Textural Spectrum of Secondary Sandstone Porosity
The textures in which secondary sandstone porosity occur must be classified in such a manner as to make their description objective, informative and manageable. Several descriptive groups of pore textures can be differentiated in sandstones using, in part, the nomenclature for pore textures that Choquette & Pray (1970) proposed for carbonate rocks. These groups include the following:
intergranular pore textures;
oversized pore texture;
moldic pore textures;
intra-constituent pore textures;
fracture pore textures.
Each group of pore textures consists of two or more distinct types of pore textures. The textural spectrum of secondary sandstone porosity is represented by these types of pore textures as they occur in various genetic classes of secondary porosity (Table 1).
Pores between grains form the textural group of intergranular pores (Figure 10). The intergranular pores may be lined by fringing cement or syntaxial cement. Three types of secondary intergranular pore textures can be distinguished: (1) regular intergranular; (2) reduced intergranular; and (3) enlarged intergranular.
Regular intergranular pore texture essentially reflects depositional intergranular spaces in size and shape (Figures 11 & 12), and is identical with primary intergranular pore texture. The secondary pores may occupy intergranular spaces entirely or they may share individual intergranular spaces with matrix, cement, replacement or primary porosity thus forming “complete” or “partial” secondary pores of this texture.
Secondary pores of regular intergranular texture form a significant percentage of secondary sandstone porosity. They result from the following: (1) shrinkage of intergranular sedimentary matrix (e.g. shrinkage of glauconite matrix); (2) dissolution
Figures & Tables
Secondary porosity n sandstones can be classified according to origin and pore texture. Five significant genetic classes of secondary porosity are defined by processes of origin: Fracturing; shrinkage; dissolution of sedimentary grains and matrix; dissolution of authigenic pore filling cement; and dissolution of authigenic replacive material. This publication provides information on the genetic-textural classes of secondary sandstone porosity; the textural spectrum of secondary sandstone porosity; the recognition of secondary sandstone porosity; the geological occurrence and diagenetic origin of secondary sandstone porosity; the textural stages of sandstone mesodiagenesis; the diagenesis of quartz arenites; the diagenesis of sandstones of intermediate and low mineralogical maturity; examples of porosity distribution, and reservoir aspects.