Page 156 - Geology and Geochemistry of Oil and Gas

126 DISPERSED ORGANIC MATTER

together with other hydrocarbons, from the original organic matter. Some geo-
chemists cast doubt on such a hypothesis over a high reactivity and instability of
unsaturated hydrocarbons. The high reactivity and instability of unsaturated hy-
drocarbons in natural environments are relative, however. There are published data
on the presence of unsaturated hydrocarbons in crude oils of different geologic age
and at different depths. For instance, small amounts of hexylenes, octylenes, and
nonilenes have been identified in a Canadian crude oil. Unsaturated hydrocarbons
are present in the crude oils from the North Caucasus, Emba Area (Russia), and
Sakhalin Island (Nametkin, 1955). It may be further suggested that the preservation
conditions for the rock-sorbed unsaturated hydrocarbons may be even better than
those in the accumulations. This agrees with the idea of Burst (1969) that bonded
water has the preservation effect on hydrocarbons. This is supported by the presence
of unsaturated hydrocarbons in gases released during a vacuum degassing of rocks
not only in shallow but also in deep wells.
Various organic solvents are used to extract the bitumen. They include chloro-
form, benzene, acetone, and mixtures thereof. The most commonly used solvent is
chloroform. It usually extracts the maximum amount of bitumen from the ground
rocks. These bitumens are usually more reduced as compared with the bitumens
extracted with the other solvents.
Elementary (C, H, S, N, O) and group (lubricants, resins, asphaltenes) compo-
sition is studied in the extracted bitumens. All available techniques are applied for
their studies such as liquid and gas chromatography, mass spectrometry, X-ray, UV
and IR microscopy, and electromagnetic resonance. Bitumens have been studied to
the same depths as those for the crude oils.
With all the other conditions being same, the content of bitumens in the rocks
increases with increasing organic matter content. Simultaneously, their relative content
in the organic matter of rocks declines. This is especially apparent when the organic
matter content in rocks is low. Also, there is an increase in the reduced bitumens.
There is a correlation between the bitumen content and the lithology (Table 7.3,
based on calculations by Vassoyevich (1984) conducted in 1973). Since then, despite
a tremendous increase in the accumulated data, no appreciable changes in principle
have occurred.
As mentioned before, the bitumens are composed of oils, resins, and asphaltenes.
Most oils contain all compounds found in crude oils, the first being hydrocarbons.
The elementary composition of bitumens is also close to that of crude oils, except for
a higher content of heteroelements (O, N, S) in bitumens.
Table 7.3 also contains data for rocks with the concentrated forms of organic
matter. The content of both bitumens and hydrocarbons is rather high there. This is
just another indication that such rocks may have contributed to the oil generation
and, even more so, to gas generation. The absence of large commercial oil and gas
reserves in such rocks is explained in the subsequent sections.
Organic matter in the carbonates is more bituminous, and the degree of their
reduction increases with the increasing carbonate content (Vassoyevich, 1971).
Some geoscientists recorded the oxidation of bitumens in argillaceous deposits in
the proximity of reservoir rocks (whether up or down the section). Neruchev and