Evolution Of Formation Fluid Pressure In Longmaxi Formation,Pengshui Area,Southeast Chongqing

The Longmaxi formation in the southeast of Chongqing is a very important source rock in the upper Yangtze area in the south of China.Because of the complex regional stratigraphic conditions and the drastic uplift and erosion,it is necessary to establish an evaluation method for the preservation conditions of the ordinary pressure shale gas to obtain the dynamic evolution process of the formation pressure.Based on the measured formation pressure data,well logging data,seismic data,core and rock slice observation data,and by using fluid inclusion measuring technique.The distribution and controlling factors of present-day formation pressure in the Pengshui area are expounded,the vertical and horizontal distribution characteristics and influencing factors of formation pressure in the longmaxi formation are analyzed,using fluid inclusion PVT simulation method and basin simulation method to obtain the paleo pressure of the Longmaxi formation,and a simplified simulation method for hydrocarbon-containing inclusions is proposed.The main controlling factors of the evolution of paleo fluid pressure are studied through the following aspects: undercompaction,structural extrusion,hydrocarbon generation and pressurization,thermal expansion and pressurization,then analyze the influence and significance on the preservation conditions of shale gas.The stratum structure is the main controlling factor of the current pressure plane distribution characteristics.The lithology and physical properties of the upper and lower strata and hydrocarbon generation of hydrocarbon source rocks are the main controlling factors of vertical pressure distribution.The evolution of paleo formation pressure can be divided into six stages: early under compaction pressurization,early tectonic uplift and depressurization,medium term under compaction,turbocharging of kerogen and liquid hydrocarbon,tectonic uplift and depressurization,and gas loss depressurization.And it is concluded that hydrocarbon generation,pyrolysis and early compaction are the main causes of overpressure in the Longmaxi formation.Combined with the evolution of formation pressure,the structural conditions and cap rock conditions are analyzed,and it is concluded that the preservation conditions of the Longmaxi group are well preserved in different geological periods,and it is related to the mechanical properties of rocks.