28
Figure 6. Relationship of porosity to percent
dolomite in carbonate rocks (after Murray,
1960).
Figure 7. Origin of secondary dissolution porosity in dolo-
mites.
Figure 8. Intercrystalline pores and pore throats in dolo-
mites. Relative size of pores and pore throats is not nec-
essarily correlative to dolomite crystal size because po-
rosity is a percentage of total rock volume. Pore throat
characteristics, however, do reflect the degree (extent)
of dolomitization in rocks. Dolomites with polyhedral
pores generally are referred to as "sucrosic".
Figure 9. Top
Typical lateral and vertical compartmentalization of
reservoir zones in cavernous (karsted) carbonate rocks. Bottom
Typical porosity types and fills of cavernous reservoirs. Cave roof
rocks become progressively more brecciated downward, with attend-
ing fracture, dissolution-enlarged fracture, and commonly, vuggy
porosity. Cave-fill deposits variously can be: (1) cave roof-collapse
breccia, which can have inter-clast porosity as well as intra-clast
vuggy and fracture porosity. Conversely, original inter-clast porosity
can be filled with cements and/or shale, or can be filled with porous
sand; (2) impermeable shale infiltered into the cavern from above; or
(3) porous or tight sand infiltered into the cavern from above. Cave
roof collapse and infiltering of sand and/or shale can occur soon
after karstification or later.
Figure 10. On the left is a partial stratigraphic column of what I've sometimes
encountered in the Mississippian in Kansas a few feet of carbonate below the
top of the Miss, underlain by sandstones (which are not laterally correlative for
any distance), in turn underlain by more carbonate. On the right is a possible
interpretation of such a stratigraphy that the wells in question encountered
sand-filled caverns below the top of the Miss.