Diagenesis of Quartz Arenites
The sandstones of the highest mineralogical stability, the quartz arenites, have been selected to trace in detail the role of secondary porosity during the course of their diagenesis. This choice was made for two reasons: (1) the data base for this sandstone lithology is the most comprehensive; and (2) primary porosity survives much longer in quartz arenites than in sandstones of lower mineralogical maturity. Secondary porosity may be expected to play a relatively less important role in quartz arenites compared to the latter. Thus a baseline for the significance of secondary porosity can be established.
It is necessary to follow the path of typical diagenetic histories of individual sandstones in order to better understand the vertical distribution of primary and secondary porosity. Four types of diagenetic histories will be discussed: (1) quartz arenites free of carbonate; (2) quartz arenites with sedimentary and eogenetic carbonate; (3) quartz arenites with mesogenetic carbonate; (4) quartz arenites with sedimentary, eogenetic and mesogenetic carbonate.
The diagenesis diagrams (Figures 53, 54, 55, 59, 63, 67) chart the composition of hypothetical quartz arenites from the time of deposition to the advent of raetaraorphism. These diagrams have been constructed by piecing together petrographical and geological observations of numerous samples.
The loss of primary porosity in quartz arenites is best examined in samples that remain free of carbonate or other readily soluble constituents throughout their diagenesis. The primary intergranular porosity and the rock volume of the quartz arenite shown in Figure 53 are reduced by mechanical compaction during 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.