Genetic-Textural Classes of Secondary Sandstone Porosity
Secondary porosity in sandstones is mainly the result of dissolution of non-silicate constituents, predominantly carbonate minerals. The soluble precursors of this dissolution porosity occur in three textural forms: sedimentary material, authigenic cement, and authigenic replacement. Fracturing and shrinkage of rock constituents may also create significant secondary porosity in sandstones, though usually in subordinate amounts.
It is useful, for geological interpretations, to base the definition of the principal classes of secondary porosity on textural origin. In addition, the textures in which secondary porosity occurs must be classified in such a manner as to make their description easy and objective.
We recognize five principal classes of secondary porosity that can be differentiated on the basis of process of origin and textural relationships (Figure 2)
porosity created by fracturing;
porosity created by shrinkage;
porosity created by dissolution of sedimentary constituents;
porosity created by dissolution of authigenic cementing minerals;
porosity created by dissolution of authigenic replacive minerals.
Porosity created by fracturing encompasses any newly formed fractures (Figure 3) including those formed due to stresses resulting from shrinkage of rock constituents or whole rocks. If fractures become filled by cement or sediment then these fillers or their replacive successors may be selectively dissolved giving rise to second cycle fracture porosity. Such reopened fractures would be included in the appropriate class of dissolution porosity.
Shrinkage porosity, forms through dehydration and/or recrystallization of a number of minerals such as glauconite (Figure 4) and hematite, or aggregates such as mud. It excludes shrinkage features. Shrinkable