Page 147 - Geology and Geochemistry of Oil and Gas

118 DISPERSED ORGANIC MATTER

Insoluble Portion of Organic Matter

Some authors (Tissot and Welte, 1981) name the portion that is insoluble in either
organic solvents or alkalis, the kerogen (the definition excludes bitumen). This in-
soluble portion is similar to coal. Chemically and petrologically, it includes the same
components as coal. The microcomponents of coals are presented in Table 7.1.
Major components of insoluble portion of organic matter are carbon, hydrogen,
and oxygen. Nitrogen and sulfur are present in a lesser amount. Tissot and Welte
(1981), using Van Krevelen’s (1961) graphs, identified three kerogen types on the
basis of their elementary composition; they attribute a genetic meaning to their
classification (Fig. 7.1).
Type I kerogen has a high initial H/C atomic ratioX1.5; the O/C ratio is o1. The
kerogen is composed mostly of a lipid matter. Oxygen is present mostly in the ester
bonds. When pyrolyzed, the volatile and extractable compounds including oil are
produced. This kerogen type forms from the lipid-enriched organic matter.
Type II kerogen has a relatively high initial H/C atomic ratio (1–1.5) and a low
O/C ratio. Polyaromatic molecule cores, heteroatomic groups of ketones, and car-
boxylic acids are of great importance. The bitumen associated with this kerogen
includes many cyclic structures, and its sulfur content is higher than in the other
component parts.
Type III kerogen has a relatively low H/C atomic ratio (o1.0) and a high O/C
ratio (0.2–0.3). It comprises a substantial extent of polyaromatic cores, heteroatomic
and carboxylic groups, but does not contain ester groups. Apparently, the land
plants constitute its main source.
All these kerogen types played different roles in the formation of oil, as discussed
later.
As shown in Table 7.1, many microcomponents can be utilized to determine
paleotemperatures. Main alteration in organic matter buried with the sediments is in
the creation and, sometimes, preservation of substances that are stable during dia-
genesis and catagenesis. It is assumed that changes occur mostly due to the effect of
heat. These processes are slow (millions of years), which is the case for most geologic
processes (Ammosov et al., 1987). Although the duration of natural processes is
long, the speed of specific reactions may vary significantly. Role of the time factor in
the natural processes needs to be understood better. For instance, as Lopatin (1971)
o
pointed out, a temperature increase of 10 C causes the speed of kinetic reaction to
double.
This simplified model has been utilized to reconstruct the geochemical evolution of
many petroliferous basins. A number of writers, however, disagree with this concept.
For example, Allen and Allen (1993, p. 284) believe that the above rule is app-
licable only in the temperature range of 50–60 1C, above which the speed of kinetic
reaction substantially changes (for instance, it is just 1.4 at 200 1C). Tissot et al.
(1987) have limited the applicability of the Lopatin’s technique by the activation
energy of no higher than 10–20 kcal/mol. Price (1985, pp. 233–240), on the other
hand, believes that alterations of heterogeneous compounds in nature follow more
complex laws.