Page 59 - Geology and Geochemistry of Oil and Gas

36 OIL- AND GAS-BEARING ROCKS
surface (artesian), or of ‘‘elision’’ type (i.e., with an inverse pattern of hydrostatic
pressure). It is not possible for such energy system to exist without a base (bottom).
Often, no distinction is made among the terms: ‘‘reservoir rock’’, ‘‘reservoir’’, and
‘‘trap’’. These terms are defined below:
Reservoir rock is a rock capable of containing oil and gas and yielding them
during production. The reservoir rock is characterized by: rock type; permeability
type (intergranular, fracture, and/or combination of the two); the total, intercom-
municating, and effective porosity; specific surface area; wettability of rock (oil-wet
versus water-wet); fracture type (width, etc.); and fracture distribution.
Reservoir is a natural subsurface container for oil, gas, and water. Its existence is
predicated on the relationships between the reservoir rock and associated poorly
permeable rocks. Reservoir is characterized by reservoir-rock type, relationship with
impermeable rocks, reservoir capacity, its hydrodynamic conditions, reservoir en-
ergy, and structure.
Trap is part of the subsurface reservoir where an oil or gas accumulation can form
and be preserved. Its parameters include the reservoir type, reservoir-rock type,
conditions of its formation, structure, and capacity. In a special case where the
reservoir is lithologically limited from all directions, its parameters may coincide
with those of the trap (i.e., the entire reservoir is represented by a single trap).
The following features are used in describing a reservoir: (1) type of the reservoir
rock comprising the reservoir; (2) relationship between the reservoir and the sur-
rounding impermeable rocks; (3) reservoir capacity; and (4) depositional environ-
ment. In terms of the relationship between the reservoir and its surrounding
impermeable rocks, Brod (1957) proposed to identify three major types of the res-
ervoirs: bedded, massive, and lithologically limited in all directions. A similar clas-
sification, with characterization of the reservoir energy, was proposed by Eremenko
(1941, Table 2.1).
Bedded reservoir is a reservoir that is restricted at its top and base by low-
permeable rocks. The reservoir rock thickness in such a reservoir is more or less

TABLE 2.1

Classification of reservoirs
Reservoir type Stratigraphic position Direction of oil and gas Possible maximum
flow energy accumulated in
oil and gas
Bedded Consistent with Along the bedding Insignificant
stratigraphy
Bedded, fault blocks Consistent with Restricted but possible Much greater than in the
(differentiated) stratigraphy in any direction bedded; may be even
major
Massive Inconsistent with Vertical Significant, rarely
stratigraphy insignificant
Lithologically limited Consistent with Locally restricted Major
from all directions stratigraphy