TEXTURE AND RECOGNITION OF SECONDARY POROSITY IN SANDSTONES

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 I 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 I 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 I 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