Abstract: Factors Controlling Porosity Relations in Limestone Reservoirs

Alonzo D. Jacka

Both primary and secondary porosities commonly are represented in limestone reservoirs. Primary porosity is in intrabiotic cavities, sheltered voids among frame builders, or algal blades and in grainstones.

Formation of secondary porosity begins with selective dissolution of unmicritized aragonitic shells and ooids in freshwater diagenetic environments. Solution of aragonitic shells commonly occurs before lithification of matrix is completed and partial collapse of molds forms intersecting crumbly fractures. Progressive solution transforms fossil molds into irregular vugs and crumbly fractures into solution channels. These limestone reservoirs have secondary-solution-cavity porosity, but generally have dense matrices with little or no effective porosity.

Preservation of both primary and secondary porosity predominantly reflects differential rates of calcite-cement precipitation within voids. Calcite cements are precipitated on the following substrates at progressively decreasing rates: (1) monocrystalline grains; (2) prismatic shell layers; (3) fibrous shell layers; (4) solution cavities within micrite matrix; (5) peloids, micritized shells and ooids, and micrite envelopes.

Calcite cement precipitates on crystalline shell materials as optically continuous overgrowths on individual crystals. Rates of precipitation reflect degrees of epitaxial response with greatest rates being on largest crystals.

Calcite cements are precipitated much more slowly on micrite-walled solution cavities and on peloids, micritized shells and ooids, and micrite envelopes than on crystalline-shell materials. These substrates may require a nucleation step to initiate cementation whereas crystalline shell foundations require only crystal growth. Peloids, micritized shells and ooids, and micrite envelopes constitute much less favorable substrates for cement precipitation than micrite-walled solution cavities. After all other voids have been filled by cement, porosity tends to be preserved among peloids, micritized shells and ooids, and on micrite envelopes.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC