and third quartile, and individual points indicate outlier values beyond this limit. The number of samples per category is listed. The arrow for the freshwater microbial indicates that some values extend beyond 600 C, and indeed some samples in this category have negative clumped isotope values, which do not correspond to any
temperature. Microbial methane samples from serpentinization sites are not shown since these samples have negative 13CH3D values. Data from Stolper et al. (2014b); Inagaki et al. (2015); Stolper et al. (2015); Wang et al. (2015); Douglas et al. (2016); Young et al., (2017) and this study.
Figure 7: Plot of (A) 13C vs. T18 and (B) D vs. T18 values for samples from natural gas reservoirs or seeps. The regression line is fit to the entire dataset depicted. Here we only plot samples of an inferred mixed source from natural gas wells. Data from Stolper et al.
(2014b); Stolper et al. (2015); Wang et al. (2015); Young et al., (2017) and this study.
Figure 8: Plot of [C1]/[C2+C3] vs. T18 values for samples from natural gas reservoirs or seeps. Not all natural gas reservoir samples had gas composition data available. We only plot samples of an inferred mixed source from natural gas wells. Data from Stolper et al.
(2014b); Stolper et al. (2015); Wang et al. (2015) and this study.
Figure 9: Plot shows the difference in T18 values for three natural gas samples between methane extracted without heating the steel cylinder and methane extracted while heating the cylinder to 85 C. In a sample from the Gulf of Mexico with a relatively low T18
value the difference between treatments is within error. For two samples from the Eagleford Shale, however, heating the sample cylinder led to substantially lower T18 values.
Figure 10: Plots of (A) D vs. 13C and (B) D vs. T18 for three suites of gases inferred to be mixtures of methane from high- and low-temperature formation environments. Mixing models for the Antrim Shale (Stolper et al., 2015) and Southeast Alaska (Douglas et al., 2016) were described previously. The mixing trend for the Diana/Hoover Fields was based on end-members from inferred pure microbial (Stolper et al., 2014b) and thermogenic methane (this study) from nearby fields in the Gulf of Mexico. The non-linearity of mixing of T18 values for the Antrim Shale and Diana/Hoover Fields is subtle, but is much more pronounced in Southeast Alaska, where the end-members differ much more in D and 13C.
Figure 11: (A) Comparison of T18 values vs. estimated reservoir temperatures for deep subsurface microbial methane samples. T18 values are either within error of, or
significantly higher than, reservoir temperatures. T18 values higher than reservoir temperatures may indicate uplift of methane forming strata to cooler thermal environments, or migration of gas from deeper strata. Methodologies for reservoir temperature measurements or estimates are detailed in Section 2.4. We applied a
conservative 20 ºC error to reservoir temperatures, with the exception of drill-stem tests from the Qaidam Basin, where we applied a 10% error. (B) Comparison of the thermal history of the Milk River Formation with methane T18 values. The T18 values suggest that methane predominantly formed during the deepest burial of the sediments, prior to
subsequent uplift. Data from Stolper et al. (2014b); Inagaki et al. (2015); Wang et al.
(2015); Young et al., (2017); and this study.
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