煤层甲烷碳同位素与含气性关系

Relationship between the methane carbon isotope and gas-bearing properties of coal reservoir

煤层气甲烷碳同位素值是反映煤层气成因及赋存条件的有效参数。通过对沁水盆地沁南东区块煤层甲烷碳同位素和煤储层含气性测试资料分析,剖析了3号煤层甲烷碳同位素分布特征,建立了煤层甲烷碳同位素与镜质组反射率、煤层埋藏深度和煤储层含气性之间的相关关系和模型,揭示了煤层甲烷碳同位素分布的控制机理。研究结果表明:本区3号煤层自然解吸气甲烷碳同位素为-28.89‰^-53.27‰,平均-36.48‰。与全国其他地区同等演化程度的煤层气相比总体偏重,表现出煤层具有较好的保存条件;3号煤层甲烷碳同位素与镜质组反射率和煤层埋藏深度之间呈对数函数关系,且随着镜质组反射率和煤层埋藏深度增加而变重,与全国煤层甲烷碳同位素统计规律一致,主要受控于煤层气形成的热动力学机制之下的同位素分异效应和煤层气解吸—扩散—运移过程中甲烷碳同位素的分馏效应;煤层甲烷碳同位素与煤储层含气性之间存在相关性,且随着煤层气含量、煤储层压力和含气饱和度增加,3号煤层甲烷碳同位素也相应变重,且呈对数函数关系,反映控制煤储层含气性的因素与控制煤层甲烷碳同位素的因素存在一致性。

Coalbed methane carbon isotope is an effective parameter to reflect the coal-bed gas origin and occurrence conditions.Through the analysis of the measured data of coalbed methane carbon isotope(δ13C1)in the Qinnan-East block of Qinshui Basin and its coal reservoir gas-bearing properties,the distribution property of methane carbon isotope(δ13C1)in No.3coal seam was analyzed,then the correlations and their models among the coalbed methane carbon isotope(δ13C1)and vitrinite reflectance,burial depth and gas-bearing properties of coal reservoir were established respectively,and its controlling mechanism of coalbed methane carbon isotope distribution was finally revealed.The research results show that theδ13C1value of naturally desorbed gas of No.3coal seam is from-28.89‰to-53.27‰and the average value is-36.48‰in this area.Theδ13C1value is heavier compared with the same evolution degree coalbed methane in other parts of China andshows that the coal seam has a good preservation condition.Theδ13 C1 value of No.3coal seam and vitrinite reflectance,burial depth have a non-linear Logarithmic function relationships,and increases with the increasing of the vitrinite reflectance and burial depth of coal seam,which is suited to the national statistical regularity.Coalbed methane carbon isotope in this area is mainly controlled by the isotopic differentiation effect under the thermodynamical mechanism and the fractionation effect in the process of the coalbed methane desorptiondiffusion-migration.Coalbed methane carbon isotope(δ13C1)also has a good correlation with gas-bearing properties of coal reservoir.Theδ13C1value of No.3coal seam becomes heavier with increasing of the coalbed methane content、coal reservoir pressure and gas saturation,which best fits the logarithmic relation.And the control factors of gas-bearing properties in coal reservoir are consistent with that of its methane carbon isotope(δ13C1).