"Double Intense Effect" Of Tectonic Activity And Its Control On Deposition In Dongying Formation,Nanpu Sag

The Nanpu Sag, located in the northeast of the Huanghua Depression, Bohai Bay Basin, is an oil-rich sag. Based on the detailed analysis of the structural formation, boundary fault activity, basement subsidence and extension of the Nanpu Sag, the longitudinal comparison between the Dongying and Liushagang Formations, and the lateral comparison between the Nanpu Sag and the neighbouring regions were carried out. Then, it was found out that the tectonic activity of the Nanpu Sag during the sedimentary period of the Donying Formation was different from other depressions or sags in the Bohai Bay Basin, and even most depressions or sags during the fault-depressed diversionary stage. On the one hand, the activity of the boundary faults were intense, especially the EW-trending faults, including the middle segment of the Xinanzhuang Fault and the Gaoliu Fault. On the other hand, the subsidence centers were located in the Linque and Caofeidian sub-sags, distant from the boundary faults, suggesting that the downwarp action was strong. On the structral-stratigraphic framework sections, both the formations close to the boundary faults and located in the sag center were thick, suggesting that both the downwarp action and fault controlling action had significant controls on deposition. Therefore, the tectonic activity during the sedimentary period of the Donying Formation was characterized by intense boundary fault activity and intense downwarp action, which was called "double intense effect" Under the control of the "double intense effect", the basement subsidence was intese, and the basement subsidence rate during the Ed3x sedimentary period was even higher than that during the intense fault rifting stage.Under the control of the "double intense effect", different from most depressions or sags in the Bohai Bay Basin, which was mainly filled by fluvial facies, coastal shallow-lake facies and braided river deltas during the sedimentary period of the Donying Formation, the depositional systems of the Nanpu Sag was similar to that during the intense fault rifting stage. Semi-deep lake and deep lake were widely developed in the sag center, distant from the boundary faults. The Nanpu Sag was undercompensation or compensation basins. Intense basement subsidence provided enough accommodation space for the clastic materials, combined with the abundant sediment supply, the very thick Dongying Formation was deposited during a short time. Because of the intense downwarp action, the depocenters were located in the Linque and Caofeidian sub-sags in the central region of the Dongying Sag. At the same time, because of the intense activities of the boundary faults, several high thickness zones were developed along the bondary faults. The deep slope along the boundary faults and the intense basement subsidence in the central region provided proper gradient condition and enough accommodation space to the nearshore subqueous fans and slump fans. The fan delta system with great thickness and wide distribution was deveopled in the northern steep slope zone. The braided river delta system was widely developed along the southern gentle slope zone.Under the control of the "double intense effect", the very thick dark mudstones were deposited in the semi-deep lake and deep lake. Due to the high geothermal gradient and high buried depth, the Ed3 mudstones, characterized by large thickness, wide distribution, high abundance of organic matter and good organic type, had entered middle mature stage, becoming high-qulity source rocks. Hydrocarbons generated by the Ed3 source rocks account for nearly 10% of the total resources in the Nanpu Sag. Because of the low maturity, the Ed2 mudstones, characterized by wide distribution, large total thickness and thick single layer, cannot become high-qulity source rocks. However, though the faults in the Nanpu Sag were well developed, the Ed2 mudstones can effectively seal the oil and gas, constituting the regional cap rocks. The sedimentary facies types of the Dongying Formation were rich. The channel mouth bar sandstones, subaqueous distributary channel sandstones and slump turbidites were favorable reservoirs. Therefore, the high-quality reservoirs in the Dongying Formation were well developed, with wide distribution and abundant types. The high-quality hydrocarbon source rocks, high-quality reservoirs with abundant types and regional seal rocks can form "self-generation and self-reservoir" pool-forming patterns in the Dongying Formation. Moreover, the oil and gas generated by the Shahejie Formation can migrate vertically along the deep faults into the reservoirs in the Dongying Formation. Therefore, the Dongying Formation has great exploration potential.Finally, the formation mechanism of the "double intense effect" is discussed. During the sedimentary period of the Donying Formation, the Bohai Bay Basin had entered the fault-depressed diversionary stage. The downwarp action caused by thermal subsidence decreased gradually from the Bozhong Depression to surrounding areas. The downwarp action in the Nanpu Sag, which is adjacent to the Bozhong Depression, was intense. In addition, after the large-scale magmatic eruption during the late Esl sedimentary period, the isostatic subsidence of the shallow crust around the magma chambers may also be the reason for the intense downwarp action during the early Ed sedimentary period. During the Esl-Ed sedimentary period, the accelerated westward subduction from the Pacific plate to the Eurasian plate triggered the dextral slipping of the Tanlu Fault (the east boundary fault of the Bohai Bay Basin) and the activity of the northern segment of the Lanliao Fault through the Huanghua Depression. The continued dextral slipping of the Tanlu and Lanliao faults created a SN trending extensional overlap at the northeast part of the Huanghua Depression. The forward extensional action of the NNE trending Cangdong Fault which was the boundary fault of the Huanghua Depression weakened dramatically, and the strike-slip component action enhanced sharply under the SN trending extensional stress field, which lead to the migration of the subsidence center and the fault activity from the Cangxian Fault to the central region of the Huanghua Depression. During the Ed sedimentary period, the subsidence center and the fault activity migrated into the Nanpu Sag. As a result, the activity of the boundary faults enhanced significantly in the Nanpu Sag, especially the EW trending boundary faults.