Origin and source of CO2 in natural gas from the eastern Sichuan Basin

Abstract

The CO2 contents of natural gas from the eastern Sichuan Basin were mainly less than 2%, and they were generally large when the H2S contents were over 5%. The natural gas in the eastern Sichuan Basin showed high δ 13C1 values uncorrelated with H2S contents, and the oil-associated gas generally displayed carbon isotopic reversal between methane and ethane, whereas the coal-derived gas samples displayed positive series. The δ 13C values of CO2 in the natural gas could be divided into two types, one with low values (−24‰–−12‰) and the other with high values (−8‰–4‰). The natural gas had low R/Ra values and broad CO2/3He values distinct from the mantle-derived gas, and the CO2 was typical of crustal origin. Natural gas with high δ 13CCO2 values is distributed in the north region of the eastern Sichuan Basin, and CO2 in H2S-poor and H2S-free reservoirs was mainly derived from the thermal decomposition of carbonate rocks under Permian magmatism and high heat flow, whereas CO2 in the gas reservoirs with high TSR extent was derived mainly from the dedolomitization of carbonate reservoirs under acidic formation water, and the CO2 of TSR origin generally entered into the secondary calcite. The CO2 in natural gas in the south-central region of the eastern Sichuan Basin, which had been rarely affected by the Emeishan mantle plume, displayed low δ 13C values and was of organic origin. Though the natural gas in Wolonghe gas field had experienced TSR, the low reaction extent and weak acidity of formation water could hardly have resulted in the dedolomitization, and therefore CO2 in the natural gas was mainly of TSR origin with low δ 13C values.