D/H RELATIONSHIP OF KEROGENS, OILS AND HYDROCARBON BIOMARKERS RELEASED DURING 5-YEAR HYDROUS HEATING EXPERIMENTS

Hydrogen from water is known to be involved in oil generation, although isotopic effects and the amount of exchange between carbon-bound H and water H are poorly constrained. To properly interpret D/H in oils and oil components, it is critical to understand how much of the original organic hydrogen is transferred to, and is thus isotopically characterizing, bulk oil and its chemical components. Previous isotopic studies of kerogens and oils from field samples have shown that alkanes can retain distinct H-isotope ratios over millions of years, suggesting low exchange rates for some organic H, whereas H in aromatic, branched, and cyclic compounds can exhibit significantly faster exchange with water H.

We present data from hydrous pyrolysis and aqueous heating experiments designed to simulate aspects of natural oil generation. An earlier set of hydrous pyrolysis experiments at >310°C for up to 144 hours in isotopically distinct waters had yielded artificial oils from a variety of source rocks (containing kerogens of type I, II, IIS, and III; Schimmelmann et al., 1999, Geochim. Cosmochim. Acta 63: 3751-3766). We extend the previous study with experiments at lower temperatures and a 5-year reaction time: the same source rocks, three oils (two crude oils from the Monterey Fm., and NBS-22 oil) were subjected to heating in contact with isotopically different waters (initial dD values ~ -110‰ to +1260‰ vs. VSMOW) in sealed ampoules at four temperatures ranging from 100° to 240° for 5 years (1998-2003). We present compound-specific D/H data that constrain the extent of H exchange and allow estimates of kinetic fractionation factors. An improved understanding of diagenetic effects on organic H will help to better utilize the H isotopic paleoenvironmental information preserved in organic matter.