Direct control of normal fault in hydrocarbon migration and accumulation in northwestern Bozhong subbasin, Bohai Bay Basin, China

Abstract Fault growth and reactivation activities have been interpreted as potential controlling factors in vertical hydrocarbon migration, but the coupling and direct comparison of fault growth/reactivation history and hydrocarbon distribution patterns remain poorly understood. The spatial distribution of oil fields in the Western subsag of the Bozhong subbasin in the Bohai Bay Basin is closely related to fault geometry and presents a unique opportunity to investigate the coupling mechanism of the evolution of normal faults and petroleum migration and accumulation. The evolution history of two major normal faults, Faults F1 and F2, was investigated via three-dimensional seismic reflection data, and Fault F2 was associated with the spatial distribution of oil. Quantitative throw analyses, including throw versus distance plots, throw versus depth (T-z) plots, throw contour projections, and expansion indexes (EI) were employed. Fault F2 exhibited a large displacement between the foot- and hanging walls along most of the fault plane and thickened hanging walls of the upper fault surface. Laterally, the fault comprised three segments that linked along the strike at local throw minima. Vertically, the fault formed through the coalescence of two parts via dip linkage. Both the isolated- and coherent-fault models may have contributed to the evolution of Fault F2 during different periods and can be summarized in several steps, including blind fault propagation as individual segments, free surface breaching, lateral and dip linkage, and subsequent frequent reactivation as a coherent structure. A key result of this study was that petroleum migration and accumulation could be directly controlled by multiple growth and reactivation activities of faults.