1. Investigating equilibrium D/H fractionation in hydrocarbons.
Alex L. Sessions, Division of Geological & Planetary Sciences, Caltech Pasadena CA
Background: Stable isotopes, such as 13C and 2H, serve as natural tracers within hydrocarbon molecules that provide a wealth of information on their origins. For hydrogen isotopes (D and H), a unique problem is that their abundance is affected by the slow but persistent exchange of C-bound H over geologic timescales. We are pursuing both laboratory experiments and molecular modeling to estimate the effects of this exchange on equilibrium D/H distributions in hydrocarbons within geologic reservoirs. This data should lead to improved predictive ability for understanding hydrocarbon generation and migration.
Experiments: Laboratory incubation of selected model carbonyl compounds leads to the progressive incorporation of D into the organic molecule. From these experiments, we can derive estimates for both the rate of exchange (left) and the equilibrium partitioning of D (right).
Molecular Modeling: We use computational models of molecular vibrational frequencies to predict D/H ratios in hydrocarbon molecules that are not amenable to laboratory exchange experiments. Model results are calibrated with experimental data for cyclohexanone (shown at left).