Page 70 - Petrophysics

44 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

Thick reservoirs are formed, but the permeability is extremely variable,
both vertically and horizontally. Consequently, both primary and
secondary production may be poor and the reactive clays produce severe
water sensitivity. Examples of arkose-type formations are the Kern River
formation in California and the Granite Wash in the Oklahoma-Texas
panhandle area [ 111.

HYDROCARBON TRAPS

Hydrocarbon traps may be illustrated by considering a porous,
permeable formation that has been folded into an anticlinal trap by
diastrophism and is enclosed between impermeable rocks (Figure 2.5).
The closure of the trap is the distance between the crest and the spill point
(lowest point of the trap that can contain hydrocarbons). In most cases,
the hydrocarbon trap is not filled to the spill point. It may contain a gas
cap if the oil contains light hydrocarbons and the pressure-temperature
relationship of the zone permits the existence of a distinct gas zone at the
top of the reservoir. If a gas cap exists, the gas-oil contact is the deepest
level of producible gas. Likewise, the oil-water contact is the lowest level
of producible oil. Transition zones exist between various zones grading
from a high oil saturation to hydrocarbon-free water. For example, the
water zone immediately below the oil-water contact is the bottom water,
whereas the edge water is laterally adjacent to the oil zone. The gas-oil
and water41 contacts are generally planar, but they may be tilted due
to hydrodynamic flow of fluids, a large permeability contrast between
opposite sides of the reservoir, or unequal production of the reservoir.

IMPERMEABLE ROCKS ABOVE THE PERMEABLE FORMATION

GAS-OIL CONTACT

SPILL POINT I

IMPERMEABLE ROCKS UNDER
THE PERMEABLE FORMATION
Figure 2.5. Idealized cross-section through an anticlinal trap formed by a porous,
permeable formation surrounded by impermeable rocks. Oil and gas are trapped at
the top of the anticline.