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Because the natural tendency in most carbonate sediments is that primary porosity is substan-
tially reduced by cementation and compaction during post-depositional burial (Figure 1; Halley and
Schmoker, 1983), many workers would argue that most porosity in limestone and dolomite reservoirs
is of secondary origin (e.g., Mazzullo and Chilingarian, 1992). Exceptions to this statement are cases
where primary porosity is preserved because of the early influx of hydrocarbons into pores (e.g.,
Feazel and Schatzinger, 1985). Early on in the study of carbonate sediments and their diagenesis, the
subaerial meteoric diagenetic (freshwater) model was promoted as a means of explaining porosity
evolution in carbonates, specifically in shallow-water carbonates that lie beneath unconformities in pa-
leo-vadose and paleo-phreatic freshwater zones (e.g., Friedman, 1964; Land, 1967). This model still is
heavily applied today, especially in sequence stratigraphic-related diagenetic studies of reservoirs. This
model presupposes that if porous carbonate rocks are present beneath unconformities, then that poros-
ity must have been created by freshwater dissolution during subaerial exposure. Of course, as explora-
tionists we all can probably list a large number of wells that were drilled into non-porous carbonate
rocks beneath unconformities. Hence, the corollary pertaining to subaerial exposure is not true mete-
oric exposure does not always create porosity, and even if it did, that porosity may be occluded during
later burial (Figure 1). A most critical constraint on evaluating, or more importantly, on predicting po-
rosity in carbonate rocks utilizing only the subaerial meteoric diagenetic model is that one must call
upon fluids capable of dissolving carbonate to come from above, that is, from rain water percolating
down into sediments or rocks beneath unconformity surfaces. Ostensibly, then, many might consider
that if there is not an unconformity in the section, then the carbonates will not be porous. Again, as ex-
plorationists we can all probably compile a list of wells in which porous carbonates that were not asso-
ciated with unconformities were encountered in the subsurface.
The foregoing analysis therefore begs the following questions: (1) can secondary porosity in
carbonate rocks be generated by processes other than subaerial meteoric exposure, and if so, what are
those processes?; (2) how might reservoir porosity formed by such alternative processes differ from
reservoirs created by meteoric dissolution along unconformities?; (3) how can we recognize and deter-
mine the origin of reservoir porosity?; and (4) can the subsurface occurrence of porosity formed by any
such alternative models of reservoir origin be predicted? The purpose of this contribution is to address
these questions by demonstrating the multi-faceted evolution of secondary porosity in carbonate hy-
drocarbon reservoirs. In the following discussions attention will be focused on the recognition and ori-
gin of pore types in shallow-water limestones and dolomites as observed mainly in cores and well cut-
tings samples.
SECONDARY POROSITY BENEATH UNCONFORMITIES: THE SUBAERIAL
METEORIC MODEL
The generation of secondary porosity in carbonate sediments or rocks in this model is a direct
consequence of dissolution by freshwater (ultimately rain-derived), which dissolves carbonates be-
cause the water is undersaturated with respect to calcium carbonate. The extent of dissolution and sec-
ondary porosity formation are controlled by factors such as the acidity of freshwater (e.g., rain water
percolating down through a soil zone will be more acidic than in areas where soils are not present), the
amount of porosity or fractures within the affected carbonates, the residence time of the freshwater in
the diagenetic system, the mineralogy of the sediments or rocks, and so forth (Longman, 1980; Moore,
1989). Secondary porosity generation via dissolution can occur relatively soon after deposition, in un-
consolidated sediments, in what Choquette and Pray (1970) refer to as the eogenetic zone; or it can oc-
cur much later, in rocks, in the telogenetic zone as a consequence of uplift of older, formerly buried
carbonates (Figure 2). In newly-deposited carbonate sediments that subsequently are subaerially ex-
posed, it is the difference in original mineralogies of particles in the sediments that drives the relatively
rapid, selective dissolution of particles. Fragments of corals, pelecypods and gastropods, and oolites,
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