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procedures to use but only if one knows the depositional environment of the rocks from study of subsur-
face samples. For example, carbonate sands (lime grainstones), deposited in high-energy environments
such as oolite shoals or skeletal sand shoals, commonly have high interparticle porosity (Figure 3A) and
attending relatively high permeabilities. On the other hand, however, high porosity but low permeability
may characterize a carbonate sand (limestone) reservoir wherein only the particles have been dissolved
(for example, in cases where the reservoir contains only molds of oolites referred to as oomoldic poros-
ity or by the older term oocastic porosity: Figure 3D). In such cases there may be ample hydrocarbon
storage volume in the pores in the rocks, but in the absence of fractures, there is little interconnected po-
rosity. Notwithstanding porosity associated with dolomitization (discussed later), limestones deposited
in tidal-flat environments commonly contain a specific type of vuggy porosity referred to as fenestral
pores (for example, birdseye pores, which is one type of pinpoint porosity: Figure 3C), and unless frac-
tured, such rocks may have decent porosity but limited permeability. Skeletal sands shoals wherein the
particles mainly are foraminifera, which are common in midcontinent Pennsylvanian limestone reser-
voirs (Wilhite and Mazzullo, 2000), may be highly porous and permeable because of the presence of in-
terparticle pores, and within the forams, of intraparticle pores as well (Figure 3B). On the other hand, if
intraparticle pores are the only pore types present, then porosity (and hydrocarbon storage volume)
might be high but permeability would be low (notwithstanding fracturing). As a corollary, variations in
porosity and permeability from well-to-well within a given zone may be a consequence of different de-
positional environments in that zone and/or from differing extents of porosity generation versus occlu-
sion between wells. Only study of cores/cuttings and thin sections can resolve the possible reasons for
such variations between wells.
Over-riding such generalizations about the relationships among pore types, permeability, and
depositional environments of the limestones is the importance of pore throats in the rocks (Wardlaw,
1976). In limestones, particularly in grainstones, for example, the nature of pore throats and their effect
on permeability is controlled by the size of the particles in the rocks, and more importantly, by the distri-
bution of any remaining earlier-precipitated cement in the pores that wasn't dissolved (Figure 3F). Cal-
cite cement overgrowths on crinoid fragments can significantly restrict pore throats as well (Figure 5),
which is why many crinoid-rich Mississippian limestones are of low-permeability nature. The best way
to determine the extent of pore-throat restriction in the rocks under consideration is by examining the
rocks petrographically in thin section. Clay-mineral cements are extremely rare in carbonate reservoir
rocks, and therefore, need not be considered here.
Meteoric Porosity in Dolomites
In contrast to earlier postulates on the subject (e.g., Murray, 1960; Weyl, 1960), the process of
dolomitization of a pre-existing limestone does not automatically create secondary porosity. Whereas it
is true that porosity tends to increase as amount of dolomite increases (Figure 6), it generally does so for
the following reasons. In partly dolomitized limestones exposed to telogenetic meteoric fluids, for exam-
ple, any remaining calcite (which may represent particles and/or carbonate mud matrix) inherently is
more susceptible to dissolution by freshwater because it is more soluble than dolomite. Hence, subaerial
exposure of a partly dolomitized limestone can result in the generation of the same types of pores as de-
scribed above by dissolution of remaining calcite, depending on the original texture of the rock
(mudstone, wackestone, packstone, or grainstone), its depositional environment, and degree of replace-
ment by dolomite (Figure 7). Likewise, remaining evaporites in the rocks can also be dissolved. In more
pervasively dolomitized rock exposed to telogenetic meteoric fluids, remaining calcite (or evaporite
minerals) between dolomite crystals can be dissolved during subaerial exposure to produce intercrystal-
line pores between dolomite crystals. In completely dolomitized rocks, vugs (and sometimes dissolution-
enlarged fractures) are common pore types present if the meteoric fluids were highly acidic or acted on
the rocks over long periods of time. Selective dissolution of small dolomite crystals (because solubility
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