四川志留系龙马溪组页岩等温解吸及甲烷碳同位素分馏特征

Isothermal desorption and methane carbon isotope fractionation of shale gas in the Silurian Longmaxi formation, Sichuan

本文选取四川志留系龙马溪组页岩,对所采样品进行了地球化学、岩石矿物组分及含气性分析。在实验室内采用磁悬浮天平进行质量法等温解吸实验,通过页岩气解吸过程中甲烷碳同位素(δ^(13)C_1)组成变化研究甲烷碳同位素分馏过程,比较不同温度下甲烷碳同位素分馏特征;同时赴现场收集页岩岩心解析气,进行气体组分和甲烷碳同位素分馏分析。结果表明,随着压力降低,样品中气体逐步解吸,甲烷碳同位素出现分馏特征,δ^(13)C_1值呈现增大的趋势;页岩储层性质影响甲烷碳同位素分馏,有机碳含量相对越高,δ^(13)C_1值越大,矿物组分和扩散运移也会影响甲烷碳同位素的分馏结果;温度影响甲烷碳同位素分馏,低温时甲烷碳同位素容易分馏。现场测定的甲烷碳同位素分馏变化明显,气体组成随解析时间发生规律性变化。

Several shale samples were selected from the Silurian Longmaxi formation from the Yibin region in the Sichuan Province of China. Shale samples were examined by geochemical analysis, rock mineral composition analysis and desorption experiments. Isothermal adsorption experiments were carried out by the gravity method and the carbon isotope fractionation of methane was investigated by measuring the value of δ^（13）C_1. The characteristics of carbon isotope fractionation of methane at different temperatures were compared. Moreover, the desorbed gas was collected at the same exploration sites and the gas composition and values of δ^（13）C_1 were measured. Carbon isotope fractionation of methane is influenced by shale reservoir characteristics, especially the higher the total organic carbon（TOC）, the greater value of δ^（13）C_1. Mineral components and migration diffusion may influence the adsorption capacity of carbon isotope fractionation of methane. In low temperature environments, the degree of carbon isotope fractionation is higher. The range of the carbon isotope fractionation of methane in-site is obvious and has a positive correlation between the content of CH_4. Moreover, the gas composition changes regularly with the increase of desorption time.