Page 154 - Geology and Geochemistry of Oil and Gas

124 DISPERSED ORGANIC MATTER

known oil reserves in the Gipsland and Cooper basins of Australia may have been
generated as a result of the thermal maturation of coals, even if just 1% of the
generated hydrocarbons was captured in the traps.
Baird (1986) suggested that liptinite was the main source maceral for oil, whereas
vitrinite was for gas. Inertinite was inert. Tissot and Welte (1981) believe that most
hydrocarbons of natural gases were generated as a result of thermal transformation
of kerogen during catagenesis. They also believe that the insoluble portion of the
organic matter (i.e., kerogen) has properties of a molecular sieve. This conclusion
was based on the following experiments: upon extraction, some samples were treated
with acid, and then extracted again. After second extraction with organic solvents,
bitumens with some hydrocarbons were released from the kerogen samples. Similar
studies with repeated extraction after treatment with the hydrofluoric acid have been
conducted by Akramkhodzhayev (1973), Akramkhodzhayev and Amirkhanov
(1977), and Akramkhodzhayev et al. (1978). They discovered not only saturated
but also unsaturated hydrocarbons in the second extracts. In the discussion that
followed these studies (Eremenko and Tverdov, 1980; Karimov and Kalapov, 1983)
it was found that unsaturated hydrocarbons may be retained by kerogen in sorbed
state, which is important for the problem of oil generation.
Ammosov et al. (1987), in discussing organic matter in the sedimentary rocks as a
whole, indicated that both humic and sapropelic organic matter is converted into
solid, liquid, and gaseous phases in the subsurface. Migrating liquid and gaseous
phases interact with the mineral and organic components of the surrounding rocks
and produce new compounds, called organofluidoliths. The genetic associations
(with either humic or sapropelic matter) are erased in an organofluidolith. Ammosov
et al. believe that the liquid and gaseous organofluidoliths participate significantly in
the oil generation.
It is interesting to review the distribution of oil reserves in the sedimentary rocks
as a function of R o , R a , and temperature. According to Ammosov et al. (1987),
major oil reserves are found with R a ¼ 62–93, R o ¼ 0.33–1.29, and temperature of
100 to 150 1C. As the temperature increases the process slows down. Above the
temperature of 200 1C, oil generation cannot occur (‘‘dead zone’’). Saxby (1982)
believed that the oil generation can proceed until R o ¼ 2, the condensate generation
until R o ¼ 3, and the gas generation until R o ¼ 5. Price and Barker (1985) stated
that a limitation of the oil generation interval to the R o range of 0.6–1.35 (which is
broader than the Ammosov’s) is wrong. The R o value of vitrinite, which is usually
enriched in the invaded hydrogen, is suppressed. The correct value is about 3–5
notches higher, i.e., the R o ¼ 6–8.
All aforementioned alterations in the composition of coals and coaly matter in the
sedimentary rocks are attributed by many authors to the thermal effects. All at-
tempts to find a correlation with the dynamic stress were either unsuccessful (Am-
mosov et al., 1987) or not clear (Ignatchenko, 1968; Stefanova and Shimorina, 1981).
It is important to mention here Strakhov’s (1960) opinion that not all natural proc-
esses are a result of changes in the thermodynamic conditions. Some are due to
internal energy of the system, which is released as a result of equilibration of the
system.