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

Abstract

Abstract 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 δ 13C1 and δ 13C2 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 O1k and O2w marine source rocks. The δ 13C1 of weathered crust gas varies from −31.2 to −37.8‰, but δ 13C2 is mostly around −25.0‰, which indicates weathered crust gas originates from the Carboniferous–Permian coal-measure source rocks. In contrast, δ 13C1 and δ 13C2 in the pre-salt strata are mostly less than −30.0‰. Yet its distribution of Δ(δ 13C2–δ 13C1) is from −1.7 to 13.6‰, and C1/C2 + C3 is of 6.68–4372.50. These indicate that the pre-salt gas is mainly high-over mature oil cracking gas from the O1m marine source rock. The δ 13C1 of Ordovician deep gas is averaging −39.4‰, and δ 13C2 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 δ 13C2 and Δ(δ 13C2–δ 13C1) 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.