四川盆地东北部马路背地区上三叠统须家河组天然气地球化学特征及气源

Geochemical characteristics and source of natural gas of the Upper Triassic Xujiahe Formation in Malubei area,northeastern Sichuan Basin

依据天然气化学组分及碳、氢同位素等地球化学资料,分析了四川盆地东北部马路背地区上三叠统须家河组天然气地球化学特征、天然气成因及来源。研究表明,马路背地区须家河组天然气组分以甲烷为主,含量介于92. 60%～99. 04%,平均为97. 59%,干燥系数普遍高于0. 990 0,平均为0. 992 2,热演化程度较高;与邻区须家河组天然气对比,马路背地区须家河组天然气碳同位素明显具有甲烷碳同位素偏重、乙烷碳同位素偏轻的特征,δ^(13)C_1值介于-33. 70‰～-28. 60‰,平均为-30. 88‰,δ^(13)C2值介于-36. 40‰～-28. 90‰,平均为-33. 11‰,甲烷和乙烷碳同位素多表现为倒转分布。天然气成因鉴别及气-源对比研究表明,马路背地区须家河组天然气为Ⅲ型和Ⅱ型干酪根生成的煤型气和油型气的混合热成因气,天然气主要来源于上三叠统须家河组煤系烃源岩及上二叠统吴家坪组海相烃源岩,甲烷和乙烷碳同位素倒转正是煤型气及油型气混合所致。马路背地区须家河组天然气高产富集与该区海相、陆相烃源岩双源供烃及深大断裂有效沟通海相、陆相多套优质烃源岩关系密切,沟通海相、陆相烃源岩的通源断裂在该区天然气成藏富集及后期调整改造方面具有重要作用。

Based on the analyses of geochemical data like gas composition and carbon and hydrogen isotopes,the study is made on the geochemical characteristics,origin and source of natural gas in the Upper Triassic Xujiahe Formation in Malubei area,northeastern Sichuan Basin.The results indicate that the natural gas in the Xujiahe Formation in Malubei area is mainly composed of methane,with the content ranging from 92.60%to 99.04%,averaging out at 97.59%,and its gas dry coefficient is generally higher than 0.990 0,with the average value of 0.992 2,indicating a higher degree of thermal evolution.Compared with the Xujiahe natural gas in adjacent areas,the values of gasδ13 C 1 in Malubei area are bigger(ranging from-33.70‰to-28.60‰,with an average of-30.88‰),but the values ofδ13 C 2 are smaller(ranging from-30.88‰to-28.90‰,with an average of-33.11‰),and the carbon isotopes of methane and ethane in the gas of the study area are mainly in reversed distribution:the carbon isotope of methane is relatively heavier while the carbon isotope of ethane is lighter.The identification of gas origin and analysis of gas-source correlation indicate that the natural gas in the Xujiahe Formation in Malubei area is the mixed thermogenic gas of coal-type gas and oil-type gas generated by kerogen Type III and II,and mainly sourced from the coal-measure source rocks in the Upper Triassic Xujiahe Formation and the marine source rocks in the Upper Permian Wujiaping Formation.And the carbon isotopic reversal of methane and ethane for the Xujiahe gas just results from the mixing of the coal-type gas and oil-type gas.The accumulation and high productivity of the natural gas in the Xujiahe Formation in Malubei area is closely related to hydrocarbon generation by both marine and terrestrial source rocks and the connectivity of several quality marine and terrestrial source rocks to the Xujiahe reservoir by means of deep and large faults,which have played a significant role in natural gas accumulation and later adjustment in the Xujiahe Formation in Malubei area.