Page 151 - Geology and Geochemistry of Oil and Gas

ORGANIC MATTER INSOLUBLE IN ORGANIC SOLVENTS: KEROGEN 121

Fig. 7.2. Conceptual curve showing temperature and geologic age as coalification factors (modified after
Lopatin, 1983).

determined based on vitrinite reflectance as measured in oil (R o ) or in air (R a ). This,
however, poses some problems. One such problem is the selection of a reliable
sample and a suitable area of that sample for the determination of maximum re-
flectance. A number of R o or R a values can be determined for the same sample and a
maximum value must be selected. But there is no guarantee that the selected value is
indeed maximal: there may be some areas in the samples where these values are
greater. According to Ammosov et al. (1987, p. 23), the reason for that is the
non-uniformity and incompleteness of alterations of the solid substance and the
preservation of such a state indefinitely under given thermobaric conditions. Non-
uniformity of organic matter accumulation and distribution in any sample cannot be
argued. At the same time, studies by Yu. V. Stepanov (in: Ammosov et al., 1987,
p. 82) of alterations in a series coal-sandstone–siltstone–argillite indicate a certain
direction for increasing R o . Price (1982), Price and Barker (1985), and Price et al.
(1986) recorded changes in the hydrogen content of kerogen. Hydrogen enters the
vitrinite and alters its reflectance. The coal samples from the same coal bed, con-
taining more hydrogen, display lower R o .
Ammosov et al. (1987, p. 26) believe that some errors may have been caused in the
determination of vitrinite reflectance not in the true vitrinite but in the vitrinite
homologs, such as saprovitrinite, semivitrinite and, sometimes, pseudovitrinite,
which have been confused with true vitrinite. True vitrinite reflectance is lower than
that of semivitrinite and pseudovitrinite and higher than that of saprovitrinite. Ac-
cording to Ammosov, this might have caused erroneous conclusions by Price and
others.
Despite some disagreements, most authors are of the opinion that vitrinite may be
used as a maximum paleothermometer and for determining the degree of catagenesis
in the surrounding rocks. Table 7.2 is a comparison chart for regional temperatures,
stages of catagenesis, and vitrinite reflectance.
Lithogenesis includes diagenesis, epigenesis, and metamorphism. Vassoyevich
(1975) changed the term epigenesis to catagenesis and subdivided it into different
stages.