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Railsback's Petroleum Geoscience and Subsurface Geology Categorization by time Larger Primary Trans-rock Categorization by petrographic context Whole-rock Intergranular* Intragranular* Smaller Intercrystalline* Micro, as in microporosity Conceptually impossible/ contradiction in terms Conceptually impossible/ contradiction in terms Original pore space between grains (e.g. between siliciclastic sand grains, or between ooids or bioclasts) Original pore space within grains, as in snail shells, foram tests, etc. Conceptually impossible/ contradiction in terms Original pore space in detrital clay or in microfossils. Note that this field in effect rolls around to the secondary trans-rock field at upper right: the abundant fine pores of mudrocks and chalks become significant when the rock is fractured. Secondary Fractures Categorization of pores in sedimentary rocks II Fracture Porosity left by wholesale dissolution: vugs and caverns Primary intergranular porosity Cement Grain* Vug Porosity left by dissolution of intergranular cement Primary intragranular porosity Porosity left by dissolution of grains (feldspars, aragonitic fossils) (“moldic” when complete) Secondary intragranular porosity Secondary intergranular porosity Legend: al n nt on o a i s n mi mm cca O Do Co Sandstones Pores between secondary crystals, as in dolostones Limestones Dolostones Chalks Pore space in diagenetic clay (space may be primary, but pore is secondary) *In the discussion of sedimentary rocks, “grains” include siliciclastic grains, fossils, ooids, and pellets, and they are effectively any sedimentary depositional Mudrocks (shales, mudstones) material distinguishable with a handlens. (Note that igneous and perhaps metamorphic petrologists use "grain" to be nearly synonymous with "crystal". In sedimentary petrology, a grain can consist of one or many crystals, and crystals of cement are definitely not grains).