Geochemical characteristics of shale gas from lower Silurian Longmaxi formation in the Sichuan Basin and its geological significance

The shale of Lower Silurian Longmaxi Formation has become a key target for shale gas exploration and development in China recently. Based on the analysis of chemical component and stable isotope composition of natural gas in Carboniferous Huanglong Formation derived from Longmaxi Formation and Longmaxi shale gas itself, the reason of carbon and hydrogen isotopic reversal in shale gas is discussed in this paper. The shale gas is mainly composed of hydrocarbon gas in which CH4 content is in range of 95.52%-99.59%, C2H6 of 0.23%-0.72%, and C3H8 of 0%-0.03%. The drying coefficient (C1/C1-5) is more than 0.99, indicating typical dry gas. δ13C1 values range from -37.3‰ to -26.7‰, δ13C2 from -42.8‰ to -31.6‰, and δ13C3 from -43.5‰ to -33.1‰, respectively. The carbon isotope values indicate that the hydrocarbon gas is oil-derived gas. However, the reversal of carbon and hydrogen isotopes of hydrocarbon gases in shale gas occurs, i. e. δ13C1>δ13C2, δD1>δD2. The geochemical characteristic of high thermal maturity and reversal carbon isotopes in the Longmaxi shale gas is similar with the Fayetteville shale gas in US. The Longmaxi shale gas is mixing of gases from decomposition of kerogen at high thermal maturity and cracking of soluble organic matter retained in the shale, suggesting there would be abundant shale gas resource for high productivity. ©, 2015, Science Press. All right reserved.