Page 29 - Geology of Carbonate Reservoirs

10 INTRODUCTION

mainly of biogenic constituents, component grains may have undergone size and

shape changes as they were eaten by organisms, and the stratification of carbonate

rock bodies is extensively modified by burrowing and boring organisms. The third
major difference is that carbonates are susceptible to rapid and extensive diagenetic
change. Carbonate minerals are susceptible to rapid dissolution, cementation,
recrystallization, and replacement at ambient conditions in a variety of diagenetic
environments. Finally, although not stressed by Ham and Pray (1962) , fractured
reservoirs are probably more common in carbonate rocks than in siliciclastics (as
indicated in Table 1.1 ), but work by Laubach (1988, 1997) and Laubach et al. (2002)
suggests that fractures are more common in siliciclastic reservoirs than was previ-
ously recognized. In short, porosity and permeability in carbonate reservoirs depend
on a broad array of rock properties, on diagenetic episodes that may continue from
just after deposition through deep burial, and on fracture patterns related more to

the geometry of stress fields than to rock type. Choquette and Pray (1970) high-
lighted some of the differences between carbonate reservoirs and those in siliciclas-

tics. A summary of their findings is given in Table 1.1 .
Three other significant differences between carbonate and terrigenous sand-

stone reservoirs are: (1) electrofacies maps from gamma ray and resistivity log data
do not indicate depositional facies in carbonates as they can do with terrigenous
sandstones; (2) Focke and Munn (1987) demonstrated that a strong relationship
exists between pore type and petrophysical characteristics in carbonate reservoirs
such that saturation calculations using the Archie equation will vary greatly

depending on the chosen “ m ” exponent and its dependence on the proportion of
vuggy and moldic pores compared to interparticle pores; and (3) carbonates form
in temperate as well as tropical environments. Because temperate carbonates have
decidedly different mineralogical and component grain type compositions from
tropical carbonates, their reservoir characteristics could also be different than
expected.

Differences between sandstone and carbonate reservoirs influence the way
we study them. Sandstone porosity is mainly interparticle; therefore it is related
geometrically to depositional texture and fabric. Because permeability usually
correlates rather well with interparticle porosity in sandstones, it can be related to
depositional texture and fabric, as illustrated in a study of pore geometry in sphere
packs and in terrigenous sandstones (Berg, 1970 ). Assuming that porosity and
permeability are closely related, laboratory measurements made on small core
plugs of terrigenous sandstones may be assumed to be representative of large rock
volumes. That is, small samples are representative of large populations if the popula-
tions are homogeneous. Carbonates do not always exhibit interparticle porosity;
they may have a variety of pore sizes, shapes, and origins, and measured porosity
values do not always correspond closely with permeability. In short, carbonate pore
systems are not usually homogeneous. While a 1 - inch perm - plug will provide reliable
data on sandstone porosity and permeability, entire core segments 4 inches in diam-
eter and 1 foot long may be required for reliable measurements on carbonates.

Relatively simple porosity classification schemes are useful for siliciclastics but a

compound scheme of genetic classification augmented by measurements of pore
geometry is needed for carbonates. Carbonate porosity classifications are discussed

in Chapter 2 .