The Porosity Diagenesis of Sandstones of Intermediate and Low Mineralogical Maturity
1979. "The Porosity Diagenesis of Sandstones of Intermediate and Low Mineralogical Maturity", Secondary Reservoir Porosity in the Course of Sandstone Diagenesis, Volkmar Schmidt, David A. McDonald
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Sandstones of lesser mineralogical maturity than quartz arenites exhibit a similar relationship between the maturation of organic matter and the development of mesogenetic decarbonatization porosity as described above. However, both primary and secondary porosity are reduced by chemical compaction at lower temperatures and at faster rates than observed in quartz arenites. Chemical constituents derived from the dissolution of minerals such as feldspars, micas, amphiboles, and pyroxenes precipitate as mesogenetic cements mainly consisting of clay minerals and zeolites (Hayes, 1979). Chemical compaction and associated intrastratal precipitation of cements may collectively be termed framework diagenesis, analogous to quartz diagenesis. Mesogenetic carbonatization in these sandstones involves more replacement and less cementation when compared with quartz arenites. Carbonatization tends to culminate during the early phase of mature stage “A” at about the same level of thermomaturation as in quartz arenites.
In general, the development and disappearance of mesogenetic secondary porosity follows a similar pattern in sandstones of intermediate and low mineralogical maturity to that observed in quartz arenites. The detail of the diagenetic histories of sandstones of lesser mineralogical stability is often very complex and much more work remains to be done to establish the basic relationships as clearly as is now possible with quartz arenites. The Cretaceous sandstones of the Labrador Shelf, the Tertiary sandstones of the Mackenzie Delta, and the Mesozoic and Tertiary sandstones of the Cook Inlet are prime candidates for such work. These sandstones of intermediate to low mineralogical maturity contain large hydrocarbon accumulations in reservoirs of secondary sandstone porosity.
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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.