Page 162 - Geology and Geochemistry of Oil and Gas

STUDIES OF SOLUBLE AND INSOLUBLE PORTIONS OF ORGANIC MATTER 131
stated that S 1 and, hence, the bitumen represent just a remnant preserved in rocks
after the hydrocarbon expulsion. This remnant may be continuously replenished to a
certain extent due to the continuity of processes. The degree of replenishment de-
pends on many factors including the composition of the original organic matter. The
type and the speed of reactions depend on the extent of coalification of the original
organic matter. As Neruchev et al. (1986) indicated, most stable components must
gradually accumulate in the organic matter.
Thus, the genetic potential (S 1 +S 2 ) and the transformation coefficient are also in
doubt. Taking into account what was mentioned about the S 1 , the genetic potential
is incomplete, because a substantial (maybe even major) fraction of the chemically
active organic matter is not considered.
At the same time, S 2 includes some components that cannot be converted into
hydrocarbons in the natural environments (and, probably to a large extent, in the
laboratory) and, therefore, cannot affect the hydrocarbon reserves. Hence it is quite
risky to associate this ‘‘potential’’ with actual hydrocarbon resource potential. The
same applies to the transformation index [S 1 /(S 1 +S 2 )], which probably cannot in-
dicate the degree of transformation of organic matter. Rather, it is just an indication
on the incompleteness of the processes of organic matter transformation.
It is important to determine whether the particular bitumen is autochtonous
or allochtonous. Comparisons have been made of the hydrocarbons in the crude oils
(in the reservoirs) and in the rocks. The former are enriched in the saturated hy-
drocarbons, moderately enriched in aromatics, and strongly impoverished in polar
(N, S, O) components.
Using thin sections, Larskaya (1983, p. 200) made an attempt to combine pet-
rographic and chemical parameters for a more reliable identification and description
of source rocks. Carbon isotopes were also used in studying bitumens and kerogen.
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The bitumens are enriched in d C by 2–3% compared with the kerogen. The crude
oil has the same carbon isotope composition or is slightly lighter (by 1–2%) than the
genetically associated bitumen.
Akramkhodzhayev et al. (1978) introduced a technique for determining whether
bitumen is autochtonous by comparing its carbon composition with that of the
bitumen extracted by heating kerogen.
Attempts have been made to evaluate the amount of hydrocarbon generation
from the changes in [S 1 =ðS 1 þ S 2 Þ] ratio with depth. The formation of accumulations
must correspond to the abnormally high values of this ratio against the background
of an average curve. Such evaluations, however, did not gain any substantial rec-
ognition due to their inaccuracy.
Uspenskiy (1970) proposed to determine the amount of generated hydrocarbons
(oil source potential) from the elementary composition of kerogen. It is possible to
determine the composition of the initial and final kerogen using the kerogen from
the same formation obtained from different depths (corresponding to various
catagenetic stages). It is also possible to determine the losses of elements during the
formation of hydrocarbons and their removal due to heating (in the form of com-
pounds such as CO, CO 2 , H 2 O, CH 4 , and H 2 S). Thus, according to Uspenskiy, one
can calculate the amount of released hydrocarbons.