Abstract: Factors Controlling Porosity Relations in Pennsylvanian and Permian Carbonate Reservoirs of Permian Basin

Alonzo D. Jacka

In Pennsylvanian and Permian carbonate deposits of the Permian basin it is possible to compare and contrast major differences that characterize limestone and dolostone reservoirs. Nearly all Pennsylvanian production is from limestones, whereas most Permian production is from dolostones.

Both primary and secondary porosity are present in Pennsylvanian limestones. Primary porosity occurs in algal mounds, bryozoan bioherms, and oolitic grainstones. Micritization of skeletal material and ooids has played a very important role in preserving primary porosity. Formation of secondary porosity in limestones begins with selective dissolution of unmicritized aragonitic shells and ooids in freshwater diagenetic environments. Commonly, dissolution preceded complete lithification of matrix, and partial collapse of molds formed crumbly fractures. Progressive solution stoping converted molds into irregular solution vugs and crumbly fractures into solution channels. Limestones with secondary porosity have very dense matrices with little or no porosity.

Preservation of both primary and secondary porosity in limestone reservoirs reflects differential rates of precipitation of calcite cements within voids. Greatest rates of precipitation of calcite cement occur on unmicritized, crystalline shell foundations, with the highest rate occurring on monocrystalline echinoderm components. Lowest rates of calcite cement precipitation occur on micritized shell components and micrite-walled solution cavities.

In contrast to limestone reservoirs, most dolostone reservoirs have matrix porosity. This reflects dolomitization of aragonitic lime muds to form neomorphic fabrics with intercrystalline porosity. Primary porosity and secondary solution cavity porosity are essentially lacking in Permian dolostone reservoirs.

Many Permian dolostones record dolomitization of calcitic limestones that possessed well-developed leached porosity. These are designated as paramorphic dolostones, because previous calcitic textures and fabrics have been perfectly preserved after dolomitization. during dolomitization, calcium ions that were exchanged for magnesium ions were precipitated as anhydrite cement which occluded all porosity within secondary solution cavities. These paramorphic dolostones now constitute seals over reservoirs formed by neomorphic dolomitization of aragonitic lime muds.

APG Search and Discovery Article #90974©1975 AAPG Mid-Continent Section Meeting, Wichita, Kansas