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.