Page 204 - Geology and Geochemistry of Oil and Gas

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Chapter 10

CLASSIFICATIONS OF OIL AND GAS ACCUMULATIONS

10.1. CLASSIFICATION OF TYPES OF OIL AND GAS ACCUMULATIONS AND TRAPS,
RESERVES, FLUID QUALITY, PRODUCTION RATES

Oil and gas fill the pores, vugs, and fractures in reservoir rocks. Usually, not the
entire reservoir but only its part called ‘‘trap’’ is filled with petroleum. There are
numerous classifications of traps depending on their relationships with the reservoir,
lithology, rock origin, tectonics, trap size and shape, and the type of energy dis-
tribution. Criterion of usefulness of a classification or a definition is its applicability.
Particular purpose, limited in scope, gives rise to incomplete classifications. The
development of a natural classification for the traps and/or reservoirs is very dif-
ficult, first of all, due to the lack of knowledge about the formation of hydrocarbon
accumulations at high pressures and temperatures.
The most definitions of a trap include the following concepts:
trap is a part of the reservoir where oil and gas can accumulate (Levorsen, 1954),
where relative equilibrium of movable components is maintained by the gravity
(Brod and Eremenko, 1950),
where the potential of oil and gas locally reaches a minimum, and
which is limited by a closed equipotential surface.
With increasing temperature and pressure, the effect of gravity on the fluid dis-
tribution decreases.
The existing trap classifications can be divided into three groups.
The first and the most common classification group is called the ‘‘Oil and gas
classifications’’ by many authors. Yet, these authors do not stress the parameters
(characteristics) of the accumulations, but instead of the crustal elements in which
they are housed, i.e., traps (their genesis, structure, shape, and type of reservoir).
The second classification group associate the trap shape with the conditions of its
formation: folds, faults, stratigraphic unconformities, lithological barriers, and
combinations of the above.
Finally, the third group of classifications stressed the hydrodynamic head.
In every case, the trap is considered to be a part of the reservoir, with substantial
emphasis on its geometry, which is undoubtedly important when selecting locations
for exploratory wells.
The accumulation represents a temporary accumulative stage against the back-
ground of migration, and the trap forms such part of the Earth’s crust (reservoir),
where this accumulative stage may occur under certain geologic environments (in-
cluding the thermobaric and thermodynamic conditions).
Not considering the genetic issues, the trap should be characterized by the
type of the reservoir it is associated with (including the lithology),