Research On Pore Characteristics And Shale Gas Occurrence Of Longmaxi Formation In Southern Sichuan Basin

Longmaxi formation of Lower lower Silurian is one of the most favorable reservoirs for shale gas exploration in China.It is very important to study the pore evolution and shale gas occurrence.This thesis studied the pore characteristics and shale gas occurrence by making Longmaxi formation as the research subject.The study area is located in southern Sichuan Basin,geological structure is relatively simple and the stratigraphic distribution is stable.The rock organic carbon content of Longmaxi formation is about 2.44%.The thermal evolution degree of the organic matter is about 2.9%.The shale is mainly composed of quartz,clay minerals and plagioclase.Clay minerals are mainly composed of illite-smectite and illite.The pore of Longmaxi formation shale in the study area is mainly composed of matrix pore.The contribution of different mineral components to the shale pores is different.The unit mass of organic matter provides the largest pore volume,followed by the clay minerals and the brittle minerals.The pore size of the shale is relatively stable,the total pore volume of the shale is 0.0081cm3/g,the specific surface area is 1.58m~2/g,the The advantage size of mesopore is from 2 to 25 nm,and the advantage size of micropore is from 0.3 to 0.9nm.The porosity of Longmaxi formation shale conforms to the fractal law,and has self-similarity.The pore structure of macropore is more complicated than mesopore and micropore.The stratification of Longmaxi formation shale developed well in Southern Sichuan Basin.The initial porosity of the shale in parallel stratification direction is higher than the shale in vertical stratification direction and the unit mass of stratification provides the initial porosity of 0.14% for the shale.With the increase of the buried depth,the impact of stratification is weaker.The porosity of the shale is mainly provided by the matrix when the buried depth is more than 3000 meter.The initial permeability of the shale in parallel stratification direction is well above the shale in vertical stratification direction,and the shale permeability decreases with the effective pressure.According to the degree of pressure sensitivity,permeability changes can be divided into sensitive stage,transition stage and insensitive stage.The pressure makes a negative influence on the changes of the shale pore volume,and the pore volume decreases with increasing pressure.With the increase of burial depth,the adsorbed gas content of shale increases first and then decreases,but the shale reservoir with small buried depth is easy to pass through the surface and the adsorbed gas content is small.The higher the reservoir pressure coefficient is,the higher the free gas content is.The total gas content of the shale increases with the increase of the depth.Under the same depth,the reservoir pressure gradient is higher and the decrease of total gas content is slower.When the depth is small,shale gas is dominated by the adsorbed gas.When the depth is large,the shale gas is dominated by the free gas which indicates that the larger the reservoir pressure coefficient is,the smaller the critical depth is.