Research On The Characteristics Of The Lower Paleozoic Marine Shale Gas Reservoir In The Northeast Chongqing

This study is mainly aimed at the Paleozoic in Northeast Chongqing,and the main shale reservoirs in this study area are Longmaxi Formation of Lower Silurian and Lujiaping Formation of Lower Cambrian,both of which have good geological conditions for shale gas accumulation,but Longmaxi Formation is relatively better.Based on the field investigation and the summary of previous research data,combined with XRD,mercury injection and liquid nitrogen test technology,this paper analyzes and compares the reservoir characteristics of Longmaxi Formation and Lujiaping Formation shale in the northeast of Chongqing,and discusses their respective reservoir characteristics.The mineral composition of the two groups of Lower Paleozoic shale samples in the northeast of Chongqing is mainly quartz and clay minerals,but the proportion of quartz in Longmaxi group shale is significantly higher than that in Lujiaping group shale,and the content of brittle minerals is higher,which is more fracturing.In terms of clay mineral content,the two sets of strata are dominated by illite and illite.The average organic matter content of Longmaxi Formation shale is 3.28%,which is higher than that of Longmaxi Formation shale,and the average organic matter content of Lujiaping Formation shale is 5.27%;both kerogen types are mainly type I,and vitrinite reflectance(RO)is between 3.01%and 3.70%,with an average of 3.17%,which belongs to over mature stage.There are many types of pores in the two groups of shale in the study area,and the organic matter pores in Longmaxi group are obviously more than those in Lujiaping group.Pores of all scales contribute to the total pore volume of shale,and the specific surface area is mainly contributed by micropores and mesopores.The average total pore volume of Longmaxi Formation shale is 0.0193cm3/g;the average specific surface area is 2.5523m~2/g;the average total pore volume of Lujiaping Formation shale is 0.0243cm~3/g;the average specific surface area is 1.4456m~2/g.The average fractal dimension of mercury porosities in Longmaxi Formation shale is2.7071,and that in Lujiaping Formation shale is 2.7637;the average fractal dimension of liquid nitrogen porosities in Longmaxi Formation shale is 2.6966,and that in Lujiaping Formation shale is 2.7821.The fractal dimension of pore size of mercury intrusion and liquid nitrogen in Longmaxi Formation shale is smaller than that in Lujiaping Formation shale,indicating that the pore size of Lujiaping Formation shale is more complex.The average gas content of Lujiaping Formation shale is 1.15m~3/t and Longmaxi Formation shale is 2.32m~3/t.The gas content of Longmaxi Formation shale is higher than that of Lujiaping Formation shale.The results of isothermal adsorption show that under the same conditions,the adsorption capacity of Lujiaping Formation shale is significantly stronger than that of Longmaxi Formation shale.The adsorption capacity of shale is negatively correlated with temperature,TOC content,illite content in clay minerals,and mixed layer.The thickness difference between the two sets of shale reservoirs in the northeast of Chongqing is large,in which the rich organic interval of Longmaxi Formation is30-55m,and that of Lujiaping Formation is 120-210m.Since the Formation of shale reservoir,the northeast of Chongqing has experienced many periods of strong structural trans Formation.The shale of Lujiaping Formation and Longmaxi Formation has been greatly damaged by the structure,and the top and bottom plates and sealing layers have been seriously damaged,which is not conducive to the storage of shale gas.No matter from the point of view of shale gas bearing or fracturing,Longmaxi Formation shale in Northeast Chongqing is more promising than Lujiaping Formation shale.Generally speaking,Longmaxi Formation has a better prospect of shale mining,while Lujiaping Formation has a relatively low prospect of shale mining.