Page 125 - Geology and Geochemistry of Oil and Gas

CLASSIFICATION OF CRUDE OILS 97
crude oil in the accumulation may be affected by various secondary factors, such as
catagenesis, hypergenesis, and migration. These factors may change the crude oil
composition. Determination of the trend and the scale of these changes and their
controlling factors may be very important for the development of hydrocarbon
accumulations and for the forecast of their chemical composition. The effect of
secondary (i.e., subsequent to the formation of accumulations) factors on the crude
oil composition is the basis of geochemical oil classification.
It is difficult to select the criteria, which would help to determine the role of
processes controlling the changes in crude oil composition (i.e., role of catagenesis,
hypergenesis, and migration). The reason for this is that some parameters changed
similarly in response to different processes. For instance, an increase in the density of
crude oil or content of resins may occur as a result of hypergenesis or of migration
resulting in the selective adsorption. Decrease in density, decrease in the resin con-
tent, and increase in the paraffins content may be caused by catagenesis as well as by
the ‘‘filtration’’ during migration.
The geochemical classification of crude oils includes the following types:
Type 1. Catagenetically altered crude oils.
Type 2. Hypergenetically altered crude oils.
Type 3. Crude oils which underwent various alterations during migration.

The following are typical for type 1: (a) lowering of the crude oil density; (b)
lowering of the content of resins plus asphalthenes; (c) simplification of complex
hydrocarbon structures; (d) lowered role of high-molecular hybrid molecules; (e)
lowering, sometimes to zero, of metal–porphyrin complexes; and (f ) increase in the
content of gasoline fraction.
Type 1 crude oils are mostly encountered in geosynclines and, only rarely, in
platforms. The scope of the catagenetic effect on crude oil composition is determined
by the following two factors.
The first one is related to the temperature and pressure. Catagenetic alterations of
the crude oil composition are caused not just by a simple increase in the formation
temperature, when the temperature exceeds the oil generation temperature in the
source rock. It is believed that the maximum generation of oil occurred within the
temperature interval of 60–1201C. Oil accumulations formed at a lower temperature
after the hydrocarbons have migrated from the structural lows (high-temperature
generation foci) to the uplifted areas (traps). In the process of the further geologic
evolution an accumulation may enter into zones of higher-than-generation temper-
ature. If the oil stays there long enough it may be subjected to the aforementioned
catagenetic alterations, up to the transformation to gas-condensate mixtures. This,
however, does not occur frequently.
The second factor is associated with the structural characteristics of the hydro-
carbon molecules. The composition of oil from 140 accumulations has been studied
by T.A. Botneva. The accumulations were selected from different petroleum prov-
inces of Russia, Azerbaijan, and Belorussia. These accumulations were stratigraph-
ically diverse (Pliocene, Eocene, Jurassic, and Devonian) and over 4 km in depth. No