塔河油田天然气的碳同位素特征及其成因类型

Carbon isotope features and genetic type of natural gas in Tahe oilfield

塔河油田天然气组分以烃类气体为主,平均含量占气体总体积的94.09%;非烃气体以N2和CO2为主,平均含量分别占4.2%和1.19%。烃类气体中以甲烷占绝对优势,平均含量为76.23%;重烃含量较高,平均含量为17.86%;干燥系数(C1/C1-5)平均为0.81,整体上属典型的湿气。塔和油田所有样品的δ13C2都轻于-30‰,属于典型的油型气范畴;天然气的1δ3C1<δ13C2<1δ3C3,呈明显的正序排列,显示天然气为典型的有机成因。塔河油田伴生天然气主体属于典型油型气,未发现其他母质类型天然气的混入。甲烷碳同位素组成反映其母质演化程度已处于成熟-过成熟阶段,且以高过成熟阶段为主。结合地质背景进行分析,源岩应属寒武—奥陶系。部分系列碳同位素的非线性分布模式显示,塔河油田的天然气具有两期充注特征:早期充注的为原油伴生气;晚期充注的为高温裂解气。

Natural gas in Tahe oilfield is mainly composed of hydrocarbon gas with an average content of 94.09%. The non-hydrocarbon gas is mainly composed of N2 and CO2, with average contents of 4.2% and 1.19%, respectively. Hydrocarbon gas is mainly composed of methane with an average content of 76.23%; while the heavy hydrocarbon has an average content of 17.86%. The drying coefficient （ C1/C1-5 ） averages 0.81, indicating typical wet gas as a whole. The δ^13C2 values of all samples in this area are less than -30‰, indicating that it is of petroleum gas; and the δ^13C values are in obvious positive sequence, i.e. δ^13 C1 〈 δ^13C2 〈δ^13C3 , indicating that it is typically of organic origin. The associated gas in Tahe oilfield mostly belongs to typical petroleum gas, without any mixture of gas from other sources. The carbon isotope composition of methane reveals that the evolution of its source rocks is in the range of mature to over mature stages, and is mainly in highly over-mature stage. Analysis in combination with geologic setting indicates that the source rocks are of Cambrian-Ordovician. The non-linear distribution of some series of carbon isotopes shows that the gas in Tahe oilfield has been charged in two stages. Associated gas has been charged in the early stage, while pyrolysis gas has been charged in the late stage.