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Structural Characteristics of Fault Systems and Controlling Factors of Hydrocarbon Accumulation in the Cenozoic Faults Intersection Area of the Southern Bohai Sea, Eastern China

Abstract

The Cenozoic fault systems in the southern Bohai Sea eastern China are controlled by the Mesozoic basement faults, the NNE- trending Tan-Lu strike-slip fault zone, the NWW- trending Zhangjiakou-Penglai strike-slip fault zone and the Cenozoic rifting, these fault zones and tectonic movement formed several different trending normal, strike-slip faults and derived subsidiary faults. These faults with different trending and properties formed a large numbers of fault intersections and overlapping areas. The exploration practice shows the fault intersections area are advantage accumulation places of hydrocarbon. In order to reveal the controlling factors of oil and gas migration and accumulation in the fault intersection area, three-dimensional seismic data of about 7,000 square kilometers and tens of exploration wells from CNOOC were used to analyze the structural characteristics of the fault intersection area and the main controlling factors of hydrocarbon accumulation. The results show that there are three kinds of fault interaction zones divided by faults proprieties, including the reactivated basement faults intersect with Cenozoic normal faults, the strike-slip faults intersect with normal faults and the strike-slip faults intersect with strike-slip faults. Among them, the intersections by normal faults and strike-slip faults are the advantage places of the hydrocarbon accumulation, they can be divided into two types, one is parallel main strike-slip faults intersect with the major normal faults at both sides of the normal fault, and another is two strike-slip faults overlap and intersect with its derived subsidiary normal faults. The major normal faults and the strike-slip faults intersection shows that the long axis direction of the reservoir is consistent with the trending of normal faults and the lateral distribution is controlled by the strike-slip faults. The reservoirs at subsidiary normal faults and the strike-slip faults intersection area are distributed along the strike-slip fault and depending on the length of the overlap zone. The rapid vertical activity of the major normal faults in the Paleogene formed several depressions and provided space for the deposition of source rocks, while the Neogene accelerated thermal subsidence provides conditions for the maturation of source rocks and the formation of cap rocks. Since 12 Ma the normal fault reactive formed reverse drag anticlines in the Neogene strata, several structural ridges formed at the left step overlap zone of the right lateral strike-slip faults at the same stage. The reverse drag anticlines and the structural ridges became traps for hydrocarbon accumulation. The major normal faults, subsidiary faults that cut the source rocks, the Paleogene fan deltas sedimentary and the Neogene channel sandstone provide the pathway for the vertical and lateral migration of oil and gas.