Page 37 - Geology of Carbonate Reservoirs

18 CARBONATE RESERVOIR ROCK PROPERTIES

deposits are produced. The removal of mud by winnowing is important because the

leading carbonate rock classifications are based on the presence or absence of mud.
Rocks with high mud content usually represent sediment “ sinks, ” or areas where
water movement has been slow and mud has settled out of suspension. If fractures,
dissolution diagenesis, or alteration to microporous crystalline fabrics are absent,
muddy carbonates are not good reservoir rocks. They may be good source rocks
instead if they contain enough sapropelic (lipid - rich) organic matter. Carbonate
sands and gravels with little or no mud represent either effective winnowing or a
lack of mud production. Well - sorted and mud - free carbonate rocks have high depo-
sitional porosity and permeability.

2.2.2 Fabric
Depositional, diagenetic, or biogenic processes create carbonate rock fabrics. Tec-
tonic processes such as fracturing and cataclasis are not part of the depositional and

lithification processes but may impart a definite pattern and orientation to reservoir
permeability. Fractured reservoirs are discussed in Chapter 7 .
Depositional fabric (Figure 2.3 a) is the spatial orientation and alignment of
grains in a detrital rock. Elongate grains can be aligned and oriented by paleocur-
rents. Flat pebbles in conglomerates and breccias may be imbricated by unidirec-

tional current flow. These fabrics affect reservoir porosity and can impart directional
permeability, ultimately affecting reservoir performance characteristics. Elongate
skeletal fragments such as echinoid spines, crinoid columnals, spicules, some fora-
minifera, and elongate bivalve and high - spired gastropod shells are common in
carbonate reservoirs. Presence or absence of depositional fabric is easily determined
with core samples; however, determination of directional azimuth requires oriented
cores. In some cases, dipmeter logs and high - resolution, borehole scanning and
imaging devices may detect oriented features at the scale of individual beds or
laminae (Grace and Pirie, 1986 ).
Diagenetic fabrics (Figure 2.3 b) include patterns of crystal growth formed during
cementation, recrystallization, or replacement of carbonate sediments and fabrics
formed by dissolution. Dissolution fabrics include a wide range of features such as
molds, vugs, caverns, karst features, and soils. Mold and vug characteristics may be
predictable if dissolution is fabric - or facies - selective; however, caverns, karst fea-
tures, and soils may be more closely associated with paleotopography, paleoaquifers,
or unconformities than with depositional rock properties. Without such depositional
attributes, dissolution pore characteristics are harder to predict. Intercrystalline
porosity in dolomites and some microcrystalline calcites are fundamental properties
but they are diagenetic in origin. The size, shape, orientation, and crystal “ packing ”
(disposition of the crystal faces with respect to each other) create an internal fabric
that greatly affects reservoir connectivity because they determine the size, shape,
and distribution of pores and connecting pore throats.
Biogenic fabrics are described in connection with carbonate buildups, or reefs,

and with the internal microstructure of skeletal grains. A classification of reef rocks
was conceived to cope with variability in reservoir characteristics within a single
reef complex (Embry and Klovan, 1971 ). They described three end - member bio-
genic fabrics, including (1) skeletal frameworks in which interframe spaces are fi lled