Atomic H/C Ratio of Kerogen as an Estimate of Thermal Maturity and Organic Matter Conversion

Abstract

In recent years, the atomic H/C (hydrogen to carbon) ratio of kerogen as a way to assess the quality of organic matter in source rocks has been overlooked in favor of the more easily determined Rock-Eval hydrogen index. Rock-Eval pyrolysis provides fast, inexpensive, quantitative (mg HC/g rock) data without requiring kerogen isolation from the rock. Because of the general scatter in the data, many source rock interpreters consider Rock-Eval pyrolysis to be a screening analysis. In this paper I describe the benefits of using H/C ratios in source rock evaluations and present new correlations between atomic H/C ratios and thermal maturity, organic matter conversion, and expulsion volumetrics. Atomic H/C ratios of pyrolyzed kerogens have been correlated to the extent of thermal conversion of organic matter for both type I and type II kerogens. The excellent agreement between stoichiometric calculated hydrogen and carbon loss to observed losses from hydrous pyrolysis maturation experiments suggests that kerogen H/C ratios are excellent indicators of thermal maturity for end-member kerogen types. These data also offer a method to estimate percent organic matter conversion, provided that both the initial and present H/C ratios of the kerogen are known. Present H/C ratios can be measured, and initial H/C ratios can be reasonably estimated, from microscopic organic analysis of kerogen. For oil-prone source rocks, typical immature type I kerogens have H/C ratios of 1.35-1.50, whereas type II kerogens have H/C ratios of 1.20-1.35. Correlations of the amount of expelled oil in hydrous pyrolysis experiments to atomic H/C ratio of the spent kerogen offer exploration geologists a quick estimate of oil expulsion volumes. Based on hydrous pyrolysis experiments, measured H/C ratios, and calculated original TOC (total organic carbon) values, first-order volumetric approximations were made on three basins containing mature source rocks. Results compared favorably with published approximate-oil-in-place estimates for the Williston basin (Bakken shale), Los Angeles basin (Nodular shale), and the Illinois basin (New Albany Shale).