Page 127 - Geology of Carbonate Reservoirs

108 DEPOSITIONAL CARBONATE RESERVOIRS

and burial history; therefore it can also be useful in tying flow units, baffl es, and
barriers to the larger stratigraphic architecture, enabling one to generate predictive

maps and sections of ranked flow units at fi eld scale.

5.1 DEPOSITIONAL POROSITY

Depositional processes are those by which sedimentary constituents accumulate to
form rocks. Depositional sedimentary constituents include detrital grains, crystalline
precipitates (as precipitated micrite), and biogenic material such as skeletal compo-
nents and microbialites. Some cements are so intimately associated with microbial-
ites that for practical reasons they can be included as depositional components.
Carbonate rocks are detrital, biogenic, or chemical in origin. Detrital carbonates
exhibit four basic pore types (see Figure 2.13 ): (1) intergranular, (2) intragranular,
(3) shelter or keystone, and (4) fenestral pores. Intergranular pores, those between
grains in a detrital rock, may be both fabric and facies selective. Porosity is highest

in rocks with the least mud; therefore the Dunham rock classification is a proxy for
this pore type. Petrophysical characteristics of intergranular pores can be estimated
using Lucia ’ s (1983) classification for interparticle porosity and his x - y plot of poros-

ity, permeability, and particle size. If the relationship between petrophysical proper-

ties and Dunham rock classification can be established, and if it is consistent within

facies, then the Dunham classification alone can be used to describe both petro-

physical and depositional characteristics of each flow unit in the reservoir.
Intragranular pores, those within grains, may exist naturally in skeletal grains or
in diagenetically altered grains of any origin. Usually they occur in porous, skeletal
allochems, which make up varying amounts of detrital sediments. Bryozoans, for
example, have internal pores, as do corals, sponges, stromatoporoids, and many other
reef - building organisms. Some mollusks such as rudistid clams have large internal
cavities that may contribute greatly to total porosity, but little to permeability. In a
bioclastic grainstone reservoir consisting of sand - sized bryozoan fragments, the
bryozoans may contribute significantly to total porosity (Ahr and Walters, 1985 ). In

that case, intragranular porosity corresponds with the distribution of skeletal grain-
stones and packstones and, in turn, with facies maps.
Shelter and keystone pores in detrital rocks are not common enough to be sig-

nificant contributors to total reservoir porosity. Shelter pores are formed when large
grains such as bivalve segments act as shelters or “ umbrellas ” and prevent detrital
grains from filling pore space beneath the shelter of the large grain. Keystone pores

are formed when the pounding of breaking waves expels air from beach sands. As
sand grains are repacked under this hydraulic pounding, the odd grain may fall in
a position similar to the keystone in an arch and prevent the sand packing arrange-
ment from reaching maximum density.
Fenestral or “ bird ’ s - eye ” pores result from desiccation or from expulsion of gas
during decay of organic matter in muddy sediment. The pores may be millimeter to
centimeter in size, they are elongate and planar in a direction parallel to bedding,

and they are especially common in tidal - flat environments where sediments are
alternately wet and dry. In such cases, fenestral pores become facies selective and
the facies map is interchangeable with the reservoir porosity map. Fenestral pores

may not be in flow communication with each other; consequently, they may have