--> Abstract: Predicting Porosity Relations in Pennsylvanian Limestone Reservoirs, by Alonzo D. Jacka; #90970 (1977).

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Abstract: Predicting Porosity Relations in Pennsylvanian Limestone Reservoirs

Alonzo D. Jacka

Predicting porosity relations in Pennsylvanian limestone reservoirs requires reconstruction of (1) the original carbonate sedimentary facies, biofacies, and depositional environments; (2) the geologic and structural history; and (3) the diagenetic history.

Although primary porosity is present in Pennsylvanian limestone reservoirs, most porosity is secondary and records selective dissolution of aragonitic shells and ooids by meteoric waters. Primary porosity thus reflects original sedimentary facies and biofacies patterns. Most primary and secondary porosity becomes occluded by precipitation of carbonate cements.

Occlusion and preservation of porosity predominantly reflect differential rates of cement precipitation within voids. Calcite cement is precipitated on the following carbonate substrates at progressively decreasing rates: (1) monocrystalline particles (echinoderm components); (2) prismatic shells; (3) fibrous shells; (4) solution cavities within micrite matrix; and (5) peloids, micritized shells and ooids, and micrite envelopes. On crystalline shell materials calcite cement forms optically continuous overgrowths on individual crystals, with most precipitation occurring on largest crystals. Cementation is much slower in micrite-walled solution cavities and on peloids, micritized shells and ooids, and micrite envelopes than on crystalline shell materials. These micritic foundations may require a nucleation step to initiate cementation, whereas precipitation on crystalline shell substrates represents only crystal growth.

Meteoric vadose-phreatic ground-water dynamics constitute another very important equation in the comprehension of porosity relations. Because many eustatic sea-level fluctuations occurred during the Pennsylvanian, most carbonate deposits were subjected to subaerial exposure and freshwater diagenesis soon after deposition. Diagenetic reactions occur at much greater rates in phreatic than in vadose zones, and there is a strong tendency for porosity to become occluded in phreatic zones and preserved in vadose zones.

In attempting to predict porosity relations it must be emphasized that formation, occlusion, and preservation of porosity reflect diagenetic overprints superimposed upon original lithofacies and biofacies, but diagenesis always exerts the predominant control.

AAPG Search and Discovery Article #90970©1977 AAPG Southwest Section Meeting, Abilene, Texas