Page 30 - Geology of Carbonate Reservoirs

UNIQUE ATTRIBUTES OF CARBONATES 11

TABLE 1.1 A Comparison of Terrigenous Sandstone and
Carbonate Reservoir Characteristics
Reservoir Characteristic Terrigenous Sandstones Carbonates
Amount of primary 25 – 40% 40 – 70%
porosity
Amount of ultimate Half or more of Small fraction of original porosity,
porosity primary porosity, commonly 5 – 15%
commonly 15 – 30%
Type of primary porosity Almost exclusively May be interparticle, intraparticle,
interparticle intercrystalline, moldic, vuggy,
cavernous, fenestral, or
“ constructed void ”
Type of ultimate porosity Almost exclusively Highly variable owing to different
primary interparticle origins or pore types
Typical pore size Diameter and throat Diameter and throat size may not
sizes related to be related to depositional texture
depositional texture
Typical pore shape Dependent on particle Varies from strongly related to
shape; typically a particle shape to totally unrelated
“ negative ” of
particles
Uniformity of pore size May be relatively Variable from fairly uniform to
and shape distribution uniform in extremely heterogeneous — even
homogeneous sand within a body made up of a single
bodies rock type
Influence of diagenesis Minor — usually Major — can create, obliterate, or

reduction by totally modify porosity;
compaction or cementation and solution
cementation important
Influence of fracturing Not of major Major importance in reservoir

importance in properties if present
reservoir properties
Visual estimation of Semiquantitative Semiquantitative estimates may be
porosity and estimates may be easy or impossible; instrumental
permeability relatively easy measurements commonly
required
Adequacy of core Core plugs of 1 - inch Core plugs commonly inadequate;
analysis for reservoir diameter may be may require whole core analyses
evaluation adequate to (∼ 4 - inch by 1 - foot segments) for
determine matrix large pore sizes
porosity
Porosity – permeability Relatively consistent; Highly varied; may be independent
relationships may be dependent on of particle texture
particle texture
Reliability of log Standard practice that Not reliable because logs cannot
characteristics as may provide reliable generally detect differences in
indicators of proxies for carbonate grain types or textures
depositional facies depositional facies
(electrofacies mapping)
Source : Adapted from Choquette and Pray (1970).