Adsorption-induced negative carbon isotope sequence in over-mature coal-type gas from the southwest Ordos Basin

The carbon isotope sequence of alkanes is a key indicator used to distinguish organic from inorganic gas. A negative carbon isotope sequence (i.e., δ¹³C₁ > δ¹³C₂ > δ¹³C₃ > δ¹³C₄) is a characteristic feature of inorganic gas. Some gas samples from the Qingyang gas field in the southwestern Ordos Basin exhibit a negative carbon isotope sequence, but the geological conditions necessary for the development of inorganic gas are not present. There is currently no reasonable explanation for this phenomenon. This study takes into account the geological background of the Ordos Basin and comprehensively investigates the causes of the anomalous carbon isotope sequence of alkanes in the Qingyang gas field; this is done through an analysis of natural gas geochemical characteristics and adsorption/desorption experiments on high-rank coal. Our results show that: (1) Natural gas from the Qingyang gas field is over-mature coal-type gas derived from Carboniferous‒Permian formations. Its negative carbon isotope sequence is mainly related to the adsorption of gases by coal. During the over-mature stage, the content of heavier hydrocarbon gases (C₂₊) is very low, and the adsorption capacity of coal for C₂₊ gases is stronger than that for methane. Heavier hydrocarbon gases (e.g., ethane), with lighter carbon isotope signatures, preferentially desorb, resulting in a relatively light observed carbon isotope composition. Owing to its high abundance, the isotopic composition of methane is impacted relatively little by adsorption. (2) The anomalous geochemical characteristics of over-mature coal-type gas result in the failure of the negative carbon isotope sequence method for identifying inorganic gas; this also invalidates the criterion for classifying oil-type gas and coal-type gas based on thresholds of δ¹³C₂ = - 28‰ and δ¹³C₃ = - 25‰. Additionally, previously proposed empirical formulae (e.g., δ¹³C₂‒Ro and δ¹³C₃‒Ro) are not applicable to over-mature natural gas. Furthermore, the δ¹³C₂-δ²H₁ cross-plot method, used for determining the type of source rock, is rendered ineffective because over-mature samples deviate from the coal-type gas range. (3) The methane carbon isotope signature (δ¹³C₁) and gas dryness coefficient (C₁/C_1-5) are reliable indicators of source rock maturity.