Comparative Analysis And Periphery Of Shale Gas Enrichment And Depletion Factors In The Wufeng-Longmaxi Formation In The Sichuan Basin

The Lower Silurian Longmaxi Formation shale in the Sichuan Basin and its surrounding areas are currently the main horizons for marine shale gas exploration and development in China,and also the preferred detection area for marine shale gas in China.To clarify the controlling factors of shale enrichment and to innovate the evaluatio n method of shale gas enrichment is the main work of the candidate area of shale gas.The paper takes Changning,Weiyuan and Jiaoshiba areas as the main research areas.Based on the investigation of a large number of domestic and foreign literatures,unconventio na l oil and gas geology and sedimentology were used as the theoretical basis.Field Outcrops,Seismic Data,and Logging Data were comprehensively utilized combining TOC tests,R_otests,Whole-rock Diffraction Analysis,Nitrogen Adsorption Experiments,Carbon Dioxide Adsorption Experiments and other series of experiments.By analysising the geological and geochemical conditions,reservoir quality and preservation conditions in the typical area of southeastern Sichuan,the main controlling factors of shale gas accumulation in the Longmaxi Formation were clarified.Meanwhile,a prediction model for the content of adsorbed gas and free gas was established so as to achieve the purpose of prediction and analysis of gas content in favorable areas.The results showed that the TOC content in Longmaxi Formation shale is high and the R_ois moderate.The TOC in the shale gas-rich interval decreases gradually from bottom to top.The minera l composition of shale reservoirs is dominated by quartz and feldspar brittle minerals with higher content of clay minerals.The brittle mineral content gradually decreases but the clay mineral content gradually increases from bottom to top.The higher content of brittle minerals at the bottom is conducive to the development of pores.The porosity of the Longmaxi Formation shale reservoir in the southeastern Sichuan is generally higher than4%.Reservoir types mainly contain organic pores,intergranular pores and microfract ure.The pore volume and specific surface area of shale reservoirs are relatively high,mainly provided by micropores and mesopores.The Longmaxi Formation shale has dense top and bottom floors and regional caprocks.The thickness of the top and bottom floor is large.At the same time,the last time of strata uplift is relatively late and the amplitude is low.Finally,the Langmuir equation and the ideal gas state equation are used to predict gas content.To sum up,the following conclusions are obtained:(1)The TOC of the Longmaxi Formation shale in southeastern Sichuan is usual y greater than 4%and the organic matter type is mainly type?and type?1 kerogen.R_o is between 2%and 3%,and has good vitality conditions;(2)The bottom of the shale reservoir has a high content of brittle minerals.Brittle mineral content higher than 40%,porosity higher than 4%.A large number of organic pores and micro-fractures are developed.The pore specific surface area is 32.14m~2/g and the pore volume is 0.02517cm~3/g.The pore volume and pore specific surface area are relatively high,which provides sufficie nt storage space for shale gas;(3)The stratum has a thick top and bottom floor and a dense regional caprock.The later uplift time,positive structural type and small uplift are the key to the rich accumulation of shale gas in the Longmaxi Formation.(4)The effects of TOC,R_o,shale thickness,burial depth,physical properties and brittle mineral content on gas content are clarified.With the help of MATLAB software,the multiple regression analysis of the adsorbed gas volume and the free gas volume is carried out,and the field analysis data is used for verification.The relative error is small and the model is reliable.The prediction model results show that the gas content of Jiaoshiba area is higher than that of Weiyuan and Changning.