Page 39 - Geology of Carbonate Reservoirs

20 CARBONATE RESERVOIR ROCK PROPERTIES

2.2.3 Composition
Composition of carbonate rocks usually refers to constituent grain type rather than
mineral content, because carbonates may be monomineralic and the mineral content
of polymineralic carbonates is not generally indicative of depositional environment.

Carbonate grains are classified as skeletal and nonskeletal . Extensive, illustrated dis-
cussions of constituents commonly found in carbonates of different geological ages

are found in Bathurst (1975) , Milliman (1974) , Purser (1980) , Scoffin (1987) , and
Tucker and Wright (1990) . Skeletal constituents include whole and fragmented

remains of calcareous plants and animals such as mollusks, corals, calcified algae, bra-
chiopods, arthropods, and echinoderms, among many others. Nonskeletal grains
include ooids, pisoids, peloids, and clasts. Ooids and pisoids (Figure 2.4 a) are spheroi-
dal grains that exhibit concentric microlaminae of calcite or aragonite around a
nucleus. The marine variety is formed by chemical processes in agitated, shallow
water, usually less than 2 m deep (Tucker and Wright, 1990 ). Clasts (Figure 2.4 b) are
particles produced by detrition (mechanical wear); they include resedimented frag-
ments of contemporaneous or older rock known as intraclasts and lithoclasts, respec-
tively, following Folk (1959) . Clasts indicate erosion and resedimentation of lithifi ed

or partly lithified carbonates, some of which may have been weakened by bioerosion
(rock boring and grinding by specialized organisms) or by weathering. Peloid (Figure
2.4 c) is an all - inclusive term coined by McKee and Gutschick (1969) to include
rounded, aggregate grains of microcrystalline carbonate. Peloids are produced by
chemical, biogenic, and diagenetic processes and are important constituents of shallow
marine platform sediments. Pellets differ in that true pellets are compacted bits of
fecal matter that have distinctive shapes or internal structures (Figure 2.4 d). Pellets
can be useful in determining the environment of deposition (Moore, 1939 ). Peloids
that were probably formed as fecal pellets are prominent constituents of Wilson ’ s
(1975) “ standard microfacies 8 ” in the “ restricted platform ” environment.

2.2.4 Sedimentary Structures
Sedimentary structures are useful aids for interpreting ancient depositional environ-
ments. They may affect reservoir characteristics because their internal fabrics are
usually oriented and there may be regular patterns of grain size change within them.
A complete discussion of sedimentary structures and their hydrodynamic signifi -
cance is beyond the scope of this book. Instead, representative categories of sedi-
mentary structures are grouped in Table 2.1 according to origin. Brief descriptions
are included on the characteristics that distinguish the types of sedimentary struc-

tures, their environmental significance, and their potential influence on reservoir
performance. Some common sedimentary structures are illustrated in Figure 2.5 .
Extensive discussions and illustrations of sedimentary structures can be found in

Allen (1985) , Purser (1980) , Reading (1996) , Reineck and Singh (1973) , and Tucker
and Wright (1990) .

2.3 CLASSIFICATION OF CARBONATE ROCKS

There are many classification schemes for carbonate rocks. In 1904 Grabau devised

one of the most comprehensive, but it is cumbersome and has never been popular