Gas genetic type and origin of hydrogen sulfide in the Zhongba gas field of the western Sichuan Basin, China

Abstract Natural gases and associated condensate oils from the Zhongba gas field in the western Sichuan Basin, China were investigated for gas genetic types and origin of H 2 S by integrating gaseous and light hydrocarbon geochemistry, formation water compositions, S isotopes (δ 34 S) and geological data. There are two types of natural gas accumulations in the studied area. Gases from the third member of the Middle Triassic Leikoupo Formation (T 2 l 3 ) are reservoired in a marine carbonate sequence and are characterized by high gas dryness, high H 2 S and CO 2 contents, slightly heavy C isotopic values of CH 4 and widely variable C isotopic values of wet gases. They are highly mature thermogenic gases mainly derived from the Permian type II kerogens mixed with a small proportion of the Triassic coal-type gases. Gases from the second member of the Upper Triassic Xujiahe Formation (T 3 x 2 ) are reservoired in continental sandstones and characterized by low gas dryness, free of H 2 S, slightly light C isotopic values of CH 4 , and heavy and less variable C isotopic values of wet gases. They are coal-type gases derived from coal in the Triassic Xujiahe Formation. The H 2 S from the Leikoupo Formation is most likely formed by thermochemical SO 4 reduction (TSR) even though other possibilities cannot be fully ruled out. The proposed TSR origin of H 2 S is supported by geochemical compositions and geological interpretations. The reservoir in the Leikoupo Formation is dolomite dominated carbonate that contains gypsum and anhydrite. Petroleum compounds dissolved in water react with aqueous SO 4 species, which are derived from the dissolution of anhydrite. Burial history analysis reveals that from the temperature at which TSR occurred it was in the Late Jurassic to Early Cretaceous and TSR ceased due to uplift and cooling thereafter. TSR alteration is incomplete and mainly occurs in wet gas components as indicated by near constant CH 4 δ 13 C values, wide range variations of ethane, propane and butane δ 13 C values, and moderately high gas dryness. The δ 34 S values in SO 4 , elemental S and H 2 S fall within the fractionation scope of TSR-derived H 2 S. High organo-S compound concentrations together with the occurrence of 2-thiaadamantanes in the T 2 l reservoir provide supplementary evidence for TSR related alteration.