Page 229 - Geology and Geochemistry of Oil and Gas

198 CLASSIFICATIONS OF OIL AND GAS ACCUMULATIONS

decline, gas solubility in water drastically decreases (depending on changes in the
reservoir conditions), and gas is released as a free phase forming a gas-in-water
emulsion. A substantial portion of the released gas energy is spent for the formation
of emulsion. The emulsion has huge surface area and, correspondingly, huge surface
energy. Thus, the gas accumulation is insulated by the gas-in-water emulsion, which
is only slightly movable and is elastic at reservoir conditions.
Three major reasons have been proposed in the above three explanations to elu-
cidate the formation and existence of the accumulations: (1) the difference in the
speeds of immigration and emigration, (2) the capillary forces (and wettability), and
(3) changes in the fluid properties due to changes in the formation temperature and
pressure.
The problem is yet to be solved. How common are such (Type VI) accumulations?
What is their contribution to the total undiscovered resources? At this time there is
no answer to these questions. The existing exploration, testing and production
techniques are attuned almost exclusively to the accumulations where the gravity
forces predominate. Discoveries of Type VI accumulations in such conditions is
highly accidental. Their exploration, testing and production techniques must be
established after the known accumulations of this type are studied in detail.
VII The ‘‘critical-state’’ accumulations. The ‘‘critical-state’’ accumulations have been
described above. It is important to emphasize again that the name is tentative.
May be they should be called ‘‘near-critical to critical’’ accumulations.

10.5. VERTICAL ZONATION OF HYDROCARBON ACCUMULATIONS

A commonly accepted definition of an ‘‘oil and/or gas field’’ is still missing.
Commonly, a field is defined as all oil and/or gas accumulations (pools) associated
with some structural feature of the Earth’s crust (see Eremenko, 1991). The authors
of this book define an oil and/or gas field as an area of the Earth’s crust that contains
accumulations (pools) of oil and/or gas, the lateral extent of which is amenable for
arranging the infrastructure to develop the field in its totality. On the other hand, the
term ‘‘pool’’ refers to any elementary, single accumulation of oil and/or gas.
This definition of a field is quite different from the one previously proposed by
one of the authors (Eremenko, 1968, 1984). The new definition encompasses not
only all accumulations, but also the entire rock sequence comprising the field (in-
cluding ‘‘dry’’, i.e., water-saturated, hydrocarbon-free formations). Knowledge of
this information is of decisive importance for the development of a field. When the
accumulation characteristics are known, one can select the main target, the intervals
for joint production, and the intervals for the injection of produced water.
There may be an accumulation located on the flank of monocline far from the rest
of accumulations. Such an accumulation, depending on the development environ-
ment, may be treated as part of the same field or considered as a separate field. The
vertically overlapping accumulations, which belong to different structural elements,
are included in the same field. Such different structural conditions may include
abrupt unconformities, e.g., monocline over an anticline, rootless anticline over a