Geochemistry Characteristics of Hydrogen Sulphide-Bearing Gas Pools in Sichuan Basin

Abstract

Sichuan Basin is the oil and gas bearing basin that is discovered in China, with the most hydrogen sulfide gas pool and the greatest reserve. In this article, gas pools with high hydrogen sulfide in Sichuan Basin were taken as the object of study; the geochemistry characteristic of natural gas was systematically studied, and the origin of natural gas with hydrogen sulfide in Sichuan Basin was discussed. The hydrogen sulfide content in the hydrogen sulfide gas pools of Sichuan Basin is 0.6%–14.5%; the sulfur isotope values of hydrogen sulfide gas have a close relationship with the sulfur isotope values of anhydrite in the same reservoir, and the sulfur isotope fractionation between the anhydrite and the H2S is 4‰–14‰, showing that the H2S in natural gas and the anhydrite in reservoir have the same sulfur source, the H2S in natural gas is the product of TSR reaction. The relationship among the hydrogen sulfide content in the gas pool, the hydrogen isotope of methane and the sulphur isotope of hydrogen sulfide is not obvious; but the high hydrogen sulfide content corresponds to the heavier ethane carbon isotope composition, and the carbon isotope of carbon dioxide is lower than −10‰, which belongs to the organic origin and has further verified the fact of TSR occurrence in the reservoir. There is the phenomenon of carbon isotopic reversal of methane and ethane in some areas of the Huanglongchang gas field, Wubaiti gas field and Puguang gas field in the Northeast Sichuan as well as Weiyuan gas field in South Sichuan, which is mainly caused by the later period of cracking-based gas-production process of the crude oil. TSR has less influence on these gas pools.