Geology Reference
In-Depth Information
Fig. 17.2.
Temporal distribution of reefs with reservoir quality
calculated for stages
.
The figure includes subsurface reefs of
producing reservoirs as well as outcrop data from reefs with reservoir potential (high porosity, thickness of at least 10 m) but
lacking hydrocarbon accumulation. After Kiessling (2002).
result in rapidly changing porosity and permeability
values over short distances. Reefs and organic banks
and shallow-marine oolitic and bioclastic sand shoals
are often cited as common hydrocarbon exploration
targets, but reservoirs are known from a wide range of
depositional environments including lacustrine and
karst settings. Marine reservoirs occur in carbonates
formed in lagoonal and open parts of platforms, inner
and outer ramps, and slope and basinal settings (e.g.
debris deposits, turbidites and chalks; Fig. 17.3).
late diagenetic processes (Chap. 7). The following cur-
sory list gives an idea of reservoir characteristics within
different depositional environments:
•
Near-shore environments, tidal flats
. Carbonate
muds and sands. Reservoir geometry ribbon- or sheet-
like. Grain size fine to medium. Intercrystalline and
intergranular porosity. Reservoirs in shoreline sands and
high-energy offshore bars of inner ramps and platforms
and in tidal flat sediments. Tidal sediments have usu-
ally low porosities and permeabilities, but dolomitiza-
tion can create higher porosities and permeabilities. As-
sociated evaporites may act as seals. Examples are Or-
dovician reservoirs of the Williston Basin and reser-
voirs in the Permian Basin of Texas.
Favored settings for carbonate reservoirs on shelves
and on ramps are
•
subtidal and intertidal complexes with updip facies
changes and downdip dolomitization (Paris Basin),
•
lime sand bodies on shelves with incomplete cemen-
tation (Persian Gulf),
•
Restricted and nonrestricted lagoons
. Muddy and
grainy sediments. Reservoirs discontinuous, often as
stacked lenses. Pores small, permeability low. Inter-
granular and moldic porosity. Porosity enhanced by
dolomitization (e.g. central lagoonal part of Malam-
paya buildup).
•
organic buildups that may be dolomitized, includ-
ing reef mounds on the shelf, at platform-margins,
outer shelf/ramp pinnacles and atolls (Devonian,
Alberta; Malampaya, Philippines),
•
talus accumulations near reefs and banks (Midland
Basin), and
•
leached zones below unconformities with or with-
out fracturing (Paradox Basin).
•
Open platforms.
Carbonate sands, muds and small
platform reefs and mounds. Variable grain sizes. Inter-
granular, moldic, vuggy, intragranular and intercrys-
talline porosity common. Low to good permeabilities.
Well stratified reservoirs. Reservoir geometry sheet-
and ribbonlike. The development of sand bodies re-
quires sand-sized sediments and removing of fines by
Reservoir rocks formed in different settings exhibit
different depositional criteria that influence initial po-
rosities and permeabilities. These properties are fur-
ther differentiated and strongly controlled by early and
