HYDROGEN ISOTOPIC (D/H) COMPOSITION OF ORGANIC MATTER DURING DIAGENESIS AND THERMAL MATURATION

Changes in the D/H ratio of sedimentary organic matter (SOM) during thermal maturation have been difficult to interpret because the effects of hydrogen exchange and kinetic fractionations are confounded in natural samples. Recent analytical developments have significantly improved our understanding of the responsible mechanisms. In this paper, we review experimental and field data that document a progressive increase in the D/H ratio of most organic hydrogen at the bulk and molecular levels, and suggest that the transfer of hydrogen from water to organic matter is the most important mechanism leading to those changes. SOM and water in natural petroleum systems approach a pseudoequilibrium D/H fractionation of about −80 to −110‰. D/H ratios of organic hydrogen can preserve quantitative information about paleoclimate throughout diagenesis, and some qualitative information through catagenesis.