Page 168 - Geology and Geochemistry of Oil and Gas

INITIAL ORGANIC MATTER AND ITS TRANSFORMATION 137
On the basis of laboratory studies, Zhuze (1957, in: Eremenko and Chilingar, 1996,
p. 154) developed a concept of clay-mineral transformation from montmorillonite to
kaolinite during rock compaction. He questioned the limitations imposed on the role
of clay minerals, as just catalysts in the transformations of organic matter. This
concept, in turn, has been questioned by Klubova (1973), who believed that such a
process could not proceed in nature due to insufficient supply of potassium in rocks.
A number of authors recorded the clay-mineral alterations during catagenesis. As an
example, Nazarevich et al. (1985) described montmorillonite alterations during meso-
catagenetic (MC) stage and cessation of this process upon reaching the MC 4 substage.
Brilling (1984, in Eremenko and Chilingar, 1996, p. 154) noticed that in the process of
alterations, the particles (sheets) change their orientation, which results in locking-up or
opening of pores; hence, the filtration in clays acquires a random, unstable nature.
The following questions arise in studying the oil generation in source rocks
(mostly shales):
1. What is the nature of source rocks and where and how are they distributed in
sedimentary basins?
2. What is the physical state of hydrocarbons in source rocks?
3. What forces (and at what stage) cause them to move to the reservoir rocks
(primary migration/expulsion)?
4. When and how do hydrocarbons move within the reservoir rocks to form oil and
gas accumulations?
5. What is the relation between (a) the oil and gas composition in the accumulations
and (b) the environment of hydrocarbon generation and of formation of accu-
mulations?
Vassoyevich (1976) introduced the term ‘‘microoil’’: The origin of oil begins with
the living matter where the biochemical compounds are born that initiate formation
of petroleum hydrocarbons or, to a smaller extent, where these hydrocarbons are
born. Upon deposition at the bottom of a basin, and partially forming in the se-
diments due to the activity of organisms, these hydrocarbons and prehydrocarbons
form a young microoil.
Although Brod (1957) did not accept the term microoil, his views were similar to
those of Vassoyevich. According to Brod, while building their bodies, cellular mem-
branes and other cellular structural elements, plant and animal cells and, especially,
some bacteria synthesize hydrocarbons. After death of the organisms and inclusion
of their remains in the depositional cycle, the hydrocarbons contained in them may
be decomposed by the microbial activity. The relative rate of hydrocarbon decom-
position is lower than that of the other organic compounds. Thus, under favorable
conditions some hydrocarbons may accumulate.
Quoting Uspenskiy (1970), Vassoyevich stated that the components of oil were
not born all at once. In his view, it would be better to discuss not the source but the
hydrocarbon generation stages that would correspond to the stages of lithogenesis.
Brod (1957) also mentioned that hydrocarbon compounds dispersed in rocks do not
constitute crude oil. Various organic compounds formed from the moment of initial
deposition, whereas mixtures of liquid hydrocarbons (i.e., crude oils) apparently
formed during the formation of accumulations in reservoirs.