Evolution And Mechanism Of Reservoir Fluid Pressure During Shale Uplifting Of Wufeng-Longmaxi Formation In The Changning Area

As an efficient and clean energy source,shale gas has made marked breakthroughs after extensively exploration and exploitation in Sichuan Basin,south China.It has been demonstrated that the black shale of the Upper Ordovician Wufeng-Lower Silurian Longmaxi Formation in the Changning area,southern Sichuan Province,underwent a complex history of burial and uplift,is the key control on the mechanism of shale gas accumulation and enrichment.Previous researches suggest the pressure coefficient of the Wufeng-Longmaxi Formation in Changning area varies between 1.2 and 2.1.Besides,gas production has a good positive correlation with overpressure.Therefore,overpressure of the Wufeng-Longmaxi Formation in Changning are the favorable factor for the high yield of shale gas.It is necessary to study the formation and distribution of the overpressure,along with the geological evolution of Wufeng-Longmaxi Formation in Changning area.Fluid inclusions within calcite and quartz cements developed in the veins of the Wufeng-Longmaxi Shale in Changning area were studied in this study.By integrating the basin modelling,fluid inclusion petrology and microthermatry techniques,the development of shale fractures and the paleo-pressure of fluid inclusions were deeply investigated.The results show that: 1)The Wufeng-Longmaxi shales in Changning area were mainly at the stage of oil generation during the middle of the Jurassic period;2)From the Middle Jurassic to the end of the Cretaceous,the shale was at the stage of cracking-gas generation,in which pressurize was continuing to increase and the paleopressure coefficient calculated from the PVT modeling of fluid inclusions ranged from 1.35 to 2.31;3)From the end of the Cretaceous to the Quaternary,the shale was at the stage of uplifting,degrading,cooling and depressurization.So far,the Wufeng-Longmaxi formation is at the stage of overpressure preservation or leakage,which has evolved into the current state of pressure distribution.Finally,the formation mechanism of shale overpressure is determined by the intersection of acoustic difference logging and density logging.Well N216 in the Changning area was taken as an example,in order to demonstrate that the overpressure evolutinon of the Wufeng-Longmaxi Formation in the study area.Mechanisems of gas generation,expansion and preservation were suggested to be the primary causes of the presentday overpressure in the Changning area.