GEOCHEMICAL CHARACTERISTICS AND INFLUENCE FACTORS OF DESORBED GAS OF SHALE: A Case Study of JYA Well in Jiaoshiba area, Sichuan Basin

Through the desorption of shale core samples from Wufeng-Longmaxi Formation in JYA well of Jiaoshiba area, the total gas content in shale is calculated, and the desorbed gas samples are collected at different times for the determination of compositions and stable carbon isotopes. The results are shown as follows:1) The gas content in the study area is high, which is mainly controlled by mineral compositions, organic carbon content and water saturation, dominated by methane, a kind of typical dry gas. 2) The desorbed shale gas compositions change regularly with desorption time. The contents of CH₄ and C₂H₆ increase gradually, CO₂ content first decreases then increases, and N₂ decreases gradually. 3) The δ¹³C₁ and δ¹³C₂ in the desorbed gas show different degrees of fractionation, mainly controlled by the "adsorption-desorption" process. 4) The desorption processes of shale gas with various lithofacies are different. At the first stage, the δ¹³C₁ of mud-rich siliceous mixed shale samples changes from heavy to light, while that of the silicon-rich shale samples from light to heavy, which may be due to the inorganic pores that are dominated in the mud-rich siliceous mixed shale with large pore diameter and large proportion of free gas in the total gas content.