STABLE AND CLUMPED ISOTOPE CONSTRAINTS ON THE ORIGINS OF NATURAL GASES FROM COLD SEEP ENVIRONMENTS

Marine cold seeps are found at oceanic margins and are commonly associated with hydrocarbon reservoirs. Here we apply molecular and isotopic tracers, including methane clumped isotopologues, ¹²CH₂D₂ and ¹³CH₃D, to gas from thermogenic seeps in the Barents Sea, and two locations in the Santa Barbara Basin (SBB), the Coal Oil Point (COP) seeps and S. Ellwood Trend (SET) seeps. We compare the compositions of the seeping hydrocarbons to the underlying reservoirs to understand the changes in gas composition associated with subsurface migration and the processes responsible for those changes.

We find that our molecular and isotopic seep data can be explained by variations in seep rate and the thermal maturity of the source rocks. The seep rates in the Barents Sea are slow and seeped gases primarily consist of methane. As this seeped gas migrates from the underlying reservoir, a microbial component is added, lowering the isotopic composition of the mixed methane compared to that of the reservoir gas. The methane samples from the Barents Sea are also in equilibrium with respect to clumped isotope compositions at bottom-water temperatures, likely through a redox reaction cycle with CO₂ or as a result of anaerobic oxidation of methane. In the SBB, seepage rates are one to two orders of magnitude higher than in the Barents Sea and the seep gases are composed of methane as well as higher order n-alkanes. The COP seeps have a lower seepage rate, cooler Δ₁₈-based apparent temperatures and lower C₂₊ concentrations than the SET seeps. There are two explanations for the observed variations. Either the cooler Δ₁₈-based apparent temperatures in the COP seeps are due to the slower seepage rate and thus longer travel time, allowing for more C₂₊ hydrocarbons to be biodegraded and the resulting methane to come closer to clumped isotope equilibrium. Or the COP seeps are reflecting a shallower and cooler source while the SET seeps reflect a hotter deeper reservoir source. Further geochemical tracer studies of the underlying reservoirs in SBB could distinguish between these mechanisms. If seepage rate is the dominant mechanism controlling the seep gases, these findings suggest the bulk isotopic composition of seeped gases from high flux seeps (but not low flux seeps) may be used in petroleum exploration to infer the composition of the underlying reservoir gas.