The dichotomy in noble gas signatures linked to tectonic deformation in Wufeng-Longmaxi Shale, Sichuan Basin

Abstract

Abstract Geochemical homogeneity in shale is often assumed when tracing subsurface fluids and characterizing sedimentary basins. This study presents measurements of the bulk gas composition, stable isotopes, and noble gas volume fraction and isotopes for shale gas samples collected from gas wells in the Wufeng-Longmaxi Shale, the southern Sichuan Basin, China. The dryness [C1/(C2\xa0+\xa0C3)] ranging from 166.3 to 251.2, combined with δ13C1 and δDC1 that vary from −28.8 to −27.3‰ and\xa0−\xa0153 to −145‰, respectively, point to a late mature thermogenic origin of hydrocarbon gas. 3He/4He ratios of gas samples are around 0.01 times the air value suggesting dominantly crust-derived He. 21Ne/22Ne and 40Ar/36Ar ratios of many gas samples are higher than the corresponding air values indicating the mixing of crustal and atmospheric noble gases. Multiple dichotomous patterns are observed in noble gas signatures of forelimb and backlimb samples, and depression and crest samples. 20Ne/22Ne ratios of some crest samples are higher than that of depression samples in the backlimb, pointing to the presence of diffusion-driven fractionation that is likely caused by the long-distance migration from depression to crest. Elemental ratios of air-derived noble gas isotopes – 22Ne/36Ar, 84Kr/36Ar, and 132Xe/36Ar are compared to the recharge water values, suggesting the interactions of oil, gas, and water phases in the shale over geologic time. Forelimb samples generally display older ages than backlimb samples, 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. The basement-involved deformation also causes pore collapse especially in the forelimb leading to a lower porosity that results in a more pristine noble gas signature in the forelimb due to the reduced impact of younger recharge water.