27
sented in a given reservoir. As in limestones, the nature of pore throats in dolomites affects perme-
ability, and as a general rule, intercrystalline pore throat sizes decrease with decreasing crystal size
and extent of dolomitization (Figure 8).
Cavernous Porosity in Carbonate Rocks
Cavernous and associated vuggy porosity are major attributes of hydrocarbon production
from reservoirs such as the Arbuckle Group in Kansas (Walters, 1946; Newell et al., 1987) and Okla-
homa (Gatewood, 1970), and from its stratigraphic correlative, the Ellenburger Group, in west Texas
and New Mexico (Holtz and Kerans, 1992). Additional examples of hydrocarbon reservoirs in paleo-
caverns are given in Mazzullo and Chilingarian (1996). Only rarely are completely fluid-filled cav-
erns encountered in the subsurface. Rather, paleo-caverns usually are filled by porous (or, unfortu-
nately, sometimes tight) cave-roof collapse breccia and associated sediments and/or by overlying,
younger rocks (Figure 9). Rather than being single zones, paleo-caverns typically are labyrinthine
systems characterized by extreme lateral and vertical reservoir compartmentalization (Figure 9). Cav-
ernous porosity undoubtedly also locally contributes to hydrocarbon production from some Missis-
sippian reservoirs in Kansas. I have encountered a number of instances in Kansas, for example,
where wellsite geologists= reports picked the top of the Mississippian at a certain depth, and then
the limestone or dolomite directly below seemingly was underlain by a section of sand (which I pre-
sume to be Pennsylvanian-age siliciclastic sand) that is, in turn, underlain by more carbonate rock.
Such occurrences may indicate that the wells penetrated sand-filled paleo-caverns (Figure 10).
This article will be continued in the March--April Bulletin
Selected References will be published with the conclusion of this article.
Figure 4. Core slab with secondary vugs (arrows) re-
sulting from the partial dissolution of earlier
dolomite cement (white).
Figure 5. Thin-section photomicrograph of calcite cement
overgrowths on crinoid fragments that
occlude interparticle porosity.
Continued from pg. 26
More figures on page 28