Carbon isotope reversal of desorbed gas in Longmaxi shale of Jiaoshiba area, Sicuhan Basin
Carbon isotope reversal of desorbed gas in Longmaxi shale of Jiaoshiba area, Sicuhan Basin
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DOI：
10.1016/j.ptlrs.2016.11.001
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发表时间：
2017-06
影响因子：
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通讯作者：
Hua Qin;Xiaojun Fan;Ming Liu;Jingyu Hao;Bohan Liang
中科院分区：
文献类型：
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作者：
Hua Qin;Xiaojun Fan;Ming Liu;Jingyu Hao;Bohan Liang
通过对页岩岩心解吸气成分和碳同位素组成的分析，研究了四川盆地胶石坝地区志留系龙马溪组页岩气的碳同位素反转现象。结果表明，与井口气相比，龙马溪页岩解吸气的湿组分和重碳同位素值显著增加，各组分的碳同位素值随着解吸时间的延长而变重，不同样品的δ13C1值的最大正值变化范围为12.3~2 3.9‰，而δ13C2值的最大正值变化范围仅为0.8~2.3‰，说明甲烷碳同位素值的变化比重烃碳同位素值变化更明显。上述结果与其他研究人员进行的页岩岩心解吸实验结果一致。地层中的页岩气可能不存在碳同位素反转现象，岩心解吸过程中δ13C1值比δ13C2值变化更大的现象不是由不同组分之间的扩散速率差异引起的，而主要是由于甲烷和乙烷解吸的不同阶段所致，即乙烷处于早期解吸阶段，甲烷处于解吸后期；在生产过程中，烷烃气不同组分之间的相态差异和吸附引起的解吸阶段的差异，可能是造成龙马溪组页岩气碳同位素完全逆转的主要原因，但也不能排除干酪根裂解气和原油裂解气的混合物对碳同位素逆转可能有部分或更多的主要贡献。
Through analysis of components and carbon isotope compositions of gas desorbed from shale cores, the carbon isotope reversal phenomenon in the shale gas from the Silurian Longmaxi Formation of Jiaoshiba area in Sichuan Basin were well studied. Results showed that compared with the wellhead gas, the desorbed gas from Longmaxi shale had significantly more wet components and more heavy carbon isotope values; carbon isotope values of each component became heavier with the desorption time, δ13C1values of different samples had maximum positive variations of 12.3–23.9‰, but δ13C2values only had maximum positive variations of 0.8–2.3‰, indicating carbon isotope values of methane changed more obviously than that of heavy hydrocarbon. The above results were consistent with previous results of shale core desorption experiments carried out by other researchers. Shale gas in strata might have no carbon isotope reversal, and the phenomenon that δ13C1values changed more significantly than δ13C2values during the core desorption was not caused by diffusion rate differences among different components, but mainly due to different desorption stages of methane and ethane, i.e., the ethane was in its early desorption stage while the methane was in its later desorption stage; during the production process, phase differences among different components of alkane gas and differences in the desorption stages induced by adsorption, could be the major cause for total reversal of carbon isotopes of shale gas in Longmaxi Formation, but it also could not excluded that mixture of kerogen cracking gas and crude oil cracking gas probably had a partial or more major contribution to the carbon isotope reversal.