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OVERVIEW OF POROSITY EVOLUTION IN CARBONATE RESERVOIRS
S. J. Mazzullo
Department of Geology
Wichita State University
INTRODUCTION
Carbonate rocks (limestone and dolomite) account for approximately 50% of oil and gas produc-
tion around the world. Of the carbonates, a slightly greater percentage of world hydrocarbon reserves
has been produced from dolomites because such rocks commonly, but not always, have more porosity
and permeability than limestones (Halley and Schmoker, 1983). Unlike most sandstone reservoirs,
which typically are single-porosity systems (i.e., interparticle pores) of relative uniform (homogeneous)
nature, reservoirs in carbonate rocks commonly are multiple-porosity systems that characteristically im-
part petrophysical heterogeneity to the reservoirs (Mazzullo and Chilingarian, 1992). Hence, the specific
types and relative percentages of pores present, and their distribution within the rocks, exert strong con-
trol on the production and stimulation characteristics of carbonate reservoirs (e.g., Jodry, 1992; Chilin-
garian et al., 1992; Honarpour et al., 1992; Hendrickson et al., 1992; Wardlaw, 1996). Pore types in car-
bonate rocks can generally be classified on the basis of the timing of porosity evolution (Choquette and
Pray, 1970) into: (1) primary pores (or depositional porosity), which are pores inherent in newly-
deposited sediments and the particles that comprise them. Such pore types include interparticle pores in,
for example, carbonate sands (but also in muddy carbonates), intraparticle pores (within particles such
as foraminifera or gastropod shells), fenestral pores (formed by gas bubbles and sediment shrinkage in
tidal-flat carbonates), and shelter and growth-framework pores (common in reef buildups); and (2) sec-
ondary pores, which are those that form as a result of later, generally post-depositional dissolution. Such
pore types include all of those mentioned above, but only when it can be demonstrated that primary
pores which subsequently were occluded by cement later had all or some of that cement dissolved
(resulting in the generation of exhumed pores Figure 2), as well as vugs (large pores that transect rock
fabric, that is, dissolution was not fabric-selective) and dissolution-enlarged fractures. Most of these pri-
mary and secondary pore types can readily be identified in cores, and with the possible exception of
shelter and growth-framework pores, also in well cuttings samples.
Figure 1. Processes by which porosity is reduced in carbonate
rocks. Syndepositional marine cementation occurs only in the
eogenetic zone, and mechanical compaction is unlikely to affect
telogenetically-exposed older carbonate rocks.
Figure 2. Meteoric subaerial exposure of newly-deposited
sediments in the eogenetic zone, and of older carbonate
rocks in the telogenetic zone. Note that the freshwater lens in
the eogenetic zone can extend some distance below sea level
("B"), and that freshwater may extend down -dip for a con-
siderable distance in the telogenetic zone ("A").