Stable Isotopes in Methane and Hydrogen Sulfide as Reliable Tools in Provenance Studies of Natural Gases Trapped in Gas Hydrates of Colombian Caribbean offshore Basins

Gas hydrates are considered the ultimate natural gas reserve of the planet and are probably present in the Caribbean offshore of Colombia, based on 2-D seismic studies that have detected zones with high density of bottom simulating reflectors (BSR) in the Colombia, Sinú Offshore, and Guajira Offshore Basins (e.g., Reed et al., 1990; Lopez and Ojeda, 2006). Stable isotopes in methane (CH4) and hydrogen sulfide (H2S) can be used as source-fingerprints of gas molecules trapped inside hydrates, allowing the differentiation of CH4 from microbial and thermal processes, providing valuable context for economic recovery of natural-gas resources. It is challenging, however, to apply isotope systematics to hydrate-forming systems due to complex influences on nucleation and dissociation under varying conditions of salinity/pressure/temperature and interactions of gas molecules with clay minerals and organic matter. In this study, pressure-vessel analogue models allow nucleation of gas hydrates, attempting to simulate the conditions at potential gas hydrate sites: low temperatures, high pressures, CH4 and H2S saturation, varying salinity, clay mineralogy (smectites), and microbial biosurfactants reported to occur at natural hydrate sites. Salinity CH4-experiments show only small differences in carbon and moderate variations in hydrogen isotopic compositions between gas phases (free gas vs. hydrate gas). Also, experiments with clay minerals and biosurfactants display an enhancement in isotopic fractionations of the different gas fractions. These results and those of ongoing experiments will refine interpretation of gas provenance and will improve risk assessment at sites where recurrent hydrate formation complicates hydrocarbon drilling and transportation in pipelines.