焦石坝地区龙马溪组页岩解吸气地球化学特征及地质意义

Geochemical characteristics and geological significance of desorbed shale gas in Longmaxi Formation,Jiaoshiba area

通过对焦石坝地区龙马溪组页岩岩心进行解吸以分析其气体组分和碳同位素组成,研究了四川盆地志留系龙马溪组页岩气碳同位素倒转现象。结果表明,解吸气相对井口气组分明显偏湿、碳同位素值明显偏重;各组分碳同位素值随解吸时间变重:不同样品δ^(13)C1值最大变重幅度12.3‰~23.9‰,而不同样品δ^(13)C2值最大变重幅度仅0.8‰~2.3‰,即甲烷碳同位素值相对重烃变化更明显,与前人页岩岩心解吸实验结果一致。研究结果认为:地层状态下页岩气可能并未发生碳同位素倒转,岩心解吸过程中观察到的δ^(13)C1值比δ^(13)C2值变化更明显,不是不同组分扩散速率差异造成,而主要是由于甲烷与乙烷处于不同解吸阶段导致,即乙烷处于其解吸早期阶段而甲烷处于其解吸较晚阶段;生产过程中吸附作用引起的烷烃气不同组分相态差异与所处解吸阶段差异可能是导致四川盆地龙马溪组页岩气碳同位素完全倒转的主要原因,但不能否认干酪根裂解气与原油裂解气的混合对页岩气碳同位素倒转做出的部分甚至大部分贡献。

This study is focused on the carbon isotope reversal of shale gas in Silurian Longmaxi Formation, Sichuan Basin. Core desorption test is conducted on the shale in Longmaxi Formation, Jiaoshiba area, so as to analyze its gas components and carbon isotope compositions. The results indicate that the desorbed gas is obviously wetter than the wellhead gas in component, and the carbon iso tope of the former is significantly heavier; the carbon isotope values of each component became greater with the increase of desorption time. In terms of different samples, the maximum variation of δ^13C1 value is from 12.3%0 to 23.9%0, while that of δ^13C2 is only from 0.8%oto 2.3%0, i. e. , the carbon isotope of methane presents more obvious variations than heavy hydrocarbon, which is consistent with the results of previous shale core desorption experiments. It is concluded that δ^13C1 value observed in core desorption process has more obvious variations than δ^13C2 value, which is not caused by the difference in diffusion rate, but mainly results from the differences in desorption stages, i. e. , ethane is in its early desorption stage while methane in the late desorption stage. In the pro duction process, the differences in the component phase of alkane hydrocarbon gas and those in desorption stage caused by adsorp tion-desorption process may be the major reasons for complete reversal of the carbon isotopes of shale gas in Longmaxi Formation, Sichuan Basin. However, it cannot be denied that the mixing of kerogen pyrolysis gas and crude oil cracking gas make a partial or even a major contribution to the reversal of carbon isotopes in shale gas.