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.
Highlights
Origins and sources of natural gas have always been the focus of attention for geochemists
Considering the Ordovician natural gas in the Jingbian gas field of the Ordos basin as an example, some scholars believed that the weathering crust reservoir at the top of the Majiagou formation directly contacts with the Upper Paleozoic Carboniferous–Permian (C–P) coal-measure source rock, from which the natural gas is considered as the coal-type gas, and from C–P source rock [1,4,5]; some other scholars regarded that the natural gas is of mixed origins, with contributions from both the C–P coal-measure source rock and marine Ordovician Majiagou source rock [6]; and some other scholars even believed that it should be oil-type gas mainly from the Majiagou source rock [7]
Forty-seven gas samples were considered in this study. They were obtained from the PetroChina Changqing Oilfield Company, including 7 western margin gas samples, 13 pre-salt gas samples, 7 Ordovician deep gas samples, 16 weathered crust gas samples, and 4 Cambrian gas samples
Summary
Origins and sources of natural gas have always been the focus of attention for geochemists. Insights into origins of the natural gas in the Lower Paleozoic of Ordos basin, China 131 field are relatively large, spanning the distribution ranges of both the coal-type gas and oil-type gas, which makes it more difficult to distinguish genesis of the natural gas. To solve this problem, multiple methods have been adapted and some research progresses have been made. In view of this, based on the C–P coal-type gas and the natural gas in various Lower Paleozoic strata, this study compared and analyzed the components and carbon isotopes of different natural gases in the Lower Paleozoic and the C–P coal-type gas, discussed the origins of the natural gas, and clarified the sources of the natural gas in combination with geological analysis to provide a scientific basis for the deployment of the natural gas exploration in the Lower Paleozoic or deep strata of the Ordos basin
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