Geology Reference
In-Depth Information
ties of different pore geometries are measured by spe-
cial core analysis (SCAL), e.g. drainage and inbibition
capillary pressure measurements and relative perme-
ability curves of hydrocarbon to water. These pore-ge-
ometry dependant parameters are a crucial input for
dynamic 3D reservoir models.
Pore systems are effective and ineffective with re-
gard to reservoir qualities.
Effective porosity
is charac-
terized by interconnected pores including interparticle
pores caused by leaching of interparticle cement, micro-
pores between calcite or dolomite crystals, and moldic
pores associated with interparticle pores. Intercrys-
talline microporosity is common in many productive
Mesozoic and Tertiary reservoirs in the Middle East
(Moshier 1989; Budd 1989; Fig. 7.4). Effective poros-
ity depends on pore throat diameter, pore size, reser-
voir pressure, temperature, and the viscosity of oil or
gas. Burrows may cause effective or
ineffective poros-
ity.
Intraskeletal porosity, e.g. in chambers of foramin-
ifera, is often ineffective.
not necessarily increase total porosity. In many cases
porosity is reduced below unconformities. Permeabil-
ity is more strongly changed than porosity. It may in-
crease or decrease. Pore systems evolve with the time
of subaerial exposure. Shorter periods of subaerial ex-
posure (10,000 to 400,000 years) are often associated
with greater porosity than long intervals (1 to 20 mil-
lion yr). Many carbonates subjected to subaerial expo-
sure have little or no porosity in the deep subsurface
due to compaction, cementation and stratal collapse
reducing burial porosity. But unconformity-related di-
agenesis may also enhance reservoir porosity by creat-
ing pore systems that are resistant to deeper burial com-
paction. Caverns, breccias and vugs can form in the
deep subsurface because of dissolution by basinal flu-
ids or the controls of shales overlying carbonates.
• Karst reservoirs
. Platform carbonates, affected by
an interplay of eustatic transgressions and regressions
with local tectonic uplifts, are major sites of karst-
ification, often connected with dolomitization. Both
processes may lead to the development of karst reser-
voir rocks (Cater and Gillcrist 1994). Reservoirs pro-
duced by near-surface karstic processes, the collapse
of paleocaves and later burial compaction and dissolu-
tion may be hundreds to thousands of meters across.
These reservoirs are complex and heterogeneous and
characterized by fractures, breccias and fill sediments.
Breccias and cave sediments need specific classifica-
tions. Loucks (1999) distinguishes highly fractured
crackle breccias and chaotic breccias with rotated and
displaced clasts. Predicting the size and distribution of
paleokarst features is generally not possible if well data
are used. Understanding paleokarst heterogeneity re-
quires outcrop data and the interpretation of karst hori-
zons within a sequence stratigraphic framework. For
an overview on karst reservoirs see Fritz et al. (1993).
17.1.3.2 Diagenetic Controls on Reservoir
Properties
Most reservoir porosity is caused by various diagenetic
processes including near-surface dissolution, early and
late dolomitization and burial fracturing. This
second-
ary porosity
often occurs together with preserved fa-
cies-controlled
primary porosity
. The understanding of
the porosity development through time and space is
crucial in evaluating reservoir qualities. Studies of po-
rosity development must take into account marked dif-
ferences in the accumulation of sediments in different
carbonate settings. Fast growing platform-margin reefs,
e.g., usually have high initial porosities, but resulting
reef limestones often exhibit low porosities because of
rapid early cementation (e.g. Malampaya). In contrast,
low-growing interior platforms may have high porosi-
ties due to widespread dissolution and dolomitization.
Common diagenetically controlled reservoir types in-
clude:
•
Dolomite reservoirs.
Early dolomitization often re-
sults in better preservation of reservoir properties dur-
ing diagenesis (Arab ABC, Middle East). Dolomitiza-
tion may increase porosity, but can also decrease it,
e.g. in near-surface dolomitization. Dolomite reservoirs
are very heterogeneous at different scales (Weber 1986).
At a scale of 10 m to 10 km heterogeneities are repre-
sented by major lithostratigraphic boundaries, large
faults and extensive fractures. At a scale of millimeters
to meters heterogeneities are caused by the rock fab-
ric. Reservoir characteristics of the major dolomite res-
ervoirs depend upon the original sediment fabric, the
mechanism forming the dolomite, and the extent to
which early formed dolomite was modified by post-
dolomitization diagenetic processes (e.g. karstification
and fracturing). Porosity and permeability in platform
•
Reservoirs related to near-surface dissolution at
subaerial unconformities.
Reservoirs associated with
unconformities have been estimated to contain 20 to
30% of known recoverable hydrocarbons (Halbouty et
al. 1970). Subaerial exposure and meteoric diagenesis
are important processes in the development and pres-
ervation of porosity. The creation and preservation of
pore systems depend on the depositional facies and is
related to the duration of subaerial exposure (Hopkins
1999; Saller et al. 1999). Diagenetic processes during
subaerial exposure rearrange the pore network, but do
