The geometry of secondary pores if compared to the geometry of primary precursor porosity can be described in three basic morphologies.
Geometry identical to precursor primary porosity.
Geometry modified from precursor primary porosity.
Geometry unrelated to precursor primary porosity.
Identical geometry is shown by exact replicas of unaltered primary pores or of residual primary pores. (Figure 87). The latter may have been reduced by such processes as compaction, pressure solution and partial cementation by silica overgrowth. These secondary pores occur wherever the soluble precursor consisted only of pore-filling cement. A considerable yet, overall, subordinate amount of secondary porosity falls into this morphologic class. Pore throats of the reopened unaltered primary intergranular porosity tend to be of concave-triangular cross section. In sandstones with reopened residual primary porosity two types of pore connections occur together: pore throats of straight-triangular or concave-triangular cross secion and irreducible pore throats at grain contacts with lamellar cross section.
Modified geometry occurs most commonly as enlarged modified replicas of the precursor primary porosity and 84, 85, 86). The sandstones with high secondary porosities are often closely associated with hydrocarbon source rocks, and contain condensate and natural gas. Quartz diagenesis and framework diagenesis of other grains is responsible for the decline of secondary porosity in the mature stage “B” of burial diagenesis.
Identical geometry is shown by exact replicas of unaltered primary pores or of residual primary pores. (Figure 87). The latter may have been reduced by such processes as compaction, pressure solution and partial
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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.