Page 155 - Geology and Geochemistry of Oil and Gas

ORGANIC MATTER SOLUBLE IN ORGANIC SOLVENTS 125
7.2. ORGANIC MATTER SOLUBLE IN ORGANIC SOLVENTS

Organic matter (OM) in a deposit includes a substantial amount of organic com-
pounds, which are easily dissolved in the organic solvents and are easily hydrolyzed.
Tissot and Welte (1981, p. 84) showed that some free and hydrolyzed organic com-
pounds contain fatty acids, hydrocarbons, sugars, and amino acids. Their total con-
tent is low and amounts to 15–40% of the organic matter even in the near-surface
layer of sediments. At a depth of 0–15 m, they constitute 6% in the Saanich Bay,
British Columbia, 25% in the Bering Sea, and 40% in the central part of the Black
Sea. In all cases, their content rapidly decreases with depth due to the bacterial
activity and becomes less than 1% at a depth of 35 m. Microbially processed organic
matter is transformed to an insoluble state through the condensation and polym-
erization during diagenesis to a depth of a few tens or a few hundreds of meters. Lipid
compounds (including hydrocarbons) are quite stable. They transfer from the living
matter into the sediments and, subsequently (after diagenesis), into rocks without or
with only slight changes. These compounds have a relatively high molecular weight.
Tissot and Welte (1981, p. 124) proposed to call ‘‘chemofossils’’ the molecules that
have been synthesized by the living plants or animals and enter into the sediments
with only slight changes and with the preserved molecular structure of hydrocarbons
or other lipids. Most important chemofossils are alkanes, fatty acids, terpenes, ster-
oids, and porphyrins. Quantitative hydrocarbon determinations in the present-day
sediments have been performed by Gorskaya (1950) (Black Sea) and Hunt (1981).
It was established in the laboratory that if subjected to the effects of temperature
or physicochemical reactions, kerogen is capable of releasing hydrocarbons. Usually,
these hydrocarbons have lower molecular weight and a simpler structure than
chemofossils. Thus, compounds extracted from rocks by organic solvents may have a
double origin: inherited from the original organic matter (chemofossils) and newly
formed compounds.
The soluble part of the dispersed organic matter is called bitumen. Petroleum
geochemists studied the bitumen in detail because of the established similarity of its
light portion with oil. The kerogen was studied in detail by the coal geochemists.
The kerogen-sorbed saturated and unsaturated hydrocarbons have been discov-
ered by Akramkhodzhayev (1973) while studying bitumens. They are of special
interest. The unsaturated hydrocarbon release-curve from a heated and solvent-
treated kerogen is bimodal. The first maximum is observed at 100 1C, whereas the
second one at 300 1C or above. The first maximum cannot be explained by the
thermal disintegration of complex hydrocarbon, ester, and other side chains of
kerogen (Nametkin, 1955). It contradicts a physicochemical law of an increase in the
intensity and speed of chemical reactions with increasing temperature. Of interest
is the total decrease in the released hydrocarbons in the temperature range of
100 –150 1C. This also cannot be explained by the hydrocarbon formation as a result
of the organic matter decomposition. The most plausible interpretation is desorption
of hydrocarbons from organic matter.
Thus, it may be suggested that the unsaturated hydrocarbons released from
kerogen at 100 1C (and even 300 1C) are products of biosynthesis and are inherited,