Insights into origins of the natural gas in the Lower Paleozoic of Ordos basin, China

Abstract Based on natural gas compositions, carbon isotope ratios, and geological analysis, the genesis origin of gases in Lower Paleozoic Ordos basin are discussed. Due to differences distribution genesis, were divided into five types, namely western margin gas, weathered crust pre-salt Ordovician deep Cambrian gas. The results show that δ 13 C 1 2 range from −35.6 −38.9‰ −27.2 −35.9‰, respectively, indicating oil-type mainly O k w marine source rocks. varies −31.2 −37.8‰, but is mostly around −25.0‰, which indicates originates Carboniferous–Permian coal-measure In contrast, strata less than −30.0‰. Yet its Δ( – ) −1.7 13.6‰, /C + 3 6.68–4372.50. These indicate high-over mature oil cracking m rock. averaging −39.4‰, −25.4 −33.0‰, imply coexistence coal-type It presumed C–P may migrate accumulate at reservoirs through unconformity fault. both most negative, −34.9‰ −3.2‰, over-mature Formation evolution Qingyang paleo-uplift result comes or rocks southern basin.