CHARACTERISTICS OF PRECENOZOIC AND CENOZOIC FAULT SYSTEMS UNDER A TENSILE STRESS FIELD IN WESTERN BOHAI SEA AND THEIR CONTROL OVER HYDROCARBON RESERVOIRS

Three groups of fault systems extending in NE, NW and EW directions respectively are well developed in the west of Bohai Sea. The NE and NW faults are from a complex conjugate fault system. The complexity of the fault system directly affects the differential enrichment of oil and gas in the region. Based on the study of regional stress field and the physical simulation experiment for preexisting conjugate faults under an extensional regime, this paper aims to reveal the characteristics of the Cenozoic fault system controlled by the preexisting structure and its influence on oil and gas reservoir formation. It is clarified that the coupling of deep and shallow structures in the study area is jointly determined by the regional stress field and pre-existing basement morphology. The influence of mantle thermal activity on regional stress field is obvious, which leads to the regional extension of the study area. Under the extensional action of the syncline nearly in SN direction, some of the pre-existing faults are selectively reactivated. The difference between the strike of pre-existing structure and the direction of regional extension force leads to different stress properties in different sections of a fault, and directly affects the accumulation and preservation of oil and gas. The larger the angle between the pre-existing fault and the near SN stretching direction, the stronger the extension component of the fault will be. Tensional faults are conducive to oil and gas accumulation in the shallow layer. On the contrast, once the strike slip component is enhanced and dominated, it will be conducive to oil and gas accumulation and preservation in the deep.