Abstract: Mesogenetic Porosity Alteration in Carbonate Sediments
The physicochemical subsurface environment and the mineralogic composition and fabric of carbonate sediments control the course of mesogenetic porosity alteration. Specific factors sometimes can be identified which play key roles in preserving and increasing or decreasing and eliminating effective reservoir porosity. Factors increasing porosity include the following: (1) generation of carbon dioxide during maturation of kerogen results in leaching by carbonic acid; (2) strong lithification prevents mechanical compaction, inhibits contact solution, increases susceptibility to fracturing; (3) presence of hydrocarbons inhibits chemical reactions; and (4) normal hydrostatic pressure, normal geothermal gradients, inactive hydrodynamics preserve chemical stability. Mixed charge occur when: (1) geopressures and abnormal geothermal gradients may induce fracturing, leaching, or cementation; (2) an active hydrodynamic system may cause leaching or cementation; (3) tectonic stress causes fracturing, compaction, pressure solution; (4) influx of water released by compaction and mineral dehydration may cause leaching and cementation; and (5) hydrothermal activity may cause leaching, dolomitization, and cementation. Factors decreasing porosity are: (1) admixture of fine terrigenous clastics promotes contact solution-reprecipitation; (2) high percentage of unstable carbonate minerals promotes cementation and contact solution-reprecipitation; (3) migration of dissolved-mineral constituents from adjacent clastic rocks, evaporites, and shaly carbonate and igneous rocks may ead to cementation; and (4) increasing depth of burial leads to compaction and contact solution of incompetent and shaly carbonate sediments.
No carbonate sediment is immune to complete destruction of effective porosity during mesodiagenesis. On the other hand, any clean and previously tight carbonate rock is capable of acquiring reservoir-grade porosity/permeability in the regime of mesodiagenesis. In competent carbonate reservoir rocks porosity may be preserved for several hundred million years of subsurface residence in any but extreme depths of burial.
AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC