Origin of natural gas from the Ordovician paleo-weathering crust and gas-filling model in Jingbian gas field, Ordos basin, China

Abstract

The Ordos basin, the second largest natural gas producing basin in China, is one of the most tectonically stable sedimentary basins favorable to natural gas generation and preservation. The origin and potential source of the Jingbian gas field, one of the five giant gas fields in the basin containing the Ordovician paleo-weathering crust as reservoir strata, remains unclear. The question is whether gas is generated from Carboniferous–Permian coal measures or Ordovician carbonates. This study provides abundant gas geochemical evidence which clarifies its origin and source. Among 82 natural gas samples collected from the Ordovician paleo-weathering crust in the Jingbian gas field, C 1–C 4 gaseous hydrocarbons are dominant with low amounts of non-hydrocarbons (CO 2, H 2S and N 2). The content of C 1–C 4 ranges from 79.26 to 99.93% with a gas dryness (C 1/C 1−4) of 0.981–0.999, suggesting that natural gases are dry and possibly originate from a high to over-mature source. The carbon isotopic compositions of methane and its homologues in natural gases from the Ordovician paleo-weathering crust vary widely, with δ 13C 1, δ 13C 2 and δ 13C 3 values of −38.9‰ to −29.1‰, −37.5‰ to −22.2‰ and −30.0 to −20.9‰, respectively. The δ 13C 2 and δ 13C 3 values in the Jingbian gas field are much more negative than those of typical coal-type gas in Sulige, Wushenqi, Yulin, Daniudi, and Kela2 gas fields. The carbon isotopic signature suggests that the natural gas in the Ordovician paleo-weathering crust in the Jingbian gas field most likely originates from Ordovician carbonates containing sapropelic organic matter. The measured δD values of methane range from −177 to −155‰ and support our hypothesis of the natural gases originating from marine sapropelic organic matter. The thermal maturity of the natural gases in the Ordovician paleo-weathering crust is at a high- to over-mature stage, suggesting that the gases originated from secondary cracking of oil and gas at high temperature and pressure. Thermochemical sulfate reduction (TSR) might partially play a role in the alteration of gas dryness and δ 13C 1 in those areas where gypsum is present as an effective cap rock. Mixing of high- to over-mature secondary cracking gas from the Ordovician source and coal-type gas from Carboniferous–Permian coal in the Jingbian gas field is minor.