Origin of marine sour natural gas and gas-filling model for the Wolonghe Gas Field, Sichuan Basin, China

Abstract

The chemical and isotopic composition of natural gases from the Wolonghe Gas Field, Sichuan Basin, Southwest China, was investigated to assess the potential gas sources and reconstruct the gas-filling history of the reservoirs. All natural gases in this field are dominated by gaseous hydrocarbons (C1–C3) with varied amounts of non-hydrocarbon components (CO2, H2S and N2). The H2S content varies with reservoir intervals. It ranges from zero to 1.84% with an average of 0.37% in the Carboniferous reservoir, from 0.05% to 0.76% with an average of 0.30% in the Permian reservoir, and from 1.09% to 18.83% with an average of 5.39% in the Lower Triassic reservoir. The gas dryness coefficient (C1/C1–3) ranges from 0.97 to 1.0 with an average of 0.99. The carbon isotopic compositions of methane and its homologues in the Wolonghe Gas Field vary widely, with δ13C1⩾δ13C2<δ13C3 in the Carboniferous and Permian gas reservoirs, and δ13C1<δ13C2<δ13C3 in the Lower Triassic Jialingjiang Formation gas reservoirs. The δD1 values range from −140‰ to −100‰, with an average of −124.5‰. The δ34SH2S values in the field extend from 5.7‰ to 31.0‰, with δ34SH2S values in the Lower Triassic Jialingjiang Formation reservoirs being much higher than those in the Carboniferous and Permian reservoirs. The sour natural gases in the field originated from cracking of oil and were sourced from the marine sapropelic organic matter at high maturity levels. The natural gases underwent alteration by thermochemical sulfate reduction (TSR). Although the limited extent TSR occurred in the Carboniferous and Permian gas reservoirs, the TSR alteration is likely to have caused the high gas dryness and the 13C enrichment of CH4. The present low content of H2S in the Carboniferous and Permian gases would be related to the loss of H2S dissolved in water under the reconstruction of these gas reservoirs. In contrast, the high H2S in the Jialingjiang Formation gases is determined by both the favorable TSR conditions in the Lower Triassic reservoirs and the favorable preservation conditions provided by the Lower-Middle Triassic gypsum.The Wolonghe natural gas system is characterized by multi-pay zones, multi-source supply and multi-type layers. The gas generated from Silurian source rocks migrated into the Carboniferous reservoir through faults and fractures and was trapped beneath the Permian Liangshan Formation shale, whereas the natural gas in the Permian and Lower Triassic Jialingjiang Formation was generated and preserved in its own formations with the overlying gypsum acting as a cap rock.