为明确四川盆地页岩气碳同位素特征及地质意义,采用气相测谱等分析手段,对涪陵、长宁、彭水、威远地区页岩气样品进行分析。结果表明:研究区页岩气成分以甲烷为主,含量为96.10%～99.40%,乙烷、丙烷等含量低;非烃气体中以N2与CO2为主;含有微量氦气,含量为0.01%～0.03%,CO2的碳同位素含量为-12.5‰～8.9‰,表明CO2的来源包含有机成因与无机成因。目前四川盆地页岩气田均发现碳同位素倒转排序(δ13C3<δ13C2<δ13C1),其机理主要为不同来源的烃类气体混合、气体与矿物反应、烷烃的解吸/扩散过程的分馏作用。页岩气碳同位素倒转程度与页岩层系的封闭性密切相关,因为页岩层系的自封闭性直接影响气体排烃及天然气扩散程度。此外,页岩气产量与倒转程度有良好正相关性,表明页岩气碳同位素特征对页岩气产量预测、保存条件与富集规律评价有重要指示作用。研究结果对四川盆地海相页岩气深入勘探具有一定指导意义。
In order to determine the carbon isotope characteristics and geological significance of shale gas in Sichuan Basin, shale gas samples in Fuling, Changning, Pengshui and Weiyuan were analyzed with gas spectrometer and other analytical methods. As found in the results, the shale gas in the study area was mainly composed of methane with a content ranging from 96.100% to 99.400%, and mixed with a small amount of ethane and propane; the non-hydrocarbon gases were dominated by N2 and CO2, and traces of helium from 0.01% to 0.03% were present; the carbon isotope of CO2 was -12.5‰ to 8.9‰, indicating that CO2 was sourced from organic and inorganic genesis. At present, carbon isotopic inversion sequence (δ13C3 <δ13C2 <δ13C1) has been found in shale gas fields in Sichuan Basin, which is mainly caused by mixing of hydrocarbon gases from different sources, reaction between gases and minerals, and fractional distillation in alkane desorption/diffusion. The degree of carbon isotope inversion in shale gas is bound up with the confinement of the shale strata, as the self-confinement of the shale strata directly affects the degree of hydrocarbon expulsion and gas diffusion. In addition, there is a positive correlation between shale gas production and inversion degree, indicating that the carbon isotope characteristics of shale gas are important indicators for shale gas production prediction and preservation conditions and enrichment pattern evaluation. The results of the study are of guidance for the further exploration of marine shale gas in Sichuan Basin.
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