Journal of Petroleum Geology, Vol.15, No.4, pp. 419-434, 1992

©Copyright 2000 Scientific Press, Ltd.

THE CAUSE AND MECHANISM OF VITRINITE REFLECTANCE ANOMALIES

Hao Fang* and Chen Jianyu*

* Department of Petroleum Geology, China University of Geosciences, Wuhan 430074, Hubei, The People's Republic of China.

Abstract

The unexpected phenomena that measured vitrinite reflectance (Ro) values are lower (suppression) or higher (enhancement) than expected values at a given regional rank are defined as vitrinite reflectance anomalies. Such phenomena have been found in many basins in China, both in coal seams and in oil source rocks, and representative examples are presented in this paper. The cause and mechanism of vitrinite reflectance anomalies have been investigated by comparing the hydrogen contents and chemical structure of vitrinite macerals from immature Type I, Type II and Type III kerogens, and by studying the effects of early diagenetic oxidation on the thermostability of humic organic matter (OM). The results demonstrate: (1) depending on the anoxic or oxic nature of the depositional conditions, that lignocellulosic tissues can form a series of vitrinite macerals with different initial hydrogen contents and slightly different initial reflectivities; (2) that vitrinite macerals with different initial hydrogen contents have different reaction kinetics, with hydrogen-poor, oxygen-rich vitrinite maturing at an enhanced rate compared to vitrinite with a higher hydrogen content and a lower oxygen content, (3) that variations in the hydrogen content and the thermostability of vitrinite macerals are the main causes of Ro suppression and enhancement.