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

©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.

¹Manuscript received July 13, 1995; revised manuscript received September 17, 1996; final acceptance March 20, 1997.

²Chevron Petroleum Technology Company, 1300 Beach Blvd., La Habra, California 90631.

I gratefully acknowledge the management of Chevron Petroleum Technology Company for permission to publish. Special thanks are extended to B. Katz, L. Snowdon, R. W. Jones, D. Wavrek, K. Peters, and M. McCaffrey, who suggested numerous ways to improve this paper.