Geology Reference
In-Depth Information
et al. 1991; Campbell and Stafleu 1992; Eisenberg et
al. 1994; Kerans et al. 1994; Antonelli and Mollema
2000)? Geometries are derived from mapping, detailed
facies analysis and seismic outcrop modeling. Smaller
carbonate bodies are evaluated by ground-penetrating
radar methods.
•
Size of carbonate bodies
and flow units. At what
scale do potential flow units vary? What thickness and
lateral extension can be expected for carbonate bodies
formed by different processes (e.g. allochthonous and
autochthonous oolitic sand shoals, platform-margin
reefs, pinnacle reefs and mud mounds)?
•
Predictive interwell interpretation
(Barnaby 1995;
Pomar and Ward 1999). What facies and petrophysical
characteristics can we expect in interwell areas? An-
swering these questions requires outcrop studies that
consider common well spacing (several hundreds of
meters).
•
Correlation.
What stratigraphic units can be used
in regional subsurface correlation?
•
Heterogeneity.
Which lateral and vertical variabil-
ity of petrophysical parameters can be expected, e.g.
in reefs formed in different settings, or in different parts
of platforms and ramps? At what scale do depositional
and/or diagenetic heterogeneities occur?
facies data by multivariate methods (Tucker et al. 1998).
Understanding heterogeneity requires integrated
analysis of cores, cuttings, petrophysical data, logs and
seismic data supplemented by reservoir-related outcrop
studies (Smith et al. 2003). Outcrops show the com-
mon facies types, vertical and lateral facies variations,
and offer the possibility for analysis of high-resolution
sequence stratigraphy. However, they may have a very
different diagenetic history compared to subsurface
reservoirs. Cores show sedimentary and diagenetic
structures, rock types and their composition, visible po-
rosity, and oil staining.
Thin section and microfacies data show depositional
fabrics and the frequency and distribution of grain types
and matrix responsible for primary porosity, allowing
visible porosity to be estimated and porosity reduction
by enhanced diagenesis to be evaluated.
17.1.5.2 Relevant Microfacies Data
The first part of this topic deals with many criteria
that are responsible for heterogeneities of carbonate
reservoir rocks (Chap. 4, 5 and 7). The following text
summarizes these criteria.
• Fine-grained matrix
Different matrix types (micrite, Sect. 4.1.1; mi-
crospar, Sect. 4.1.3; calcisiltite, Sect. 4.1.4) as well as
auto- and allomicrites (Pl. 6, Pl. 7) can provide differ-
ent intercrystalline microporosities. Matrix micro-
porosity may be high (10-20%) due to subaerial expo-
sure or retention of intercrystalline porosity during re-
crystallization of original lime mud (Moshier et al.
1988). Diagenetic processes, e.g. recrystallization, re-
sulting in pervasive microporosity and leading to mi-
crospar formation, may outweigh depositional controls
on porosity distribution in micritic reservoirs (Jordan
and Abdullah 1988). Take a look at 'homogeneous' and
'inhomogeneous' micrites (Pl. 6/1) both in the matrix
and as constituents of grains and examine SEM micro-
photographs (Pl. 7/3, 5, 6) for the geometry and con-
nections of micropores!
17.1.5 Microfacies, Lithofacies and
Reservoir Rock Types
Modeling carbonate reservoirs and bridging the gap
from seismic to subseismic heterogeneities requires
differentiating rock types that reflect depositional and
diagenetic constraints of petrophysical properties.
17.1.5.1 Reservoir Heterogeneity
Problems encountered in carbonate reservoirs are due
to the inherent heterogeneity and complexity of the
system, caused by highly variable in-situ sedimenta-
tion and sediment transport as well as subsequent al-
teration by diagenetic processes. Carbonate reservoirs
are characterized by the extreme heterogeneity of po-
rosity and permeability, particularly in cyclic shelf car-
bonates, as a result of a complex depositional and di-
agenetic history. The variability of well-log responses
and downhole petrophysical parameters and the lateral
variability of reservoir quality is related to carbonate
textures and pore-system dimensions (Tanguy and
Friedman 2001). Different approaches are used to un-
derstand reservoir heterogeneities, including reservoir
rock type modeling (Sect. 17.1.5.3) and grouping of
• Grains
Grains form the primary depositional framework.
Their arrangement, frequency and size determine pri-
mary porosity. Grains react differently to subaerial, ma-
rine and burial diagenesis, depending on their primary
mineralogical composition.
Skeletal grains.
Higher porosities may occur in in-
tervals with high proportions of bioclastic particles. The
evaluation of moldic and dissolution porosity is assisted
