ASSESSING RESERVOIR FLUID ISOLATION OR UPWARD MIGRATION FROM LIGHT HYDROCARBON AND CO2 COMPOSITIONS IN MULTIPLE ZONES: IMPLICATIONS FOR CARBON CAPTURE AND STORAGE

Carbon Capture and Storage (CCS) is injection of CO₂ into deep sedimentary formations to mitigate greenhouse gas emissions. To determine if CO₂ is effectively isolated from the atmosphere at potential CCS sites, information is needed about the risk, rate, and migration process of buoyant fluids from depth to the surface. The goal of this study is to constrain interpretations of fluid connectivity, or lack thereof, between reservoirs, permeable zones in the overburden, aquifers, and the vadose zone. Hydrocarbon systems provide an analog that shows the extent to which isolation or vertical migration is possible. Fluid compositions from three Gulf Coast CO₂ Enhanced Oil Recovery (CO₂-EOR) sites and one offshore site show several trends that relate to fluid source signatures and migration. Extensive fluid migration at a Mississippi Interior Salt Basin CO₂-EOR location is unlikely given that the reservoir, overburden, and vadose zone each have unique methane stable isotopes that generally match separate methanogenic processes. Hydrocarbon migration is also not observed at an Oligocene Frio Formation reservoir defined by a rollover structure juxtaposed against a regional growth fault. Methane isotopic values from the Evangeline (~650 – 2500’) and Chicot (~100 – 600’) aquifers support the conceptual model a CO₂-reduction formation process rather than vertical migration. However, light hydrocarbon seepage is possible at a Frio Formation reservoir that was highly faulted by underlying salt intrusion. Methane isotopes from shallow groundwater show a thermogenic signature similar to Frio reservoir values measured at this site. Hydrocarbon migration is also interpreted from fluid inclusions from two Texas offshore Miocene wells near intruded salt domes. There is a non-stoichiometric decrease in CH₄ concentrations upwards while CO₂ concentrations increase, which may be partially explained by anaerobic oxidation of CH₄. Results from this work indicate that while post-genetic effects and mixing can alter fluid compositions, generation process signatures may still be discernible even after vertical ascent. Stacked zones containing light hydrocarbons showing unique generation processes indicate fluid isolation over geologic timescales, which is an important prerequisite for potential CCS sites.