川中与川西地区陆相煤成气地球化学特征差异

摘要

The composition, carbon and hydrogen isotopes data of terrestrial natural gas reservoirs from central Sichuan and western Si⁃chuan both in the Sichuan Basin have been analyzed. The result has shown that the natural gas from Upper Triassic Xujiahe Formation and Jurassic System in western Sichuan is mainly dry gas;from Upper Triassic Xujiahe Formation in central Sichuan is mainly wet gas. Variation of methane content, drying coefficient and nitrogen content in natural gas is regular vertically in western Sichuan. The Upper Triassic natural gas methane carbon isotope in central Sichuan is lighter than in western Sichuan. The ethane carbon isotope value ex⁃cept part of gas samples from Daanzhai Formation in Bajiaochang, central Sichuan appears as oil gas, all gas reservoirs from Xujiahe Formation in central Sichuan, Xujiahe Formation and Jurassic System in western Sichuan are all coal gas. The main reason of above dif⁃ference is:in western Sichuan, the end of third member of Xujiahe Formation has been entered into hydrocarbon generation threshold, while Xujiahe Formation in central Sichuan just starts until the early Jurassic. The secondary gas reservoir in western Sichuan Jurassic System was formed from Xujiahe Formation gas source under high pressure as mixed facies through faults. Gas reservoir in central Sich⁃uan Xujiahe Formation mainly as free facies converged into sand bodies with better physical property.%对四川盆地川中与川西地区陆相气藏天然气组分、碳同位素和氢同位素数据进行分析。结果表明：川西地区上三叠统须家河组和侏罗系天然气以干气为主，川中地区上三叠统须家河组天然气以湿气为主；天然气中甲烷含量、干燥系数和氮气含量等在川西地区纵向上有规律变化。川中地区上三叠统天然气甲烷碳同位素轻于川西地区天然气甲烷碳同位素；乙烷碳同位素值除川中八角场大安寨组气藏部分气样表现为油型气外，川中地区须家河组气藏和川西地区须家河组、侏罗系气藏均为煤型气。造成上述差异的主要原因是：川西地区须家河组在须三段沉积末期就进入生烃门限，川中地区须家河组直到早侏罗世才开始生烃。川西地区侏罗系次生气藏是须家河组气源在高压下以混合相通过断裂形成的，川中地区须家河组气藏主要以游离相汇聚至物性较好的砂体中。