Quantifying the helium and hydrocarbon accumulation processes using noble gases in the North Qaidam Basin, China

Limited reserves and insecure resource supply have led to global shortage crises in helium, a vital gas for cryogenic engineering and other countless industrial manufacturing processes. Despite the attention drawn by global supply disruptions, the helium accumulation mechanism in natural gas fields remains poorly understood. Noble gases are excellent tracers for studying migration and accumulation processes of fluids in the subsurface and can be used to investigate the influence of subsurface fluids on helium accumulation. We present noble gas isotope and abundance data as well as major gas compositional data from 10 producing wells in three gas fields in the North Qaidam Basin, China. Helium is more concentrated in the Mabei and Dongping gas fields (2.06-48.4x10(-4) cm(3) STP/cm(3)) than in the Niudong gas field (1.15-1.42x10(-4) cm(3) STP/cm(3)). The helium is mainly radiogenic, with He-3/He-4 ratios of 0.01-0.05 Ra, where Ra is the atmospheric value of He-3/He-4, and lacks significant contribution from the mantle (0.03-0.67%). The noble gases derived from air-saturated water (Ne-20, Ar-36, Kr-84 and Xe-130) can be explained by an oil-modified groundwater-exsolution model with excess heavy noble gases. The calculated V-oil/V-water and V-gas/V-water ratios indicate that the Mabei region is the most oil-rich area and the Dongping region has the driest natural gas, which is consistent with the geological context. These ratios further support the fractionation models. The strong linear relationship between He-4 and Ne-20 (R-2=0.98) suggested that He-4 was dissolved into groundwater before migrating into the oil or gas phase. The initial He-4 concentrations in groundwater can accumulate within 0.31-2.78 Myr assuming a He-4 flux from the entire crustal section. According to the fractionation model, helium in groundwater partitions into the gas phase when contacting hydrocarbons. The different volume ratios among oil, gas and water during the equilibration process cause much greater variability in the helium concentrations in the gas phase (e.g., 6.08x10(-4) to 2.01x10(-3) cm(3) STP/cm(3) in Mabei) than those in the groundwater phase (e.g. 9.18x10(-3) to 1.39x10(-2) cm(3) He-4 STP/cm(3) H2O in Mabei). Hydrocarbons play a critical role in helium accumulation and dilution. Helium-rich natural gas fields are characterized by old groundwater systems and moderate hydrocarbon abundance. This study has succeeded in quantitatively assessing the helium accumulation process in natural gas fields in the North Qaidam Basin and revealed that both groundwater and hydrocarbon phases control the helium accumulation in the subsurface environment. This outcome has broad implications for the prediction of hydrocarbon and helium as resources.