Page 127 - Geology and Geochemistry of Oil and Gas

CLASSIFICATION OF CRUDE OILS 99
TABLE 5.6

Distribution of crude oils with different degree of oxidation at various depth intervals
Depth Interval (m) Crude oil
No oxidation Slightly oxidized Strongly oxidized

300–999 12.3 (17.0) 32.2 (38.1) 86.6 (44.9)
1,000–1,999 33.7 (62.5) 21.5 (34.3) 4.6 (3.2)
2,000–2,999 29.8 (52.4) 28.7 (43.5) 5.9 (4.1)
3,000–3,999 12.9 (52.2) 13.4 (43.1) 2.9 (4.7)
4,000–4,999 9.6 (70.8) 4.6 (29.2) Not found
5,000–5,999 1.7 (75) 0.1 (25) Not found
Note: In parentheses: percent of crude oil samples having different degree of oxidation at a certain depth
interval.

of cyclization in naphthenes decreases and the aliphaticity coefficient (C p /C n ) dras-
tically increases. Similar alterations in crude oils occur during the migration through
carbonate rocks (e.g., microgranular limestones and dolomites). However, an in-
crease in the density and in the contents of asphaltenes and resins, and naphthene-
aromatic hydrocarbons was recorded for carbonates. These alterations may have
been caused by the oxidation of naphthene hydrocarbons and by the appearance of
ester groups instead of the ethyl ones in the paraffin chains. The great diversity in
physical properties of rocks through which the crude oil migrates affects the nature
and scale of the migration-associated alterations. The effect on the composition of
crude oil as a result of migration decreases in the following order: clays-silts and
sands-limestones-dolomites-anhydrite. It may be very difficult to determine the
geochemical type of migration-altered oil, because many parameters may exhibit
similar changes from other processes. The resins/asphaltenes ratios (and other sim-
ilar ones) were proposed as a criterion for the identification of migrated crude oils
(this particular ratio drastically increases, up to 40%, upon migration). Mileshina
et al. (1983) believe that one of the best criterion of the migrated crude oils is an
increase in the degree of aliphaticity of paraffinic and aromatic hydrocarbons (and
even in tars) by a factor of 1.5.
Determination of a geochemical type for the migration-altered crude oils is im-
portant in evaluating the type and direction of migration. For instance, in the dif-
ferential entrapment (accumulation of oil and gas in interconnected traps), assigning
the crude oil to the migration-altered type (rather than oxidized) may enable one to
determine the direction of regional migration.