The Characteristics Of Combined Strike-slip And Extension Fault System In Western Offshore Bohai Bay Basin And Its Control On Basin Fillings

Western Offshore Bohai Bay Basin(WOBBB)lies at the intersection of the ZhangjiakouPenglai Fault zone and the Huanghua-Dongming Fault Zone.The fault system behaves both extension and strike-slip under the combined effect of multi-episodes rifting and regional strike-slip fault zones.Existing research focused on the control effect of the combined strike-slip and extension fault system on basin filling is not systematic,which has become an obstacle for Paleogene lithologicstratigraphic traps exploration.This research is based on the scientific research project"Characteristics of fault system in the WOBBB and its controlling actions to reservoir formation" of the CNOOC and the national science project "New field and key technology for Bohai Exploration."We investigated the fault characteristics and basin sedimentary filling evolution in WOBBB using seismic,well log,and core data.The characteristics of the combined strike-slip and extension faults and basin sedimentary fillings in different periods were revealed in the isochronous stratigraphic framework.The controlling effect of the combined strike-slip and extension faults on basin sedimentary fillings was further discussed and revealed.The following new insights arise as the main results of this study:(1)The Paleogene strata consist of three second-order sequences:namely SQK,SQS,and SQD.A single second-order sequence lasted from 10 to 25 Ma and corresponded to an episode of rifting.The second-order sequences SQs and SQD were further sub-divided into five third-order sequences,including sqs3,sqsi2,sqd3,sqd21,and sqd2u,which lasted from 2 to 5 Ma.The thickness pattern of each sequence and the depositional-center migration were under the control of the faulting and basin tectonic configuration.(2)The faults behaved combined movements of both extension and strike-slipping.The strongest stretching was in E2S3 Period.There is a decreasing tendency for strike-slipping displacement from the west to the east and the north to the south.The significant tectonic configuration change in the strike-slip zone in E2S2 Period marked the strike-slipping initiation.The movement and evolution of the fault system can be divided into three stages,including ? the intense extension stage from Eik Period to E2S3 Period ? the combined strike-slip and extension stage from E2S2 to E3d Period and ? the compression and strike-slip stage since N1g Period.(3)Five depositional systems,including fan delta,braided river delta,subaqueous fan,beach bar,and lacustrine deposits,were recognized in rifting strata.Both the fault strike-slip and extension movements have significant control over depositional-system distribution,which were demonstrated in six aspects,including:1.The sedimentary filling difference between the steep slope and the gentle slope;2.The sedimentary filling pattern in fault-step zone;3.The control of extension fault accommodation zone on sediment transportation;4.The uplift of the strike-slip transpression bend could serve as local provenance;5.The transpression uplift zone is a favorable location for beach bar deposits,and 6.Broom-like associated fractures control the sediment transportation path.Moreover,four favorable targets for Paleogene litho-stratigraphy traps exploration were proposed,including:?Fan deltas on steep slopes,?Braid river deltas on gentle slopes,?Beach bar deposits on transpression uplift and subaqueous low bulge,and ?Subaqueous fan deposits controlled by intrabasin faults.