Carbon isotopic fractionation by desorption of shale gases

Geochemical studies of shale gas and conventional reservoirs within the Triassic Yanchang Formation of Xiasiwan and Yongning Field, Ordos Basin show that methane is isotopically depleted in 13C as compared to δ13C1 calculated by the Ro based on the relationship between δ13C1 and Ro. Geochemical fractionation during the adsorption/desorption process of shale system may play a significant part in influencing δ13C1 values of shale gas. Two shale core samples from confined coring of the Yanchang Formation were adopted segmented desorption experiments to examine this phenomenon. The results show that the δ13C1 of desorbed gas changes little in the first few phases of the experiments at low desorption levels, but become less negative rapidly when the fraction of desorbed methane exceeds 85%. The desorption process for the last 15% fraction of the methane from the shale samples shows a wide variation in δ13C1 from −49‰ to −33.9‰. Moreover, δ13C1 of all desorbed methane from the shale samples is substantially depleted in 13C than that calculated by Ro, according to Stahl and Careyʹs δ13C1–Ro equation for natural gas generated from sapropelic organic matter. This shows some gases with isotopically enriched in 13C cannot be desorbed under the temperature and pressure conditions of the desorption experiments. This observation may be the real reason for the δ13C1 of shale gases and conventional reservoirs becomes more negative in Xiasiwan and Yongning Fields, Ordos Basin. The magnitude of the deviation between the δ13C1 of shale gas and that calculated by Ro may be related to the adsorption capacity of shale or the proportion of absorbed gases. In this way, we may be able to evaluate the relative adsorption capacity of shale in geological conditions by δ13C1 of the shale gas, or by δ13C1 of conventional gas which generated by the shale with certainty. The δ13C1 of conventional gas in Dingbian and Yingwang Fields have no deviation because the TOC value of the hydrocarbon source rock is relatively low.