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Carbonate Classification by Interparticle Pore Space
(Thumbnail images with a red border are clickable links to petrographic examples)
CLASSIFICATION BY INTERPARTICLE PORE SPACE In the absence of vuggy porosity, pore-size distribution in carbonate rocks can be described in terms of particle size, sorting and interparticle porosity. The approach to size and sorting used in this petrophysical classification is similar to the grain-/mud-support principle upon which the Dunham's (1962) classification is built. Dunham's classification, however, is focused on depositional texture, whereas petrophysical classifications are focused on contemporary rock fabrics which include depositional and diagenetic textures. Therefore, minor modifications must be made in Dunham's classification before it can be applied to a petrophysical classification. Instead of dividing fabrics into grain support and mud support as in Dunham's classification, fabrics are divided into grain-dominated and mud-dominated. The important attributes of grain-dominated fabrics are the presence of open or occluded intergrain porosity and a grain-supported texture. The important attribute of mud-dominated fabrics is that the areas between the grains are filled with mud even if the grains appear to form a supporting framework. Grainstone is clearly a grain-dominated fabric, but Dunham's packstone class bridges a boundary between large intergrain pores in grainstone and small interparticle pores in wackestones and mudstones. Some packstones have intergrain pore space and some have the intergrain spaces filled with mud. The packstone textural class must be divided into two rock-fabric classes: grain-dominated packstones that have intergrain pore space or cement and mud-dominated packstones that have intergrain spaces filled with mud. Dolomitization can change the rock fabric significantly. In limestones, fabrics can usually be distinguished with little difficulty. If the rock has been dolomitized, however, the overprint of dolomite crystals often obscures the precursor limestone fabric. Precursor fabrics in fine-crystalline dolostones are easily recognizable. However, as the crystal size increases, the precursor fabrics become progressively more difficult to determine. Grainstones and grain-dominated packstones are usually composed of grains much larger then the dolomite crystal size so that dolomitized grainstones are readily identified. Dolomite crystals (defined as particles in this classification) commonly range in size from several microns to >200 microns. Micrite particles are usually <20 microns in size. Therefore, dolomitization of a mud-dominated carbonate fabric can result in an increase in particle size from <20 microns to >200 microns, and a corresponding increase in permeability as dolomite crystal size increases.
Interparticle Pore Space Classification | Petrophysics of IPS Data and Classes | Vuggy Pore Space Classification | Petrophysics of VPS Data |
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