LAUSR

Abstract The gas exploration in the 4th Member of the Middle Triassic Leikoupo Formation (T 2 l 4 ) 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 T 2 l 4 gas in the Western Sichuan Depression is typically dry. Its dryness coefficient is generally higher than 0.99. The δ 13 C 1 and δ 13 C 2 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 δ 13 C 2 value of −26.4‰. The δD C1 value ranges from −164‰ to −136‰. The gas souring index is positively correlated with the δ 13 C 2 value in comparison to the δ 13 C 1 . The T 2 l 4 gas has experienced heavy alkane-dominated TSR instead of the methane-dominated TSR. The T 2 l 4 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 T 2 l 4 gas in the Western Sichuan Depression is mainly oil-type gas that has reached the secondary gas cracking stage. The CO 2 in the T 2 l 4 gas that has high δ 13 C values are mainly inorganic. They are mainly derived from the interaction between acidic fluids and carbonate reservoirs.