川东北元坝地区陆相气地球化学特征及来源

川东北元坝地区近年来在上三叠统须家河组（T3x）及中下侏罗统千佛崖组（J2q）和自流井组（J1z）陆相地层发现工业性气藏.它们的化学成分和碳同位素组成存在明显差别，具有不同的成因和来源.须家河组天然气烷烃中以甲烷为主，干燥系数在0.96以上.其上、下部地层天然气碳同位素组成变化显著，T3x3~5气层乙烷碳同位素较重，δ13C2值（-25.2‰~-20.7‰）均大于-26‰，为典型的高 过成熟煤型气；而T3x1~2天然气δ13C2值大多数小于-28‰，主要在-32‰~-28‰，具有高 过成熟煤型气与过成熟油型气混合气的特点.J1z地层天然气干燥系数大都在0.95以上，δ13C2值在-28.2‰~-21.4‰，属不同成熟度的煤型混合气.J2q地层天然气干燥系数都在085以下，热演化程度较低.其乙烷δ13C值分布在-32.3‰~-31.4‰，为典型的（高）成熟油型气.在对天然气区域性对比基础上，通过与本区烃源岩的碳同位素组成的对比认为，T3x3~5天然气源自本层位煤系烃源岩，而T3x1~2天然气除源于所在地层气源外，可能混有下部海相天然气；侏罗系天然气主要为自生自储气.

Geochemical characteristics and origin of natural gases from terrestrial strata in Yuanba area of the Northeastern Sichuan Basin

The industrial gas reservoirs have been discovered in the strata of the Upper Triassic Xujiahe （Tax）, the Middle-Lower Jurassic Qianfoya （J2q） and Ziliujing （J1z） Formations in the Yuanba area of Northeastern Siehuan during recent years. Their chemical and carbon isotopic composition differ significantly, meaning that they have dif ferent origins and sources. Alkane gases from Xujiahe Formation are dominated by methane, and the dryness coeffi- cient is above 0.96. Carbon isotope composition of natural gases from the upper and lower strata of Xujiahe Forma- tion changes significantly. Carbon isotope of ethane in Taxa3-6 strata is isotopically heavier with δ13 Ce values （-25. 2‰~-20. 7‰） greater than -26‰, meaning that it is typically high over mature coal-type gases. In con trast, δ13C2 of the gases from T3x1-2 strata is less than -28‰, and mainly ranges between -32‰ and -28‰, showing the characteristics of mixture between over mature coal-type gases and over-mature oil-related gases. Dry- ness coefficient of natural gases from J1z is mostly more than 0.95, and δ13 C2 changes in the -28.2‰~ 21.4‰, belonging to the mixture of coal-formed gases with different maturity. However, dryness coefficient of natural gases from J2q is Below 0.85, indicating a relatively low level of thermal maturity. Their δ13 C2 value ranges from -32.3 ‰ to -31.4 ‰, belonging to the typical mature oil-related gases. Based on the comparison of carbon isotopic com position between the natural gas and the hydrocarbon source rocks, it comes to a conclusion that natural gases in T3x3-5 strata are derived from the coal bearing source rocks within this sequence, whereas those in T3xl-2 strata are from local source rocks, maybe mixed with the marine gases. Jurassic gases are mainly self-sourced.