Page 176 - Geology of Carbonate Reservoirs

DIAGENETICALLY ENHANCED POROSITY 157

from zone to zone and well to well. This process allows mapping of flow units and

flow barriers at fi eld scale.
Diagenetic processes that create or enhance porosity are dissolution, replace-
ment, and recrystallization. Dissolution of solid rock in undersaturated waters pro-
duces molds, vugs, caverns, and channels with or without collapse features (commonly
associated with caves and karst features), and solution - enhanced interparticle pores.
These pore types share a common origin and differ in degree rather than kind.
Common origin suggests similar geological settings in which the pores formed and
helps geologists eliminate settings that were unlikely to have been sites for dissolu-
tion. Geological settings where dissolution is common include the top of the mete-
oric phreatic zone, the mixing zone, and parts of the vadose zone. Dissolution can
also occur in the subsurface when rocks and water are out of chemical equilibrium.

Rock – water interaction where undersaturated fluids are present may be involved
in stabilization reactions that alter metastable carbonates to stable ones (recrystal-
lization, also called neomorphism). Weathering and soil - forming processes at uncon-
formities can involve a combination of diagenetic processes including dissolution,
precipitation, biological activity, and neomorphism. Soils and lightly weathered
zones are not usually important as reservoir rocks because matrix pore sizes in car-
bonate soils are small and capillary pressures are correspondingly high.
Dissolution requires undersaturated water, usually meteoric or mixing - zone
water, to interact with carbonate country rock. Caves are formed in both vadose
and meteoric phreatic environments as illustrated in Figure 6.4 . An idealized cave
system with both vadose and phreatic characteristics is shown in Figure 6.5 . Dissolu-
tion is common as corrosion and pore enlargement in deep - burial settings, too.
Deep - burial dissolution is also called mesogenetic dissolution , following the termi-
nology in the Choquette and Pray (1970) carbonate porosity classifi cation (Mazzullo

Figure 6.5 Sketch of an idealized cave system showing both vadose and phreatic diagenetic
characteristics. Note the brecciated material on the cave floors and the “ crackle breccia ” on

the ceiling. Although buried paleocave reservoirs may have large open caverns, they are more
likely to be filled with collapse breccias and other forms of internal sediment. (From Loucks

(1999) . Reprinted with permission from the American Association of Petroleum
Geologists.)