Genesis and source of natural gas in Well Mitan-1 of Ordovician Majiagou Formation, middle-eastern Ordos Basin, China

The Well Mitan-1 has achieved a major breakthrough in the exploration of Ordovician subsalt natural gas in the mid-eastern Ordos Basin, demonstrating a high-yield industrial gas flow in fourth member of the Majiagou Formation of Ordovician (O₁m₄). Despite this success, there are ongoing disputes regarding the origin of the natural gas found in Well Mitan-1. Measured results show that the natural gas in Well Mitan-1 is mainly composed of alkane gas (95.18 %). The gas drying coefficient (C₁/C_1-5) is measured at 0.947, and the H₂S content is 3.49 %, with a small amount of N₂ and CO₂ in the non-hydrocarbon gas. The carbon isotopic compositions of methane, ethane, and propane in the natural gas are −45.5 ‰, −26.4 ‰, and −24.3 ‰, respectively. Based on the regional geological background, the characteristics of potential source rocks and the geochemical characteristics of natural gas, it is considered that the natural gas in Well Mitan-1 is self-generated and self-accumulated oil-associated gas in Ordovician subsalt carbonate rocks. However, certain geochemical anomalies, such as the lighter methane carbon isotope value (δ¹³C₁) and coal-type gas characteristics in ethane carbon isotope (δ¹³C₂), raise questions. Further insights from thermal simulation experiments on hydrocarbon generation and the analysis of residual gas in rocks suggest a close relationship between the special geochemical characteristics of Well Mitan-1 and the presence of gypsum rocks. The ubiquitous gypsum rock serves a dual purpose: acting as an effective caprock, facilitating the retention of early-generated natural gas, and promoting the generation of heavy hydrocarbon gases (C₂₊) and H₂S. The relatively low H₂S content (less than 5 %) and higher C₂₊ content indicate that thermochemical sulfate reduction (TSR), if present, is not strong enough to significantly impact methane. The δ¹³C₂ is identified as a potentially sensitive parameter for identifying TSR.