Factors controlling the deuterium contents of sedimentary hydrocarbons

The hydrogen isotopic compositions of sedimentary hydrocarbon molecules are now being used to address a range of scientific questions, from paleoclimate to environmental reconstruction to understanding of petroleum systems. Here I review the environmental, biological, and physical/chemical factors that influence the H isotopic compositions of sedimentary hydrocarbons. A hierarchy of four main controls can be recognized: (i) the composition of environmental water that serves as ultimate hydrogen source to the biosphere, (ii) physiologic and metabolic processes in organisms that fix water hydrogen into organic molecules, (iii) hydrogen exchange processes that alter D/H ratios slowly over time under geologic conditions, and (iv) kinetic fractionations that arise during the thermal conversion of sedimen-tary organic matter to liquid hydrocarbons. Variations in the terrestrial hydrologic cycle, and in biologic fractionations, create lipids with dD values spanning a huge range, and carrying abundant information about source organisms and environments. This information is gradually lost over geologic timescales due to hydrogen exchange, although the rates of this process appear to vary by orders of magnitude in clastic versus carbonate sediments. The H isotopes of hydrocarbons in many sediments of low to moderate thermal maturity may only be partially exchanged. Finally, additional kinetic fractionations are imposed during thermal cracking to generate mobile hydrocarbons. The dD values of sedimentary hydrocarbons are thus concluded to generally reflect a complicated combination of processes. Although it is tempting to view this as a failure of the isotopic record, there are numerous opportunities for understanding environmental, diagenetic, and catagenetic processes if we can learn to quantitatively disentangle the competing fractionations. Quantitative analysis of the relative abundances of stable hydrogen isotopes, protium (1 H) and deuterium (2 H, or D), dates back more than 80 years to the earliest work of Urey and his students (Urey and Teal, 1935). However, most early studies of hydrogen isotopes were focused on environmental water, and the modern era of H-isotopic studies of organic matter arguably did not begin until 1967, when Zborowski measured the D content of fatty acids from sediments of the Mohole coring project (Zborowski et al., 1967). Shortly thereafter, Schiegl and Vogel (1970) and Smith and Epstein (1970) reported their respective surveys of the H-isotope ratios of extant plants, noting that they are universally depleted in D relative to the environmental water around them. The former authors also noted that coal and oil had similar D/H ratios as wood and marine algae, respectively, …