Page 52 - Geology of Carbonate Reservoirs

DEPENDENT OR DERIVED ROCK PROPERTIES 33

usually skeletal in origin, can support high - porosity packing arrangements as
Dunham (1962) illustrated in photographs. As we have seen earlier in this chapter,
reef growth form and skeletal microstructure are types of biogenic rock fabric that
have a major effect on effective porosity. In some reef rocks porosity values may
be high, but effective porosity may be low because some intraparticle pores are open

to fluid movement in only one or two directions. Some intraparticle pores are, in
fact, totally disconnected. They make up part of the residual porosity in a reef
reservoir. Conversely, if large interskeletal and intraskeletal pores are present and
connected, a reef reservoir may have very high effective porosity.
Porosity in modern carbonate sediments ranges from about 40% to 70% but is
about 5% – 15% in ancient rocks (Choquette and Pray, 1970 ). Porosity reduction is
complex and can involve cementation, compaction, or combinations of the two.
Some studies show that porosity in carbonate reservoirs is reduced by a factor of 2
during burial to a depth of 1740 m and that burial depth has a greater effect on
porosity reduction than the amount of time during burial (Schmoker and Halley,
1982 ). They found that porosity in South Florida carbonates decreased exponen-
tially with depth from over 40% at the surface to less than 10% at 5486 m (see Figure
5.7 for a variety of porosity vs depth curves). They also found that porosity in dolos-
tones was lower than that of limestones near the surface, but greater than limestones
at depths greater than 1700 m, and that the rate of decrease in dolostone porosity
was less than for limestones with increasing burial depth. Budd ’ s (2001) study of
shallow Cenozoic carbonates in Florida revealed that permeability is lost more
quickly during burial than is porosity in the same rocks. He also found that low

permeability, lime - muddy rocks with median permeability values of 35 md (millidar-
cies) or less did not show a clear trend of permeability change with depth, but
limestones with median permeability of 69 to over 400 md showed a clear trend of
decreasing permeability with increasing depth of burial. He concluded that the best
limestone reservoir rocks in his study were those with grain - supported textures and
higher permeability before burial. He found that depth - related permeability loss
was due mainly to mechanical compaction in shallower depths and to chemical plus
mechanical compaction at greater depths. For limestone reservoirs in general, poros-
ity and permeability loss due to cementation is probably an early diagenetic phe-
nomenon. More pronounced porosity and permeability loss with depth is caused by
mechanical and chemical compaction. Amthor et al. (1994) found that if burial depth
is not considered, limestones and dolomitic limestones have higher porosity and
permeability than dolostones in the Devonian LeDuc Formation in Canada, but
they noted that porosity and permeability decreased with increasing depth. At
depths of up to about 2000 m, limestones and dolomitized limestones had nearly
equal values of porosity and permeability, but at depths greater than 2000 m, dolos-

tones had significantly greater porosity and permeability than limestones. They
concluded that dolostones undergo less porosity and permeability loss with depth
than limestones because dolostones are more resistant to chemical and mechanical
compaction than limestones.
Most carbonate reservoirs have porosity of about 5 – 15%, as compared with ter-
rigenous sandstone reservoirs, which have porosities of 15 – 30% (see Table 1.1 ).
The percentage of sample surface area covered by visible porosity can be used to
obtain a qualitative estimate of the “ quality ” of reservoir porosity, following Archie
(1952) :