Jamie Robert Beagle1,2, Shengfei Qin3, Yunpeng Wang2, Chengsheng Chen2, Chunhui Cao4, Greg Holland5, Ben Surridge1, Zheng Zhou1
(1Lancaster Environment Centre, Lancaster University, Lancaster LA1 4YQ, UK, 2State Key Laboratory of Deep Earth Processes and Resources, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China, 3Research Institute of Petroleum Exploration & Development, PetroChina, Beijing 100083, China, 4Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou 730000, China, 5Department of Earth and Environmental Sciences, The University of Manchester, Manchester M13 9PL, UK)
Abstract: Understanding the long-term preservation of deep natural gas is critical for evaluating highly evolved petroleum systems in sedimentary basins. Tight sandstone reservoirs are commonly considered effectively closed following hydrocarbon charge because of their extremely low matrix porosity and permeability. However, their present gas compositions may record episodic fluid migration, gas-water interaction, and partial gas loss during burial and structural evolution. Noble gas systematics provide sensitive tracers of these processes because their elemental and isotopic distributions respond predictably to radiogenic production, phase partitioning, and fluid transport.
Noble gas elemental and isotopic data for He, Ne, Ar, Kr, and Xe is presented from 12 production wells in the Xinchang gas field, Sichuan Basin, China. The samples are hosted in the Triassic Xujiahe and Jurassic Shaximiao and Qianfoya tight sandstone reservoirs, which have porosities below 5% and matrix permeabilities below 10-4 mD.
Measured 3He/4He ratios of 0.0037–0.025 Rₐ demonstrate that helium is overwhelmingly derived from crustal radiogenic production. Strong fractionation of 20Ne/36Ar relative to air-saturated water, together with elevated 4He/20Ne ratios, indicates substantial radiogenic He accumulation accompanied by multistage Ne loss. Moderate enrichment in 40Ar/36Ar, combined with the relative retention of Kr and Xe, requires repeated gas-water interaction and selective degassing rather than simple radiogenic ingrowth within a continuously closed reservoir. Overlapping noble gas signatures between the Triassic and Jurassic reservoirs further indicate limited but geologically significant vertical fluid connectivity.
These results demonstrate that even ultra-low-permeability sandstones can undergo episodic openness, selective gas loss, and partial re-equilibration with groundwater following hydrocarbon charge. The preservation of deep tight gas therefore depends not only on low matrix permeability but also on the temporal evolution of structural connectivity and multiphase fluid interactions. Noble gas systematics provide a powerful means of reconstructing charge preservation and post-charge fluid evolution in mature sedimentary basins.
Keywords: Noble gases; Tight sandstone; Reservoir evolution; Charge preservation; Fluid connectivity; Sichuan Basin
Author Profile: Jamie Robert Beagle, PhD, CEP Scholar at Chengdu University of Technology (CDUT), primarily engaged in noble gas geochemistry research to understand volatile evolution in terrestrial and planetary systems.
