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Based on natural gas compositions, carbon isotope ratios, and geological analysis, the genesis and origin of gases in the Lower Paleozoic of Ordos basin are discussed. Due to differences in distribution and genesis, the gases in Lower Paleozoic were divided into five types, namely western margin gas, weathered crust gas, pre-salt gas, Ordovician deep gas, and Cambrian gas. The results show that the  δ13 C 1  and  δ13 C 2  of western margin gas range from −35.6 to −38.9‰ and from −27.2 to −35.9‰, respectively, indicating oil-type gas and mainly from the O 1 k and O 2 w marine source rocks. The  δ13 C 1  of weathered crust gas varies from −31.2 to −37.8‰, but  δ13 C 2  is mostly around −25.0‰, which indicates weathered crust gas originates from the Carboniferous–Permian coal-measure source rocks. In contrast,  δ13 C 1  and  δ13 C 2  in the pre-salt strata are mostly less than −30.0‰. Yet its distribution of Δ( δ13 C 2 – δ13 C 1 ) is from −1.7 to 13.6‰, and C 1 /C 2  + C 3  is of 6.68–4372.50. These indicate that the pre-salt gas is mainly high-over mature oil cracking gas from the O 1 m marine source rock. The  δ13 C 1  of Ordovician deep gas is averaging −39.4‰, and  δ13 C 2  varies from −25.4 to −33.0‰, which imply coexistence of oil-type and coal-type gas. It is presumed that the C–P coal-type gas may migrate into and accumulate at Ordovician deep reservoirs through unconformity and fault. The  δ13 C 2  and Δ( δ13 C 2 – δ13 C 1 ) of Cambrian gas are both the most negative, respectively, averaging −34.9‰ and averaging −3.2‰, indicating over-mature oil-type gas. Formation and evolution of the Qingyang paleo-uplift result in that the Cambrian gas mainly comes from the Cambrian or Ordovician marine source rocks in the southern basin.

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