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Pressure Evolution in Buried-Hill Reservoirs of Wen'an Slope, Jizhong Subbasin, Bohai Bay Basin

Abstract

Wen'an Slope locates in the north-central Jizhong Subbasin, Bohai Bay Basin. The buried-hill reservoirs of Wen'an Slope are the main regions for oil and gas exploration. Understanding the pressure evolution is important to reveal the process of hydrocarbon migration and accumulation. The pressure evolutions of deeply buried Carboniferous-Permian (C-P) clastic reservoirs and Ordovician (O) carbonate reservoirs are reconstructed by using the PVT modeling of fluid inclusions combined with basin modeling. The fluid inclusion petrographic and microthermometric data indicate the existence of episodic fluid flow. The paleo-pressure was simulated by using the homogenization temperatures and the liquid to vapor ratio which can be accurately determined by Confocal Laser Scanning Microscopy (CLSM) of hydrocarbon inclusions. Results show that the trap age of hydrocarbon inclusions in both reserviors concentrates around two stages: 30~25Ma and 5~2Ma, which correspond to the deposition periods of the Oligocene Dongying Formation (Ed) and Neogene Minghuazheng Formation (Nm) respectively. Both C-P and O reservoirs developed overpressure in the two stages. The pressure evolution was simulated with the constraints of the present-day measured pressure and the paleo-pressure determined by fluid inclusions. Using the Basinmod 1D, the pore fluid pressure evolution was reconstructed in some typical wells. The simulation results show that the pressure evolutions in C-P and O reservoirs have experienced 4 periods: normal pressure (before 30Ma)-overpressure (30~20Ma)-normal pressure (20~5Ma)-overpressure (5~2Ma). The present pressure data (DSTs) of C-P reservoirs show low overpressure and that in O reservoirs show normal pressure. Moreover, 2D moldeling of 3 sections indicate that higher overpressure developed in the northern Wen'an Slope and areas close to Baxian Depression. The mechanisms for C-P overpressure can be explained by disequilibrium compaction and the transfer of pressure. Overpressure developed in O carbonate reservoirs is mainly due to the charge of hydrocarbon fluid which can be proved by the hydrocarbon expulsion history of source rocks. Also, the homogenization temperatures of the fluid inclusions (<150°C) and the co-existence of solid-bitumen bearing and condensate inclusions indicate the occurrence of gas flushing, which is an evidence of the higher overpressure developed in the second stage (5~2Ma) in O reservoirs.