Page 124 - Geology and Geochemistry of Oil and Gas

96 CRUDE OILS

difference in living environment in different basins at the same geologic time
interval).
According to the proponents of the first concept, genetic types of crude oils are
rigidly associated with the facies-genetic type of organic matter. For example, a
marine, sapropelic type of organic matter always (at any geologic period and in any
basin) generated the same genetic oil type (e.g., type A). This approach does not take
into consideration the evolution of living matter. It is doubtful that the sapropelic
(marine) type of organic matter was identical in the Paleozoic, Mesozoic, and
Cenozoic sediments of different sedimentary basins, e.g., Siberia (platform) and
Caucasus (geosyncline).
The second approach is based on the concept of multiple genetic types of crude
oils and their uniqueness within each petroleum province. The contention here is that
any sedimentary basin has its own specific biomass. Besides, organic matter in the
source rocks of the same facies-genetic type (e.g., the sapropelic type) could, due to
evolution, substantially change from one geologic time interval to the next, acquiring
specific hydrocarbon composition and structure.
The identification of biomarkers that are not present in the other crude oils is of a
particular genetic interest. There are certain compounds discovered only in partic-
ular crude oils (Petrov, 1984):
(a) 1,2- and 1,3-methylalkanes, in East Siberia;
(b) the biotriocoxan, in Indonesian crude oils;
(c) the C 25 hopane, in a Siberian crude oil;
(d) the dyno-methyl-adiantanes, in North Sea crude oils;
(e) the cyclic isoprenoids with an irregular structure and trimethyl-alkyl-cyclohex-
anes with an isoprenoid chain, in the Buzachi Peninsula (North Caspian,
Kazakhstan) crude oils; and
(f ) the hopane-type six-cyclic monoaromatic hydrocarbons, in the inter-salt Pripyat
deposits (Belarus).
The genetic classification of crude oils of T. A. Botneva and N. A. Eremenko is
based on several concepts. These include (a) each oil/gas source rock in a given
petroleum province generates a specific type of oil; (b) the set of genetic types is
rather limited and changeable; and (c) the set of genetic criteria may change in
different petroleum provinces. In some cases, the crude oils belonging to different
genetic types may be different in the structure of their paraffins. In some other cases,
the difference may be in
(1) naphthene index,
(2) mono- or polycyclic aromatics,
(3) the metal and its content in the porphyrin complexes,
(4) the pristane/phytane ratio (or the other isoprenoids), and
(5) the composition of light gasoline fraction (C 5 +C 6 /C 7 +C 8 ; cyclohexane/cycl-
opentane; toluene/benzene, etc.; see Chakhmakhchev, 1982).
The genetic type of crude oil is indicative of the initial properties of the fluid, its
homologous series of hydrocarbons, and the configuration of carbon atoms; how-
ever, the crude oil composition, as determined in the laboratory, is a result of its long
evolution in the Earth’s crust. Depending on the geologic/geochemical scenario,