system evolution.

Subsidence history and forming mechanism of anomalous tectonic subsidence in the Bozhong depression, Bohaiwan basin

The Bozhong depression of the Bohaiwan basin belongs to a family of extensional basins in East China, but is quite different from other parts of the basin. The Cenozoic subsidence of the depression is controlled by a combination of lithospheric thinning and polycyclic strike-slip movements. Three episodic rifts have been identified, i.e. Paleocence-early Eocene, middle-late Eocene and Oligocene age. The depression underwent syn-rift and post-rift stages, but two episodic dextral movement events of the strike-slip faults modify the subsidence of the Bozhong depression since the Oligocene. The early dextral movement of the Tan-Lu fault associated with crustal extension resulted in accelerated subsidence during the time of deposition of the Dongying Formation with a maximum thickness of 4000 m. A late reactivation of dextral movement of the Tan-Lu fault began in late Miocene (about 12 Ma), which resulted in the intense subsidence of Minghuazhen Formation and Quaternary. In addition, dynamic mantle convection-driven topography also accelerated the post-rift anomalous subsidence since the Miocene (24.6 Ma). Our results indicate that the primary control on rapid subsidence both during the rift and post-rift stages in the Bozhong depression originates from a combination of multiple episodic crustal extension and polycyclic dextral movements of strike-slip faults, and dynamic topography.

Fluid Dynamic Field in BozhongDepression, Bohai Bay Basin

The data from regional geology, boreholes, geophysics and tests are integrated to analyze the fluid dynamic field in the Bozhong depression, Bohai Bay basin. The current geothermal gradient is determined to be about 2.95 /100 m by integrating 266 drill-stem test (DST) measurements and comparing with the global average value. The paleogeothermal gradients are calculated from the homogenization temperatures of saline inclusions, which vary both laterally and vertically. The data from sonic logs, well tests and seismic velocities are used to investigate the pressure variations in the study area. The mudstone compaction is classified as three major types: normal compaction and normal pressure, under-compaction and overpressure, and past-compaction and under-overpressure. The current pressure profile is characterized by normal pressure, sight pressure and intense overpressure from top to bottom The faults, unconformity surfaces and interconnecting pores constitute a complex network of vertical and horizontal fluid flows within the depression. The fluid potential energy profiles present a 'double-deck' structure. The depocenters are the area of fluids supply, whereas the slopes and uplifts are the main areas of fluids charge.