AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Effects of Brine Composition (NaCl, MgSO4, FeSO4) and Clay Minerals (Kaolinite, Nontronite, Montmorillonite) on the Stable Isotopic Composition of Methane and Hydrogen Sulfide in Gas Hydrates
(1) Biogeochemical Laboratories, Department of Geological Sciences, Indiana University, Bloomington, IN.
Stable isotopes of carbon, hydrogen, and sulfur in methane (CH4) and hydrogen sulfide (H2S) can be used as source-fingerprints of gas molecules trapped in hydrates. Isotopic fingerprints are useful for differentiation of methane 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. Here, a series of pressure-vessel experiments have been conducted with quantitative recovery of free-gas and hydrate-gas molecules of CH4 and H2S. These experiments allow nucleation of gas hydrates containing CH4 and H2S from solutions of deoxygenated Millipore water and from brines with varying concentrations (14 mM to 2M) of NaCl, MgSO4, and FeSO4. After addition of water or brine, the vessel is purged with low pressure N2 for 30 minutes followed by pressurization with CH4 (20 to 55 bars) or H2S (0.35 to 2.32 bars) from tanks with known isotopic composition. Methane experiments show only small differences in carbon isotopic composition (max 0.63‰); between tank gas and both free gas and hydrate gas. In the same experiments, hydrogen isotopic compositions vary by up to 11‰. Sulfide experiments show sulfur isotopic differences up to 3‰ between gas phases. Future experiments will test the influence of microbial biosurfactants reported to occur at natural hydrate sites. The results of these 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.