The 13C-depleted methane in terrigenous shale gas: A case study in the Triassic Yanchang Formation, Ordos Basin

The methane carbon isotopic composition (δ13C1) of some shale gas, and of some conventional gas in shale gas regions, is more depleted in 13CH4 than can be predicted from vitrinite reflectance (Ro) using the δ13C1-Ro equation. This is also often the case for coal bed methane. There is currently no convincing explanation for this phenomenon primarily because wellhead gas or core desorption cannot represent the original, in-situ state of the shale gas, owing to problems of desorption fractionation, lost gas, and residual gas. In this paper, we focus on shale gas in the Yanchang Formation of the Ordos Basin. To avoid the above-mentioned problems and obtain the shale gas in-situ, two shale samples, obtained from pressure coring, were used for parallel experiments. The gas desorption experiment was carried out under normal temperature and heating conditions, the core residual gas was obtained by crushing, and the components and carbon isotopic composition of desorption gas and residual gas were analyzed. The δ13C1 values for the two shale (Ro = 1.2%) desorption gas samples were −50.7‰ and −50.8‰, about 10‰ lighter than the value of −40.6‰ predicted by the empirical δ13C1-Ro formula. The δ13C1 values for the small amount of core residual gas in the two samples were −47.0‰ and −47.6‰, indicating that δ13C1 of Yanchang Formation in-situ shale gas is about 10‰ lighter than the δ13C1-Ro empirical formula predicts. This difference may be related to gas adsorption onto organic matter and clay minerals in shale. The negative δ13C1 is due to the retention of pre-biogas and early thermogenic gas. Dynamic adsorption experiments with activated carbon and a 5 Å molecular sieve show that early methane molecules preferentially occupy adsorption sites with relatively low pore radii and are likely to be replaced by late methane molecules, due to the high adsorption energy. The pore size distribution characteristics of shale in the Yanchang Formation are generally consistent with this pattern. In general, the δ13C1 of shale gas is related to adsorption, and the degree to which it is negative is related to the adsorption intensity and pore structure of the source rock.