Geochemical characteristics and origin of natural gas reservoired in the 4th Member of the Middle Triassic Leikoupo Formation in the Western Sichuan Depression, Sichuan Basin, China

The gas exploration in the 4th Member of the Middle Triassic Leikoupo Formation (T2l4) in the Western Sichuan Depression has achieved a continuous breakthrough in the recent years. However, the gas origin and source remain controversial. The study on the geochemical characteristics indicates that the T2l4 gas in the Western Sichuan Depression is typically dry. Its dryness coefficient is generally higher than 0.99. The δ13C1 and δ13C2 values range from −35.1‰ to −29.3‰ and −34.8‰ to −31.9‰, respectively, with the exception of one gas sample from Well PZ1 with the δ13C2 value of −26.4‰. The δDC1 value ranges from −164‰ to −136‰. The gas souring index is positively correlated with the δ13C2 value in comparison to the δ13C1. The T2l4 gas has experienced heavy alkane-dominated TSR instead of the methane-dominated TSR. The T2l4 gas in the Western Sichuan Depression generally displays a GSI value lower than 0.01 with the exception of two gas samples from Well PZ1 (0.036, 0.04); they indicate extremely low TSR alteration extent. Gas origin identification points out that the T2l4 gas in the Western Sichuan Depression is mainly oil-type gas that has reached the secondary gas cracking stage. The CO2 in the T2l4 gas that has high δ13C values are mainly inorganic. They are mainly derived from the interaction between acidic fluids and carbonate reservoirs.