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Abstract: Can Regional Porosity Occurrence in Carbonate Slope and Basin Facies Systems be Predicted?

S. J. Mazzullo

Subsurface carbonate depositional sequences (i.e., coeval platform-to-basin facies mosaics), which can most readily be identified by application of seismic sequence analysis, include porous units of complex geometry and occurrence. A highly sought-after occurrence of reservoir-grade porosity commonly, but not always, is associated with sequence boundaries. Therefore, porosity can be predicted to possibly occur in already-lithified platform strata beneath significant unconformities of regional extent (e.g., cavernous porosity). By this method, however, the possible existence of porosity in carbonate slope and basin deposits may be overlooked because they do not lie beneath sequence-bounding unconformities insofar as they are not usually exposed subaerially during platform emergence. Ye , such facies compose locally significant hydrocarbon reservoirs in the Permian basin by virtue of high porosities and permeabilities and water-free production histories. Short of drilling or seismic wavelet analysis, can porosity in such facies be predicted a priori?

Reservoirs in carbonate slope and basin deposits in the Midland basin include both porous limestones and dolomites. Petrographic and geochemical studies indicate that syndepositional and shallow-burial diagenesis of such deposits mainly involved porosity reduction via calcite cementation, dolomitization, and compaction. Most of the porosity present in reservoirs in these facies was created by later dissolution in the deep-burial environment as a consequence of rock interaction with connate fluids enriched in carbonic and sulfuric acid and/or organic acids generated during hydrocarbon maturation. These fluids migrated up and out from deeper parts of the basin, through slope and basin deposits and into platform carbonate strata. Insofar as fluid migration pathways can be mapped or at le st inferred by various

means, it follows that porosity created (or destroyed) by these fluids also can be mapped predictively.

AAPG Search and Discovery Article #90980©1994 AAPG Southwest Section Meeting, Ruidoso, New Mexico, April 24-26, 1994