Geochemical differences in natural gas of Sinian Dengying Formation on the east and west sides of the Deyang-Anyue rift trough and their genesis, Sichuan Basin, SW China

Abstract

Taking the natural gas reservoirs of the Sinian Dengying Formation on the east and west sides (Gaoshiti-Moxi area and north slope of central Sichuan paleo-uplift on the east; Weiyuan and Well Datan-1 block on the west) of the Deyang-Anyue rift trough in the Sichuan Basin, China, as the research object, the geochemical parameters (component, isotopic composition) of natural gas from the Dengying Formation in different areas are compared, and then the differences in geochemical characteristics of Dengying natural gas on the east and west sides of the Deyang-Anyue rift trough and their genesis are clarified. First, the Dengying gas reservoirs on both sides of the rift trough are predominantly composed of oil-cracking gas with high maturity, which is typical dry gas. Second, severely modified by thermochemical sulfate reduction (TSR) reaction, the Dengying gas reservoirs on the east side exhibit high H₂S and CO₂ contents, with an elevated δ¹³C₂ value (average value higher than −29‰). The Dengying gas reservoirs in the Weiyuan area are less affected by TSR modification, though the δ¹³C₁ values are slightly greater than that of the reservoirs on the east side with partial reversal of carbon isotope composition, likely due to the water-soluble gas precipitation and accumulation mechanism. The Dengying gas reservoir of Well Datan-1 shows no influence from TSR. Third, the Dengying gas reservoirs reflect high helium contents (significantly higher than that on the east side) in the Weiyuan and Datan-1 areas on the west side, which is supposed to attribute to the widespread granites in basement and efficient vertical transport along faults. Fourth, controlled by the paleo-salinity of water medium in the depositional period of the source rock, the δ²H_CH4 values of the Dengying gas reservoirs on the west side are slightly lighter than those on the east side. Fifth, the Dengying natural gas in the Datan-1 area is contributed by the source rocks of the Sinian Doushantuo Formation and the third member of the Dengying Formation, in addition to the Cambrian Qiongzhusi Formation.