Insight into the mechanism of helium enrichment in natural gas from the Bohai Bay basin, China: Chemical and isotopic composition perspectives

Abstract

Helium (He) is an important societal resource, yet its enrichment mechanism is unclear in the Bohai Bay Basin, despite the presence of He-rich (He > 0.1% v/v) gas. A comprehensive analysis of 112 sets of chemical and isotopic data from natural gases in the literature was conducted to further elucidate the source and accumulation mechanism of He in this basin. Natural gas is categorized into five groups based on CO2 and N2 abundances: N2 gas (CO2+N2 ≥ 95%, CO2 < N2), high-purity CO2 gas (CO2+N2 ≥ 95%, CO2 > N2), low-purity CO2 gas (50 ≤ CO2+N2 < 95%, CO2 > N2), low-purity HC gas (15 ≤ CO2+N2 < 50%) and high-purity HC gas (CO2+N2 < 15%). δ13C–CO2 (−8.3 to −3.0‰) and 3He/4He (1.66–6.45 Ra, where Ra is the atmospheric 3He/4He) suggest a mantle origin of CO2 in N2 and CO2 gases. The δ13C–CO2 of N2 gas (−8.3‰) is lighter than that of high-purity CO2 gas (−6.2 to −3.4‰), indicating CO2 dissolution and mineralization. A positive correlation between He and CH4 content in the N2 and high-purity CO2 gases suggests water-derived CH4 capture during CO2 migration. Similarly, the positive correlation between He and N2 content across all samples indicates water-derived N2 capture during gas migration. A mixture of mantle and crust-derived He is indicated by 3He/4He ratio (0.06–6.45 Ra). Two distinct types of He-rich gas were identified: He-rich N2 gas (3He/4He = 3.1–3.2 Ra) and He-rich HC gas (3He/4He = 0.36–0.48 Ra). He enrichment in N2 gas is attributed to water-derived He capture and concurrent loss of CO2 during mantle-derived CO2 migration. He-rich HC gas formation results from He-rich N2 gas dilution with hydrocarbons. The findings provide theoretical support for exploring He-rich natural gas in this basin and offer valuable insights into He enrichment mechanisms in comparable sedimentary basins globally.