Geochemical Characteristics and Origin of Sinian-Cambrian Natural Gas in Penglai Gas Area, Sichuan Basin

The Sichuan Basin in southern China is well-known for its large natural gas resource potential stored in Sinian-Cambrian systems. Recently, high-yield industrial gas flow has been discovered from the Dengying Formation (Sinian System) and Canglangpu Formation (Cambrian System) in the Penglai gas area, preluding the multilayer stereoscopic exploration in Sichuan Basin. However, the origin of the natural gas and its preserving mechanics is still debated, and thus, in this study the geochemical characteristics of the natural gas are systematically analyzed, based on the data from gas composition and hydrocarbon isotope of a series of local wells. On this basis, the geochemical characteristics of natural gas in different regions and layers are compared, and the reasons for these differences from the origin and influencing factors are analyzed. The results show the following: (1) The natural gas of the Penglai gas field is dry gas dominated by CH4, and the Sinian Dengying Formation gas has lower C2H6 content, larger dryness coefficient, heavier delta C-13, and lighter delta H-2(CH4) than the Cambrian gas, which is associated with the high proportion of hydrocarbons from the high-maturity Dengying source rocks. (2) The natural gas from some wells in the lower part of the structure is characterized by high H2S content and low CH4 content, and heavy delta C-13 in the components, which seems to be affected by the thermochemical sulfate reduction (TSR) effect. (3) The natural gas from the Penglai gas area has a relatively low maturity, which appears to be attributed to the continuous sealing ability of the caprock, which can preserve both the early generated gas and the late thermal-cracked gas.