Using noble gases to trace groundwater evolution and assess helium accumulation in Weihe Basin, central China

The severe shortage of helium resources is an impending global problem. However, the helium accumulation processes and conditions favorable for helium enrichment in reservoirs remain poorly understood, which makes helium exploration challenging. Noble gases are good tracers of subsurface fluids provenance, migration and storage, as well as indicators of the nature and quantity of associated phases. In this study the variation of major gases and noble gases data in Weihe Basin provide us with an excellent opportunity to understand the groundwater evolution and helium accumulation processes. Twelve gas samples collected from wellheads of geothermal wells can be classified into three groups, in which Group A has high concentrations of N2 (58.57% - 91.66%) and He (0.32% - 2.94%); Group B has high contents of CH4 (52.94% and 69.50%) and low concentrations of He (0.057% and 0.062%); Group C has a high content of CH4 (71.70%) and He (2.11%). Helium isotopic ratios are predominantly radiogenic in origin and therefore crustally derived. Measured elemental ratios of noble gases are compared with multiple fractionation models for Group A and B samples, implying that open system heavy oil-water fractionation with excess heavy noble gases has occurred in the basin with Voil/Vwater ratios of 0.06-0.18. The amount of helium in Group A and B samples requires the release of all 4He produced in the crust since 0.30Ma-1.98Ma into the groundwater. The Group C sample requires an additional He flux from adjacent granitic bodies. The accumulation of helium and hydrocarbon in the Weihe Basin can be explained by a 4-stage process. Accumulation of commercially viable helium requires high He flux from source rocks, the existence of a free gas phase of major gas components (CH4 in most cases, N2 or CO2) and minimal major gas addition after formation of the free gas phase.