Accumulation mechanism of mantle-derived helium resources in petroliferous basins, eastern China

A series of marginal-sea basins and fault-depression basins were formed in eastern China under the background of subduction of the West Pacific plate. Different types of helium-rich natural gas reservoirs (He>1000 ppm, 1 ppm=1 mu mol mol(-1)) have been found in these basins: helium-rich CO2 gas reservoirs, helium-rich N-2 gas reservoirs, and helium-rich hydrocarbon gas reservoirs. Based on the analysis of gas geochemical data, the source and accumulation mechanism of helium in these helium-rich natural gas reservoirs were discussed. Helium-rich natural gas has relatively high He-3/He-4 ratios (0.88-4.91 Ra, average 2.82 Ra). The He-3/He-4 ratio characteristics of mantle xenoliths and mantle-derived CO2 gas reservoirs indicate that the helium in these helium-rich natural gas reservoirs is mainly mantle-derived (>70%). The original mantle volatile is mainly CO2 with a low helium concentration (He<200 ppm), and the enrichment of mantle-derived helium in the gas reservoir is mainly related to the dissolution and mineralization of CO2. During this process, the CO2/He-3 ratio decreases from 2x10(9) to approximately 2x10(6). As CO2 dissolves and mineralizes, the concentration of conservative gases (He and N-2) increases in the remaining CO2 gas proportionally to the loss of CO2. Large amounts of carbonate minerals, such as dawsonite, which are relatively enriched in C-13, are found in CO2 reservoirs in eastern China. The relative enrichment of C-12 in residual CO2 gas is important evidence of the dissolution and mineralization of CO2. The relative abundance of mantle-derived helium and N-2 gas increases thousands of times during the dissolution and mineralization of CO2, which is the main accumulation mechanism of mantle-derived helium-rich CO2 gas reservoirs and helium-rich N-2 gas reservoirs. Helium-rich gas from the mantle is mixed with alkane gas generated by organic matter in the sedimentary basin to form helium-rich hydrocarbon gas reservoirs.