A Noble Gas Study Linking Reservoir Compartmentalization to Tectonic Deformation in Wufeng-Longmaxi Shale, Sichuan Basin

This study presents measurements of the bulk gas composition, stable isotope, and noble gas volume fraction and isotope for shale gas samples collected from gas wells in the Wufeng-Longmaxi shale, the southern Sichuan Basin, China. These gas wells are divided into two groups: forelimb and backlimb based on their relative locations to the anticline. The dryness [C1/(C2 +C3)] ranging from 166.3 to 251.2, combined with δ13C1 and δDC1 data that vary from –28.83 to –27.26 ‰ and –152.5 to –144.6 ‰, respectively, point to a late mature thermogenic origin of the gas. 3He/4He ratios of gas samples are mostly around 0.01 times the air value suggesting a dominant crust-derived He. 21Ne/22Ne and 40Ar/36Ar ratios of many gas samples are higher than the corresponding air values indicating the mixing of terrigenic and atmospheric noble gases. In addition, forelimb samples present the highest 21Ne/22Ne and 40Ar/36Ar ratios indicating a larger contribution of terrigenic noble gas in these wells. Elemental ratios of air-derived noble gas isotopes – 22Ne/36Ar, 84Kr/36Ar, and 132Xe/36Ar are compared to the recharge water values, pointing to the interactions of oil, gas, and water phases in the shale over geologic time. The study of terrigenic noble gases further suggests the addition of crust-derived noble gases from deeper formations. Unlike backlimb samples, all of the forelimb samples display ages older than 45 Ma – the age of major tectonic exhumation event in the study area – likely indicating a larger flux of external radiogenic 4He due to the higher density of deep faults in the forelimb area caused by the basement-involved deformation. In the meantime, the basement-involved deformation causes the pore collapse especially in the forelimb leading to lower porosity, which might in turn allow better preservation of noble gases in the shale by reducing the recharge of younger groundwater into the shale.