Carbon isotope effects of methane transport through Anahuac Shale — A core gas study

• Core gas extracted from a shale core of the Anahuac Formation.
• Carbon isotope ratios of methane increase upward in the caprock core.
• Core gas was not produced in situ.
• A computational model coupling diffusion and adsorption is proposed to interpret the dataset.

Compositional data and stable isotope ratios are critical datasets for the study of hydrocarbon generation, expulsion, and migration. These geochemical parameters are also important for studies of leakage detection for geological carbon sequestration. Diffusion, gas–liquid partitioning and adsorption, the three most crucial processes to affect gas transport through shale and other low-permeability formations, are known to cause stable isotope fractionation. A considerable number of studies have been conducted on stable isotope fractionation associated with diffusion and gas–liquid partitioning. However, significantly fewer data are reported in the literature that specifically addresses adsorption carbon isotope effects. Moreover, adsorption isotope effects are rarely verified or seldom demonstrated in geologic systems. In this study, a shale core taken from the Anahuac Formation in Texas Gulf Coast, which overlies several Frio reservoirs, was analyzed for composition and stable isotope ratios of the residual gas. These geochemical parameters are used to identify gas migration and associate transport mechanisms in the caprock formation. Gas samples extracted from the core contain alkane gas components (C1–C5) and show a systematic variation in carbon isotope ratios of methane along depth. An analytical model is proposed to interpret the observed carbon isotope trend on the basis of coupled processes of diffusion and adsorption. At a ratio of retardation of 0.99 between 12CH4 and 13CH4 (with 12CH4 having a preferential bonding with the clay mineral) and a diffusion time of 1.5 Ma, the modeled curve fits well with observed upward increasing δ13CCH4 values. This study demonstrates the isotope effect of adsorption in a natural system and its importance in quantifying gas-migration rate and distance in low-permeability systems.