Page 121 - Geology and Geochemistry of Oil and Gas

CLASSIFICATION OF CRUDE OILS 93

Fig. 5.2. Chemical composition of some crude oils plotted on a triangular diagram (after Nelson, 1949,
Fig. 7, p. 87).

Type A near-equal contents of paraffin-naphthene and naphthene-aromatic
VIII. hydrocarbons (PN/NA ¼ 1).
Type IX. Dominance of naphthene-aromatic hydrocarbons (PN/NA o1).

Oils belonging to types VII–IX are devoid of gasoline fraction.
The distribution of the above-described crude oil types by density and sulfur
content is presented in Tables 5.2 and 5.3. The most commonly occurring in nature
are type I crude oils, whereas the least common are types IV and IX (Table 5.4).
A number of classifications based on the identification of chemical types first on
the contents of normal alkanes and isoprenoids, and then on the cycloalkanes and
arenes (Petrov, 1984). Petrov subdivided all crude oils into two categories (A and B),
with two subtypes within each of them (Table 5.5).
Recent developments in the petroleum geochemistry resulted in the introduction of
genetic and geochemical classifications. Both classifications provide useful information
for the petroleum potential forecast. Geochemistry plays an important role in fore-
casting the quality of crude oil in the accumulation. The information important during
the field development includes the possible communication between the productive
formations, oil cross-flows, possible crude alterations at the oil/water contact, etc.