Geology Reference
In-Depth Information
portant factor. High porosity is caused by the specific
composition (calcitic nannofossils; Pl. 29/5) and micro-
pores (Feazel and Farell 1988). Porosity occlusion is
due to overburden stress.
Major hydrocarbon accumulations are common in
pinnacle reefs
and
mud
buildups
formed in deeper-water
basinal settings. The porous core facies is usually
bounded on all sides by impermeable flank and margin
deposits, basinal shales or basinal evaporites. These
deposits can form a seal. Examples are known from
the Silurian of Michigan and the Devonian of Canada.
dissolved or the low-permeability carbonates were frac-
tured. Environmental controls may also be closely
linked with diagenetic controls. An example is the early
marine cementation of isolated carbonate platform
slopes as a result of ocean currents and the enhanced
water rock ratio (e.g. Bahama platform; Malampaya).
17.1.3 Diagenetic Controls on Carbonate
Reservoirs
Meteoric, marine and burial diagenesis may either de-
stroy or enhance original porosity depending on com-
paction, cementation and dissolution. Pores are often
occluded by cements (Sect. 7.4), but can be open due
to specific diagenetic processes or cementation is in-
hibited, as in the case of oil emplacement (Neilson et
al. 1998).
17.1.2.2 Environmental Controls
Environmental controls primarily affecting the hetero-
geneity of carbonate reservoir rocks are
•
hydrodynamic conditions (Sect. 12.1.1) reflected by
grain- or mud-supported sediments (Pl. 43) with dif-
ferent primary porosities,
•
paleowater depth (Sect. 12.3) influencing sedimen-
tation patterns,
17.1.3.1 Reservoir Properties
•
high-frequency sea-level fluctuations resulting in
cyclic sedimentation (Sect. 16.1) and favoring sub-
aerial exposure and meteoric diagenetic processes,
Major reservoir property controls are permeability and
porosity. Permeability is often more important than
porosity. Porosity was discussed in Sect. 7.3. The terms
used to describe visible porosity are summarized in Box
7.4. Fig. 7.5 shows the commonly used
Choquette and
Pray porosity classification
(Sect. 7.3.2). Pore geom-
etry and permeability are discussed in Sect. 7.3.1.2,
porosity measurements and pore types seen in thin sec-
tions in Sect. 7.3.1.3. The thin-section aspect of com-
mon pore types is shown in Pl. 29 and Pl. 30.
•
wind- and storm-related processes shaping the ge-
ometry of reefs and producing talus deposits (Sect.
12.1.2),
•
climatic factors (Sect. 12.2) that contribute to dif-
ferent controls in the development of reservoir rocks
in arid and humid regions (Dmietrievsky and Kuz-
netsov 1992).
The
influence of prevailing climates
producing dif-
ferent styles of diagenesis has been demonstrated for
Miocene reservoir rocks (Sun and Esteban 1994): Reefs
and banks formed in
humid
tropical and subtropical
settings (Miocene of Southeast Asia) are characterized
by common subaerial exposure and meteoric diagen-
esis. The best reservoir qualities are developed beneath
subaerial unconformities, where the effects of fresh-
water leaching and karstification were most intensive.
In contrast, lagoonal ramp carbonates and reefs formed
in
arid
, landlocked temperate-subtropical settings
(Mediterranean, Gulf of Suez and Red Sea) with peri-
odically increased salinity are characterized by dolo-
mitization, leaching of metastable skeletal grains, gen-
eration of moldic, vuggy and intercrystalline porosity,
and widespread anhydrite cement, precipitated over a
wide range of burial depths. Porosity development in
these reservoirs depends not only on the original depo-
sitional fabric, but more importantly on the degree to
which the porosity-occluding anhydrite cement was
Porosity has a great influence on the saturation and
mobility of hydrocarbons in carbonate rocks. It is de-
termined by conventional core plug analysis, and cap-
illary-pressure curves. SEM studies of three-dimen-
sional resin casts, and the investigation of thin sections
exhibit the size and shape of the pores. A correlation of
thin-section derived porosity data and physical poros-
ity measurements is possible using digitalized thin-sec-
tion measurements (McCreesh et al. 1991).
Reservoir characteristics depend on the arrangement
of the pores and how pores are interconnected by pore
throats. Pore throat sizes and distributions depending
on the size and shape of grains and crystals are deter-
mined by capillary-pressure curves (Wardlaw and Tay-
lor 1976). Pore throats are often too small to be exam-
ined visually in thin sections (Wardlaw and Li 1981).
The size, shape, arrangement, and distribution of pore
throats are major controls on reservoir-rock producib-
ility and recovery efficiency, and must be included in
the differentiation of porosity types. Dynamic proper-
