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for example, are originally composed mostly of the mineral aragonite (CaCO

, orthorhombic), which is

very soluble. It is for this reason that formerly aragonitic particles in limestones usually are represented

by pores (in this case, fabric-selective pores) or cement-filled pores. In contrast, particles such as forams,

crinoid fragments, and bryozoans are originally composed of the mineral high-magnesium calcite

(CaCO

, hexagonal-rhombohedral), which is calcite with up to 23 mole% MgCO

in the crystal lattice.

With exposure to freshwater such particles tend merely to lose MgCO

and not to dissolve like arago-

nitic particles. Other particles, such as brachiopod shells and some pelecypods, build their skeletons out

of low-Mg calcite (also CaCO

, hexagonal-rhombohedral), which is calcite with less than 4 mole%

MgCO

. Particles of original high-magnesium calcite and low-magnesium calcite mineralogy tend not to

dissolve unless the freshwater is quite undersaturated with respect to calcium carbonate, and it is for this

reason that some particles (crinoids, bryozoans, brachiopods) often are well-preserved in ancient rocks.

The eogenetic exposure to freshwater of newly-deposited carbonate sediments, which are generally

highly porous and polyminerallic (i.e., as discussed above, composed of mixtures of aragonite, high-

magnesium and low-magnesium calcite), results in the formation of cemented limestones, of varying po-

rosity, of stable low-magnesium calcite composition (notwithstanding dolomitization). In contrast, late

eogenetic or telogenetic freshwater exposure of older limestones that have already been mineralogically

stabilized and cemented is not driven by such differences in the relative solubility of aragonite, high-

magnesium and low-magnesium calcite because the rocks already are mineralogically stabilized to low-

magnesium calcite, and further dissolution can occur only if the fluids are quite undersaturated with re-

spect to calcite (the least soluble of the aforementioned carbonate minerals). Typically, such dissolution

forms vugs and caverns, which can also form in polyminerallic carbonate sediments. Dissolution of al-

ready stabilized limestones can also result in the formation of particle-selective pores when certain parti-

cles in the rocks are slightly more soluble than other particles because of differences in particle size or

their micro-architectural arrangement of component calcite crystals. For example, crinoid fragments in

older rocks are composed of single, relatively large crystals of low-magnesium calcite, which have rela-

tively low solubility. It is for this reason that crinoid-rich Mississippian limestones, for example, typi-

cally have low porosities. In contrast, foram shells are composed of myriads of small calcite crystals,

which have relatively higher solubility, and they usually are more readily dissolved than crinoid frag-

ments. In either case, it is important to note that carbonates can be affected by meteoric dissolution not

only directly beneath unconformities on land, but also for some distance down-dip into the subsurface

("A" in Figure 2) and some distance in a seawa rd direction below sea level, depending on the extent of

freshwater lenses ("B " in Figure 2). Porosity generation by dissolution eventually ceases, generally in a

down-dip direction within phreatic zones when that water becomes saturated with respect to dissolved

calcium carbonate. At that point, porosity can be maintained, or if the water becomes even more satu-

rated with respect to dissolved calcium carbonate, it can begin to be occluded by carbonate cement (and

other cements as well, such as gypsum/anhydrite or silica).

Meteoric Porosity in Limestones

In limestones, common secondary pore types formed as a result of post-depositional dissolution

variously include exhumed interparticle, intraparticle, fenestral, shelter, and growth-framework pores, all

of which are considered to be fabric-selective pores; and also not fabric-selective vugs (Figure 3E) and

dissolution-enlarged fractures. The size of vugs (Figure 4) varies from small (but larger than component

particles in the rocks) to caverns or cavernous porosity. Vugs may originate either by wholescale disso-

lution of parts of the rock or by dissolutional enlargement of fabric-selective pores (Figure 3E).

In many cases there is coincidence between the types of fabric-selective pores present in the

rocks and the depositional environment of the rocks, which serves as an important guide in evaluating

permeability and potential recoverable reserves from the reservoir, and in deciding on what stimulation

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