Thermochemical sulphate reduction of Sinian and Cambrian natural gases in the Gaoshiti-Moxi area, Sichuan basin, and its enlightment for gas sources

Integrated analysis for natural gases, asphalt and gypsum deposits in the Gaoshiti-Moxi (Gao-Mo) area indicates that natural gases in the three reservoirs of the area underwent various degrees of Sulphate Reduction (TSR). The main evidence includes: 1) natural gases contain abundant H2S with a content of 0.6%~3% at the Sinian Dengying Formation reservoir and 0.2%~0.8% at the Cambrian Longwangmiao Formation reservoir, and heavey δ34S values of H2S (from 21‰ to 23‰), suggesting that S may source from inorganic gypsum through TSR reaction. 2) The S/C atomic ratios of the reservoir asphalt was about 0.06~0.4, much higher than the upper limit value (0.034) of the asphalt formed through organic matter thermal cracking, and even a little higher than the S/C ratios (0.06~0.12) of the asphalt from the Puguang gas field where TSR had been proven strongly. 3) Gypsum deposited widely at the lower Cambrian in the Sichuan basin provided the materials as SO4 2- and Mg2+ for TSR. Formation waters in the Dengying Formation reservoirs contain high Ca2+/Mg2+ and low Na+/K+, indicating that dissolution of the salt and gypsum are quite common. Relative lack of SO4 2- in the formation water may result from the consumption by TSR reaction. Ethane should have been oxidized during TSR reaction, resulting in the increasing dry coefficient and much heavier δ13C2 values. Therefore, TSR reaction extents can result in differences of the natural gases in both Longwangmiao Formation and Dengying Formation. Relatively weak TSR reaction of the Longwangmiao Formation leads to reverse trend (δ13C1>δ13C2) of C isotopic composition in methane and ethane of natural gas, while relatively strong TSR reaction makes natural gases have normal distribution (δ13C1<δ13C2) of C isotopic composition in methane and ethane. If without oxidation of TSR, the reverse trend of C isotopic compositions in methane and ethane for Sinian and Cambrain natural gases in the Gao-Mo area should be commonly. The inverse trend (δ13C1<δ13C2) is consistent to that of highly mature shale gases in the basin and other areas in the world, indicating that primary origin of the natural gases in the Gao-Mo area should be correlated with the late-generated gases by shales. That can explain reasonably why the δ13C of methane is generally heavier than that of reservoir bitumen. This study is important to revisiting the accumulation mechanisms of the natural gases in this area. © 2019, Science Press. All right reserved.