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CLASSIFICATION OF HYDROCARBON ACCUMULATIONS 177
accumulations (rims). The gas under these conditions, even if not underlain by oil,
extracts enough hydrocarbons from the reservoir rocks (primary condensate) to be
considered as a gas-condensate. A transition zone between the gas-condensate and
the water is a gas–water solution, which is not an emulsion because the droplets and
their bounding films are absent. Such a transition zone may constitute the entire
accumulation.
Thus, the following transformations of gas accumulations can occur with in-
creasing formation temperature and pressure: gas accumulation-gas-condensate
accumulation with an emulsion (gas–water) fringe-emulsion (gas–water) accumu-
lation-gas–water solution.
10.2.2. Oil accumulations
As with gas accumulations, oil occurs always with water. To be accepted for
refining or pipeline delivery, the oil should be ‘‘water-free,’’ i.e., water content must
be below 2%. The latter is a tentative boundary, which can be changed.
As in the case of gas accumulations, beside a small content of dissolved water,
other types of water may be present at moderate temperatures (no higher than
o
80–100 C) including: (1) capillary water; (2) ‘‘submelted’’ water; (3) water from
water-saturated lenses and interbeds within the accumulation; (4) water from water
coning and fingering; and (5) the bottom-water or edge-water. Influx of water into
the accumulation from the overlying or underlying aquifers due to poor cement job
and casing corrosion is not considered here.
At low and moderate temperatures, the oil–water contact is usually clear-cut
(distinct). 15 If, however, the temperature is higher than 801C (sometimes lower,
depending on the water and oil composition), an emulsion forms the transition
zone. Its thickness can vary and can even comprise the entire accumulation, i.e.,
a stable emulsion is present throughout the entire accumulation (Kontorovich et al.,
1975).
Gas is always present in oil and even after a long period of weathering the oil still
contains gas. As an example, some formations of the Productive Series in Azerbaijan
(Buryakovsky, 1974; Buryakovsky et al., 2001) are exposed at the surface in the
Yasamaly Valley of the Absheron Peninsula. Despite a long (a few tens of thousands
of years) erosion, the hydrocarbon gas is continually seeping from these formations
together with oil (this prevents an open-pit development).
The pressure under which dissolved gas resides in the oil is of importance. When
the pressure is close to the atmospheric pressure, some gas is released at the free
surface of oil. This gas is usually non-commercial. Gas-saturated oil remains a sin-
gle-phase liquid until the saturation pressure (i.e., the pressure of the dissolved gas)
reaches the reservoir pressure, after which a gas cap can form.
15
If anaerobic bacteria are present, they may destroy the oil–water contact and form a peculiar transition
zone. In such a case, a clear oil–water contact disappears.