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H/C Ratio of Kerogen as an Estimate of Thermal Maturity and Organic Matter
D. K. Baskin2
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
1997. The American Association of Petroleum Geologists. All rights reserved.
received July 13, 1995; revised manuscript received September 17, 1996;
final acceptance March 20, 1997.
Petroleum Technology Company, 1300 Beach Blvd., La Habra, California 90631.
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.
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