Geochemical characteristics and geological significance of shale gas from the Lower Silurian Longmaxi Formation in Sichuan Basin, China

The shale of the Lower Silurian Longmaxi Formation has become a key target for shale gas exploration and development in China recently. The origin of carbon and hydrogen isotopic reversal in shale gas is discussed in this paper based on the analysis of the chemical components and stable isotope composition of natural gas in the Carboniferous Huanglong Formation sourced from Longmaxi Formation and Longmaxi shale gas. The shale gas was mainly composed of hydrocarbon gas in which the content of CH4 was in the range of 95.52%–99.59%, C2H6 0.23%–0.72%, and C3H8 0.0%–0.03%. The drying coefficient (Cl/Cl–5) was more than 0.99, indicating typical dry gas. The values of δ13C1, δ13C2 and δ13C3 ranged from −37.3‰ to −26.7‰, −42.8‰ to −31.6‰, and −43.5‰ to −33.1‰, respectively. The carbon isotope values indicated that the hydrocarbon gas was an oil-type gas. However, the reversal of carbon and hydrogen isotopes of hydrocarbon gases in shale gas occurred, i.e., δ13C1 > δ13C2, δD1 > δD2. The geochemical characteristic of high thermal maturity and carbon isotope reversal in the Longmaxi shale gas was similar to those of the Fayetteville shale gas in U.S. The Longmaxi shale gas is a mixture of gases from decomposition of kerogen at high thermal maturity and cracking of soluble organic matter retained within the shale, suggesting there is an abundance of shale gas to support high productivity.