Geology Reference
In-Depth Information
5
10
15%
Fig. 17.3.
Spatial distribution of carbonate res-
ervoirs.
Depositional setting of 315 Late Precam-
brian to Tertiary carbonate reservoirs studied all
over the world by C & C Reservoirs, Inc. The
figure gives only a rough picture of the relative
frequency, but underlines the importance of
grainy deposits (offshore bar, high-energy ramp,
platform/ramp marginal shoal, debris flows/tur-
bidite), muddy and grainy deposits (tidal flat,
shelf lagoon, low-energy ramp) and reefs par-
ticularly reef mounds. Note that karst-related
reservoirs and pelagic reservoirs (e.g. chalks),
are much more important than indicated in this
figure! Based on internet data (www.ccreser-
voirs.com).
Karst-related detrital wedge
Shore line
Offshore bar
Tidal flat
Marine embayment
Shelf lagoon
Open shelf
Skeletal bank
Mud-rich skeletal bank
Coquina bank
High-energy ramp
Low energy ramp
Patch reef
Reef mound
Mud-rich reef mound
Pinnacle reef
Platform/ramp marginal shoal
Barrier and fringing reef
Debris flows/turbidites
Pelagic deposits
wave action or strong tidal currents. Sand bodies occur
as tidal bars, storm layers in lagoons, storm washover
sediments, in tidal channels, at bank edges, and behind
reefs. Carbonate sands have high initial porosities that
may be preserved in the subsurface. Secondary poros-
ity may develop shortly after deposition by freshwater
diagenesis or later by dolomitization. The best examples
of production from sand shoals are the Smackover For-
mation of the U.S. Gulf Coast and the Arab D Forma-
tion forming the reservoir facies of the world's largest
fields, sealed by evaporites (e.g. Ghawar field, Saudi
Arabia).
ments which act as updip seal in stratigraphic traps.
Examples of reefs acting as excellent reservoirs are the
Oligocene reef complex of Kirkuk (Iraq) and the De-
vonian Leduc reef in western Canada.
•
Slopes.
Highly variable sediments. Coarse and fine
sediments and breccias. Small but thick downslope
reefs (e.g. mud mounds). Proximal reef slopes with
intergranular and moldic porosity, distal slope with
intercrystalline and moldic porosity. Both parts may
exhibit high permeabilities. Reservoirs are ribbon- and
mound-like (e.g. Cretaceous, Mexico).
•
Platform margins
. Carbonate sands or reefs. Grain
size coarse to medium. Ribbon- and mound-like reser-
voirs. Intergranular and vuggy porosity in sand shoals.
Sand accumulations of reservoir size commonly occur
near the seaward edge of banks, platforms and shelves.
When dolomitized, intercrystalline and vuggy poros-
ity. Good porosity in backreef sediments and in dolo-
mitized reefs. The reservoir potential of reefs is widely
assumed to be high. This is an assumption made dur-
ing many exploration plays. However it is often not
true. Examples with tight margins are the Malampaya/
Camago buildup and the Central Luconia buildups
(Sarawak). Often early marine cementation at ocean-
ward slopes occludes porosity in a very early stage.
Effective porosity is due to solution and fracture, and
preserved framework pores and molds. Proximal back-
reef sands can retain significant amounts of primary
porosity. Good permeability. Forereef deposits and
aprons of mud mounds may have good reservoir po-
tential, especially if the reef itself is plugged by ce-
• Basins.
This setting is characterized by carbonate
muds with intercalations of allochthonous carbonate
sands as
debris flows and turbidites
. The porous parts
of turbidites may act as sheetlike reservoirs or conduits
for migrating oil (e.g. Cretaceous Golden Lane field,
Mexico),
Deep-sea micrites generally have low porosities and
permeabilities, but are susceptible to fracturing and can
form significant reservoirs. Primary porosity due to
loosely packed nannofossils and open foraminiferal
chambers is very high in
chalks
, but permeability is
low. However, chalks can be significant reservoirs in
the Cretaceous and Tertiary of the North Sea and U.S.
Gulf Coast due to combinations of preserved primary
porosity, moldic porosity and fracture porosity. Chalk
reservoirs have proven to be finer grained, more po-
rous, less permeable, more uniform and widespread,
and considerably more predictable in terms of petro-
physical properties than reservoirs in typical shallow-
water limestones. Burial depth is the single most im-
