Page 163 - Geology and Geochemistry of Oil and Gas

132 DISPERSED ORGANIC MATTER

The concept proposed by Uspenskiy, however, is questionable. He equated the
amount of volatile coalification products (that form when the dispersed organic
matter matures from one catagenetic stage to the next) with the difference in the
amount of volatiles per combustible mass in the technical analysis of coal. It was
found, however, that the adjacent points in the same rock sample may have a sub-
stantially different elemental composition. Consequently, the elemental analysis re-
sults for two samples collected at different depths within the same formation may
lead to totally different conclusions. This is due to non-uniform composition of the
accumulated organic matter (in terms of the number, composition and even the size
of particles) and the non-uniformity of the effect of geochemical factors that result in
different degrees of transformation of organic matter at different locations within the
rock. The organic matter itself also affects the geochemical environment. Thus,
determination of the true degree of transformation and the amount of released
hydrocarbons is a complicated problem. This pertains to all types of point (local)
determinations in rocks.
The above-presented concepts and parameters are commonly used for the iden-
tification of source rocks, determination of the degree of their maturity, and esti-
mating the amount of hydrocarbons formed during lithification. The ideas of
Potonie (1920) and Uspenskiy (1970) are still commonly accepted, and the sapropelic
organic matter is a preferred source for oil generation.
In their experiments, Rohrback et al. (1984) used deltaic (humic) and algal-
lagoonal (sapropelic) oozes and heated samples from 35 1C to 550 1C for 1 h to 625
days. The time and the composition of the input material affected the products.
Three stages have been identified:
1. Immature: C 1 through C 5+ hydrocarbons are released; C 15+ hydrocarbons are
absent.
2. Mature: The bulk of the C 15+ is released. The released CH 4 is isotopically lighter
than in the input material.
3. Overmature: The amount of released C 2 –C 5 hydrocarbons increases again. The
CH 4 formed is isotopically heavier than in the input material.
Simultaneously, the R o changed from 0.2 to 3%. Liquid hydrocarbons begin to
form at R o ¼ 0:25%, maximum is reached at R o ¼ 0:5% in the humic organic matter
and at 0.7%, in the sapropelic organic matter. Liquid hydrocarbons are formed
faster in the humic ooze. Tissot and Welte (1981, p. 421) also recorded a faster and
more complete formation of liquid hydrocarbons from the humic matter.
It is important to note sudden change in the carbon isotopic composition of
methane at Stage 2. A similarity may be observed here with the bacterial sulfate
reduction experiments by V. Mekhtiyeva (personal communication, 1994). In these
experiments, first the isotopically heavy H 2 S was formed, which became heavier
upon further experimentation. This phenomenon, however, was only observed in a
closed environment. When the new batches of the input material were supplied, the
increase in isotope composition of sulfur in H 2 S stopped.
Of course this comparison is tentative, because in one case the reactions were
chemical, whereas in the other, they were biochemical. Still, the concept arises that
the carbon in methane in natural environments does not have to become isotopically