Page 212 - Geology and Geochemistry of Oil and Gas

CLASSIFICATION OF HYDROCARBON ACCUMULATIONS 181
TABLE 10.3

Accumulation groups based on phase relations
Mild environment (a) Severe environment (b)
I. Oil accumulations with no dissolved gas; I. Oil accumulations with no dissolved gas
oil–water contact is distinct. (possibly nonexistent in this environment).
II. Oil accumulations with dissolved gas II. Oil accumulations with dissolved gas. Oil–water
(p res 4p satur ). Oil–water contact is distinct. contact is not distinct.
III. Oil accumulations saturated with gas III. Oil accumulations saturated with gas
(p res ¼ p satur ). Oil–water contact is distinct. (p res ¼ p satur ). Gas–oil transition zone appears in
the upper portion. Contact with water is not
distinct.
IV. Oil accumulations with gas cap (p res ¼ p satur ). IV. Oil accumulations; transition into gas–oil
Gas contains some condensate. Oil–water contact mixture in the upper portion. Oil–water transition
is distinct. zone instead of oil–water contact.
V. Gas accumulation with condensate underlain by V. Gas accumulation with condensate. There is a
oil. Oil–water contact is distinct. transition into oil–gas zone down the section.
Oil–water transition zone instead of oil–water
contact. Gas–oil–water zone at the base.
VI. Gas accumulation rich in condensate. VI. Gas accumulation with condensate and with
Gas–water contact is distinct. rim of oil–gas mixture.
VII. Gas accumulation. VII. Gas accumulation. Gas–water mixture
constitutes transition to water.

according to the chemical composition of hydrocarbons and quantity of H 2 S and,
sometimes, mercury (e.g., in Germany). Noteworthy is the high H 2 S content in deep
accumulations. For example, the H 2 S content in gas in the Astrakhan Field of
Russia (at a depth of 3000–4000 m) reaches 24%.
Oil and oil-gas accumulations may be subdivided according to the nature of
admixtures in the oil, water, and gas. The most important admixture is sulfur and its
compounds.
An important parameter in producing oil is its viscosity. One can use the fol-
lowing classification: low-viscosity oil — viscosity o5 mPa s, medium-viscosity oil —
5–10 mPa s, high-viscosity oil — 10–30 mPa s, and very high-viscosity oil — greater
than 30 mPa s.
When discussing the formation of hydrocarbon accumulations, many authors
gave preference to gravity forces. It should be remembered, however, that any oc-
currence of gravity in the Earth’s crust is unavoidably accompanied by the elastic
phenomena. Inasmuch as a rule, all these forces are active in the reservoir, most
common drives are of a mixed nature. But the most important drives are the water
drive and the elastic drives of free and solution gas.
Various geochemical and bacteriological processes are most active near the
oil–water and gas–water contacts. Sulfate reduction and underground hydrocarbon
oxidation is possible there. Although the most obvious hydrocarbon oxidation in oil
accumulations occurs near the oil–water contacts, it is possible that the oil biode-
grading occurs within the entire accumulation volume (Petrov, 1984).