Page 58 - Geology and Geochemistry of Oil and Gas

OIL AND GAS RESERVOIRS 35
2.4. OIL AND GAS RESERVOIRS

The natural subsurface reservoir is a container of oil, gas, and water where they
can move, and its shape is determined by the relationship between the reservoir rock
and its surrounding poorly permeable rocks (Brod, 1950, 1957). Levorsen (1967)
defines the subsurface reservoir as only that part of the reservoir rocks where the oil
and natural gas can form an accumulation. Many authors tend to tie the notion of
oil and gas reservoir to its content of hydrocarbons (Trofimuk et al., 1980;
Kuznetsov, 1992). This appears redundant to the writers because the presence of oil
or gas is not a vital property of the subsurface reservoir. The natural reservoir
remains as such regardless of the type of the contained fluids, or even if it is dry. A
comparison can be made with the overpressure. The presence of fluids is necessary
for the overpressure. No fluids of different densities, no surplus pressure. As for the
reservoirs, the writers agree with Kuznetsov who stated that ‘‘they possess two
important attributes: spatial limitation (which determines the volume and bound-
aries of the reservoir), and an internal structure that defines the type and nature of
inter-reservoir migration’’ (Kuznetsov, 1992, p. 15).
Indeed, these properties should be included in any definition of the reservoir as
well as in the classifications being developed. Some attempts have been made to
classify the oil and gas reservoirs on the basis of their relative size (local, zonal,
basin-wide, regional, etc.) (Burlin et al., 1976; Kontorovich, 1991) or their absolute
size (Weber, 1986; Allen and Allen, 1993).
Three types of reservoir limitations can be identified:
reservoir roof
lateral, and
reservoir base (bottom).
The reservoir roof (top) may cover the reservoir (1) in normal stratigraphic suc-
cession, (2) with some depositional hiatus, and (3) may change its age along the
strike. It is improbable but not impossible that a reservoir may be capped by the
surface of an impermeable overthrust. It should be kept in mind the selectivity
toward different fluids by caprock and a possibility of its transformation into a
reservoir rock during epigenesis. Consequently, it is always important to indicate the
exact nature of the fluid. The transformation of the caprock into a reservoir rock
results in either disappearance of the reservoir or its conversion into a new reservoir
(if there is another caprock above).
Lateral limitations are caused by lithologic alterations (including cementation)
and the permeability. Small accumulations may be laterally limited by faults. This is
possible, but not typical, for the larger reservoirs, because the fault zones in their
evolution can become migration paths for various fluids (oil, water, or gas). Fault
zones are actually ‘‘communication windows’’ with the other reservoirs.
The importance of the presence of the base (bottom) as a necessary reservoir
component was not always clearly recognized, because it was believed that the ac-
cumulations were formed exclusively by the buoyancy (Archimedes forces). One
must always keep in mind that the reservoir is an inseparable part of the hydro-
dynamic system. This system may be open or with a restricted communication to the