Origin of CO2 in natural gas from well KPN-1 in the Northwest Tarim Basin, China

Abstract

Abstract In well KPN-1 located in the NW Tarim Basin, China, a significant amount of CO2 has been detected. In order to determine the source of CO2, various analyses have been conducted, including the gas composition, the ratio of radiogenic isotopes (R/Ra), the δ13C of CO2 and bulk rock. Based on the relative positive δ13C value (-0.299‰ to -2.726‰), the CO2 is unlikely organic origin. Additionally, a deep mantle origin has been ruled out, as the extremely low composition of N2 and He in the gas, along with the R/Ra ratio less than 1, does not match what would be expected from this source. The most plausible source of CO2 is likely crustal in origin. Instead, it is believed to have formed through the retrograde solution of carbonate minerals under decreasing temperature and pressure conditions. The well KPN-1 experienced subsidence tectonic movements followed by uplift, which resulted in lower temperatures and pressures, facilitating the dissolution of carbonate and the subsequent production of CO2 with a positive δ13C value. The abundant CO2 produced from this process highlight the significant role of tectonic uplift in the formation of carbonate reservoirs, which should be further investigated and given more attention in future research.