Geochemical characteristics and origin of natural gas from the Silurian in Jiannan gas field

The geochemical characteristics and origin of the gas are studied according to gas composition and carbon isotope data from the Jiannan and adjacent areas and combined with regional hydrocarbon source rock data. Results show that the Silurian natural gas in the Jiannan gas field is dry. The content of non-hydrocarbon gas is low, and no H₂S exists. The carbon isotopes of n-alkanes are lower than -40‰, indicating a petroliferous gas origin from sapropel kerogens. The correlation between ln(C₁/C₂) and ln(C₂/C₃) shows that the present day Silurian gas mainly comes from the secondary cracking of crude oil. The analyses of geologic features indicate that the Silurian gas in the Jiannan gas field source from the carbonate shale in the Longmaxi Formation. The local reversal of alkane carbon isotopes can be explained by the mixing of late stage crude oil cracking gas and early stage kerogen degradation gas. The Longmaxi shale is mature and widespread in the middle and upper Yangtze region, indicating it is a good prospect for natural gas exploration in Silurian and Carboniferous.