COMPOSITION AND ISOTOPIC CHARACTERISTICS OF UPPER PALEOZOIC MARINE-CONTINENTAL TRANSITIONAL SHALE GAS IN THE SOUTHEASTERN PART OF ORDOS BASIN, CHINA: APPLICATION TO GAS ORIGIN AND MIGRATION AMONG COMPLICATED LITHOLOGIES

31 natural gas samples were collected from different lithologies including mudstone, sandstone and coal of Shanxi formation (P₁s) and Benxi formation (C₂b) in the southeastern part of Ordos Basin and the gas components, carbon and hydrogen isotope composition of gas samples were measured in this study. A remarkable difference in the ethane and carbon dioxide isotope values present between the natural gas from Shanxi formation and Benxi formation. The δ¹³C₂ value of natural gas of Shanxi Formation generally is less than -30‰ whereas that of Benxi Formation is generally greater than -28‰, and the CO₂ in Shanxi formation is mainly of organic origin while the CO₂ in Benxi formation is mainly inorganic one. The difference between Shanxi and Benxi formation reveals that the gases in the two formations was preserved in two separated gas system, which may be attributed to existence of the tight mudstone and limestone between the reservoirs as effective barrier that prevent the gases from diffusion. Gases in the Shanxi Formation suffered strong diffusion, evidenced by homogenous gas component and isotope composition among various lithologies within Shanxi Formation, while no strong secondary alteration occurred in Benxi formation. The unusually depleted δ¹³C₂ value found in Shanxi Formation was caused by the secondary cracking under extremely high thermal maturity and resulted in partial reversal of carbon isotope values (δ¹³C₂<δ¹³C₁), whereas this secondary cracking did not occur in Benxi formation, suggesting different organic matter and depositional environment may have effect on isotopic fractionation.