Large-scale accumulation of abiotic gases associated with subduction and volcanic activities in rift basins in eastern China

Abstract

A series of large-scale abiotic gas reservoirs have been discovered in a few rift basins in eastern China. However, the compositions of the abiotic gases in each gas reservoir differ significantly, dominated by CO₂ or CH₄. This study aims to explore the factors controlling the large-scale generation and accumulation of different types of abiotic gases in the reservoirs. CO₂ is the major component of the abiotic gases in the Huangqiao, Huagou, and Pingfangwang reservoirs, exceeding 60 % up to 95 % and the δ¹³C_CO2 values are generally greater than −8‰. CH₄ is the major component in the Qingshen and Songnan reservoirs with content up to 94.98 %. The δ¹³C_CH4 values are commonly greater than −30 ‰ (up to −16.8 ‰) and the alkanes exhibit a reverse sequence (δ¹³C_CH4 > δ¹³C_C2H6 > δ¹³C_C3H8 > δ¹³C_C4H10). Most of the ³He/⁴He ratios of the reservoirs are commonly greater than 1 Ra. The carbon isotope values and ³He/⁴He ratios indicate that most of the gas components (CO₂ or CH₄) are abiotic. The abiotic CO₂ is associated with mantle-degassing, and abiotic CH₄ is mantle-derived and/or attributed to Fischer-Tropsch (FT) reactions. The ratios of Fe³⁺/∑Fe of the basalt near the Huangqiao reservoir are higher than those near the Qingshen and Songnan reservoirs, suggesting the relatively oxygen-fugacity controls the major component of the abiotic gases. The northwestward subduction of the western Pacific Plate, starting from the Japan Trench, triggered extensive magmatic and volcanic activities, resulting in the release and accumulation of abiotic gases in the rift basins. The variation in oxygen fugacity associated with oxidizing materials (e.g., iron and manganese oxides) and carbonate and sulfate sediments carried by the subducted plate determined the presence of abiotic CO₂ or CH₄. As the subduction slab continues moving forward beneath the continental plate, the influence of high-oxygen-fugacity materials gradually decreases, causing the abiotic gases to transition from CO₂ to CH₄. The Huangqiao, Huagou, Pingfangwang, Songnan, and Qingshen gas reservoirs are located farther away from the subducting slab, which explains the gradual shift in the abiotic component in these reservoirs. In general, the majority of the abiotic gases are released from deep faults and magmatic eruptions. Therefore, we conclude that the abiotic gas reservoirs are located near deep faults and igneous rocks.