On the second point, whether ¹³C can be used as a signal to differentiate fracked vs. non-fracked gas, the paper [1] does discuss this a bit. This seems to be t

On the second point, whether ¹³C can be used as a signal to differentiate fracked vs. non-fracked gas, the paper [1] does discuss this a bit. This seems to be the key paragraph: Several studies have suggested that the δ¹³C signal of methane from shale gas can often be lighter (more depleted in ¹³C) than that from conventional natural gas (Golding et al., 2013; Hao and Zou, 2013; Turner et al., 2017; Botner et al., 2018). This should not be surprising. In the case of conventional gas, the methane has migrated over geological time frames from the shale and other source rocks through permeable strata until trapped below a seal (Fig. 2a). During this migration, some of the methane can be oxidized both by bacteria, perhaps using iron (III) or sulfate as the source of the oxidizing power, and by thermochemical sulfate reduction (Whelan et al., 1986; Burruss and Laughrey, 2010; Rooze et al., 2016). This partial oxidation fractionates the methane by preferentially consuming the lighter ¹²C isotope and gradually enriching the remaining methane in ¹³C (Hao and Zou, 2013; Baldassare et al., 2014), resulting in a δ¹³C signal that is less negative. The methane in shales, on the other hand, is tightly held in the highly reducing rock formation and therefore very unlikely to have been subject to oxidation and the resulting fractionation. The expectation, therefore, is that methane in conventional natural gas should be heavier and less depleted in ¹³C than the methane in shale gas. [1] https://www.biogeosciences.net/16/3033/2019/ https://www.biogeosciences.net/16/3033/2019/