Factors Controlling the Deuterium Contents of Sedimentary Hydrocarbons

Abstract

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 sedimentary organic matter to liquid hydrocarbons. Variations in the terrestrial hydrologic cycle, and in biologic fractionations, create lipids with δD 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 δD 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 environment, diagenetic, and catagenetic processes if we can learn to quantitatively disentangle the competing fractionations.

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