Textural Stages of Mesodiagenesis
The improved ability to differentiate between primary and secondary porosity permits the tracing of the diagenetic alteration of each of the two porosity classes in response to progressive burial. This provides an excellent means of relating mesogenetic sandstone diagenesis to burial history. We propose four textural stages of sandstone mesodiagenesis that are defined by four petrographically recognizable stages of the burial diagenesis of sandstone porosity (Figure 43). These are in order of progressive burial: (1) immature stage; (2) semi-mature stage; (3) mature stage; and, (4) super-mature stage.
The immature stage is characterized by mechanical compaction of clean, uncemented sands with primary intergranular porosity (Figure 44). The compaction is accomplished by deformation of ductile grains, rotation of grains, and fracturing of grains. It reduces both primary rock volume, and primary porosity and permeability. Chemical compaction as defined below is negligible or absent. Any significant chemical compaction would tend to lithify the rock and make appreciable mechanical compaction impossible. Secondary porosity may exist. It can either be inherited from eodiagenesis or may originate during the immature stage.
The semi-mature stage is marked by the onset of pervasive chemical compaction, while, at the same time, mechanical compaction of primary porosity becomes insignificant or ceases entirely. We define chemical compaction as being essentially accomplished through dissolution of sand grains at points and interfaces of contact. In the process rock volume and the percentage of intergranular primary porosity are diminished. The dissolved material may reprecipitate within the sandstone formation as pore cement further reducing the
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Secondary Reservoir Porosity in the Course of Sandstone Diagenesis
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.