The relative abundances and stable isotope compositions (d13C and 5H) of low molecular weight (C1-C5) hydrocarbons are key parameters for determining the origin of natural gases and interpreting thermal maturity in unconventional petroleum systems. Gases released from drill cuttings may provide insight into matrix-bound components of the petroleum system. Cuttings gases commonly exhibit greater compositional and stable isotopic variability relative to mud gases sampled from the same intervals. This variability can be induced by in situ methanogenesis occurring after collection in poorly preserved samples. In such cases, previous work (Rosenau et al. 2014) has shown the thermogenic isotopic signatures of methane to progressively shift to more negative d13C values, with decreasing gas wetness due to microbial generation of methane.
Here we report results from continued monitoring of carbon isotopes in headspace gases of the Piceance Basin cuttings samples from the Rosenau et al. study. New measurements, performed from September to March of 2014, showed continued variation of C1 d13C values. Surprisingly, the d13C of methane shifted to less negative values in the samples where microbial activity was evident after approximately 11 months of storage. In all but one of the samples, these values began approach the initially measured values, consistent with a thermogenic source. In the remaining sample, the rebounded d13C is higher than the initial measurement. More promising, all samples show very little variation of C2-C5 d13C values throughout the entire course of the study. Time series results from additional samples from the Fort Worth Basin shows both microbial influence as well as C1-C3 d13C increasing over time. Time series of Appalachian Basin samples show virtually no C1-C3 carbon isotope variation over one year of storage.
Overall, these data show cuttings samples can retain the carbon isotopic signature of C1-C5 hydrocarbons for spans greater than 1 year. While methane carbon isotope values are often obscured by microbial processes, that influence may subside over time. While methane d13C values can be critical data for petroleum system evaluations, the d13C of ethane and propane, if available, are often more useful for predicting system maturity and evaluating vertical trends. Thus, our results indicate cuttings samples can be of value, even in cases where samples are not well preserved.