Texture and Recognition of Secondary Porosity in Sandstones
Published:January 01, 1979
Secondary porosity in sandstones can be classified according to origin and pore texture. Five significant genetic classes of secondary porosity are defined by the following processes of origin: (1) fracturing; (2) shrinkage; (3) dissolution of sedimentary grains and matrix; (4) dissolution of authigenic pore-filling cement; and, (5) dissolution of authigenic replacive minerals. Hybrid pores are characterized either by the coexistence of several genetic classes of secondary porosity or by the coexistence of primary and secondary porosity.
Secondary porosity appears in five major groups of pore textures: (1) intergranular pores; (2) oversized pores;
(3) moldic pores; (4) intra-constituent pores; and, (5) open fractures. Some secondary porosity mimics the entire range of pore sizes and pore textures of primary sandstone porosity. Other secondary porosity bears a general resemblance to the textures of primary porosity but differs in detail. Secondary porosity may also appear in textures that are entirely different from those of primary porosity.
In most instances it is possible to identify the occurrence of secondary porosity in thin section using a set of simple petrographic criteria that include: (1) partial dissolution; (2) molds; (3) inhomogeneity of packing;
(4) oversized pores; (5) elongate pores; (6) corroded grains; (7) intra-constituent pores; and, (8) fractured grains. In medium- and coarse-grained sandstones secondary porosity can, in some instances, be observed by the naked eye.
The detailed analysis of the petrological attributes of secondary porosity may require the use of advanced analytical techniques, such as cathode luminescence petrography, scanning electron microscopy, pore-cast examination, microprobe analysis and stable-isotope analysis. In most sandstones, however, a surprising amount of important information can be obtained simply by the careful and methodical use of conventional petrographic microscopy.
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Aspects of Diagenesis
There are a number of gaping holes in accumulated knowledge within the discipline of sedimentology. Perhaps one of the largest holes has been the general subject of diagenesis in clastic rocks. It was therefore fortuitous that two symposia covering various aspects of diagenesis (mainly in clastics) were presented a year apart in different parts of the country but with the same motivation – to contribute to the closing of that knowledge gap. Sedimentologists now have a fairly good idea of the what and the how of sediment deposition. What happens after the sediments are lithified has frequently been ignored. It was the aim of both editors of this publication to approach the subject from two different viewpoints. Schluger directed a symposium which looked mainly at clastic reservoirs, and Scholle presented a symposium which examined various aspects of paleotemperature control of diagenesis.