FAULT ACTIVITIES ON THE SOUTHEASTERN MARGIN OF THE QINNAN SAG IN THE BOHAI SEA AND THEIR CONTROL OVER PALEOGENE HYDROCARBON ACCUMULATIONS

A substantial amount of oil and gas accumulations has been discovered in Es₁₂ on the steep slope of the southeastern margin of the Qinnan Sag. However, the factors which control the enrichment of oil and gas remain unknown. The situation severely restricted the further exploration and evaluation of oil and gas resources in the region. Based on the data of drilling, logging, 3D seismic and analytical tests, combined with the study of depositional background, the method of fault activity rate was adopted in this paper to quantitatively study the fault characteristics. The relationship between fault activities and Paleogene oil and gas accumulations was also discussed. It is found that from Paleogene to the present, fault activities in the study area is characterized by a "weak-strongest-weak-strong-weakest-weak" pattern with time. The periods of Es₃, Ed₃ and Ed₂, while fault activities were strong, are the main periods for source rock and cap rock deposition. In the area, stronger the fault activity, thicker the source rocks and higher the maturity. During the periods from the first and second Members of the Sha 1 and Dong 1 Formations, fault activities are weak in the region, and that is the time for reservoir formation. The period from Es₁₂ and Ed₁ to the present is the time for reservoir development, while fault activities are weak. Reservoirs were more developed in the accommodation zones. In the places with weak fault activities, it is easier for lithological traps to form, while for the area with strong fault activities, fault-lithologic traps are more favored. In Paleogene, the alternation of strong and weak fault activities with time provided favorable conditions for the formation of high quality source rocks and reservoir cap systems. By the end of the Dongying Formation, the source rocks are matured and oil and gas started migrating. The episodic fault activities reactivated the fault zone and favorable channels were available for the vertical and horizontal migration of oil and gas. During the late period of the Minghuazhen Formation, oil and gas moved into the reservoir and began to accumulate. Fault activities directly affect the preservation and redistribution of oil and gas. Fortunately, they were weakened after the oil and gas accumulation, that led to the formation and preservation of billion tons large oilfields.