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

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.