Geology Reference
In-Depth Information
7.4.2.2), and cement sequences (e.g. Pl. 35). Cement
sequences are of great importance in evaluating the tem-
poral succession of marine, freshwater and burial di-
agenesis.
Many authors consider depositional fabric and its
diagenetic overprinting to be the major factors explain-
ing differences in petrophysical properties. However,
complex diagenetic overprinting can prevent deposi-
tional facies from being related to petrophysical prop-
erties. Recognizing these difficulties and aiming for
rock units that can be used in static and dynamic 3D
models evaluating reservoir quality, Grötsch and
Mercadier (1999) and Grötsch et al. (2003) proposed
the concept of
reservoir rock types
(RRTs).
Each reservoir rock type is characterized by its pore
geometry, a specific average porosity and average per-
meability, an average gas saturation, and a saturation-
height function. A specific lithofacies may be a defini-
tion criteria but not necessarily. RRTs do not always
follow the classical texture-based carbonate classifi-
cation schemes, but instead combine effects of deposi-
tional texture and diagenetic overprinting into classes
of common petrophysical properties. The rock facies
is defined by mercury injection capillary pressure analy-
sis and pore-throat distribution curves as well as other
SCAL measurements.
The same depositional facies (lithofacies) may ex-
hibit different reservoir properties due to a different
degree of diagenetic overprinting. Correlation between
depositional fabric and petrophysical data may be good
in carbonates that have been affected only by early dia-
genetic processes, but is usually weaker in carbonates
affected by burial diagenesis.
Distinguishing RRTs requires integration of depo-
sitional lithofacies, including their environmental con-
trols, and diagenetic sequences determined in cores and
thin section, porosity and permeability from core plug
measurements, high-pressure mercury measurements
exhibiting the pore-throat distribution, and the evalua-
tion of wireline log characteristics and formation tests.
• Diagenetic textures
Mechanical compaction
(Sect. 7.5.1) alters the spa-
tial arrangement of grains and leads to an increase in
grain packing paralleled by a decrease in interparticle
porosity. For compaction criteria see Box 7.9.
Chemical compaction
(Sect. 7.5.2) is responsible
for pressure solution and stylolites that reduce poros-
ity, but also may act as conduits for fluids (Box 7.10).
Fig. 7.18 assists in describing pressure solution fea-
tures.
Of specific interest in evaluating the role of pres-
sure solution for the development of permeability and
porosity are the geometry, distribution and abundance
of stylolites, and the association of fracture sets with
stylolites (Nelson 1981; Grötsch et al. 1998).
• Dolomitization
Dolomitization can enhance or reduce porosity de-
pending on the mode and the timing of the dolomitiza-
tion process (Mazzullo 1992). Dolomite textures should
be described using the criteria summarized in Sect.
7.8.1.1, including crystal size and crystal textures. Do-
lomitization may be restricted to the matrix or to spe-
cific grain types or may have effected the total rock.
Differentiation of dolomitization stages requires ongo-
ing investigations, including cathodoluminescence and
isotope studies. Dedolomitization (Sect. 7.8.3), often
associated with meteoric diagenesis, may improve
intercrystalline and moldic porosity.
17.1.5.3 Reservoir Rock Types and Facies
Criteria
Microfacies and reservoir rock types.
Some of the
problems involved in correlating microfacies and petro-
physical patterns are caused by the typification of
microfacies. Microfacies types based solely on textural
classification (e.g. bioclastic wackestone, oolitic grain-
stone) or defined primarily to the association (e.g.
intraclastic skeletal wackestone) or proportion of dif-
ferent grain types (e.g. ooid-rich and ooid-poor grain-
stone) do not reflect all the constituents of carbonate
rocks influencing porosity and permeability.
The microfacies types discussed in Chap. 11 of this
topic are delimited by differences in qualitative and
quantitative compositional criteria. These often rather
small differences reflect variations within the deposi-
tional environment and allow paleoenvironment con-
ditions and depositional settings to be evaluated (Chap.
Lucia (1995, 1999) underlined the importance of pore
space related to rock fabrics, which control porosity,
permeability, saturation and dynamic behavior of fluid
movement through the rock. He introduced the con-
cept of rock/petrophysical classes that aims at a classi-
fication of carbonate reservoir rocks based on petro-
physical properties and major pore space groups (inter-
particle and vuggy pore spaces). The classes should
reflect unique reservoir characteristics and specific dis-
tribution patterns. Lucia's classification is based on the
fact that pore size distribution controls permeability and
saturation and that pore-size distribution is related to
sedimentary fabric, resulting in different pore types in
grain- and mud-supported limestones.
