建南气田志留系天然气地球化学特征及气源探讨

根据建南气田及邻区的天然气组分、烷烃碳同位素等资料,结合区域烃源岩研究资料,研究了该区志留系天然气的地球化学特征及其气源特征。结果表明,建南气田志留系天然气为干气,非烃气体总含量低且无H2S气体;烷烃气碳同位素均小于-40‰,属于油型气成因,其母源为腐泥型干酪根。ln(C1/C2)-ln(C2/C3)相关性表明现今的志留系气藏以原油二次裂解贡献为主。结合该气藏地质特征综合分析认为,建南气田志留系天然气来源于志留系龙马溪组碳质页岩,烷烃气碳同位素局部倒转为同源不同期的天然气混合所致,即晚期原油裂解气与早期干酪根降解气混合。中上扬子区广泛分布且已成熟的志留系龙马溪组页岩预示志留系和石炭系的天然气勘探前景良好。

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.