Faults development controlled the Neogene hydrocarbon accumulation in the mid-Huanghekou Sag, Bohai Bay Basin

Huanghekou Sag is an important hydrocarbon-rich sag in Bohai Sea and can be divided into West-, Mid-, and East-Huanghekou sags. The source rocks of the third member of Shahejie Formation in the Mid-Huanghekou sag began to generate and expel hydrocarbons on large scale during the sedimentary period of Minghuazhen Formation. The analysis of active faults and their impact on the Mid-Huanghekou sag evolution is important to understand the regional hydrocarbon migration and accumulation. Based on 3D seismic and drilling data analyses, the distribution, activity, and growth evolution of the faults in the Mid-Huanghekou sag were systematically analyzed. Combined with the distribution characteristics of regional source rocks and oil-gas resources, the controlling effect of faults on hydrocarbon accumulation in the Neogene was summarized. The study shows that the development of faults in the mid-Huanghekou sag is characterized by “weakening first and then strengthening”. From the deposition period of Shahejie Formation to the deposition period of Guantao Formation, the intensity of fault activity gradually decreased and the fault activity area gradually shrank. During the deposition of Minghuazhen Formation, the fault activity was enhanced and the faults were fully active in the region. In the early stage, faults were small-sized; and in the late stage, they developed and merged into large ones in the central area of the sag, forming network of active faults-source rocks-reservoirs. Most of oil and gas migrated to the sand bodies of shallow facies of the Guantao and Minghuazhen Formations along active faults, and further to the both sides the structural highs of reservoirs. The north and south secondary structures have different hydrocarbon accumulation characteristics. In the southern gentle slope zone, the fault activity rate is small in the late stage, which effectively favored the sealing of the oil and gas, and finally formed large-scale oil-gas accumulation in the sand body along structural ridge. In the northern steep slope zone, the fault activity rate is great in the late stage, and the oil and gas formed in certain shallow structural traps nearby faults.