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Secondary porosity in sandstones is caused by dissolution and fracturing, and is common in the sedimentary record. Secondary porosity commonly develops in the deep subsurface and thus provides an opportunity to extend exploration to depths traditionally considered unsuitable for exploration.
Two contrasting routes of diagenesis exist in nature: porosity reduction and porosity enhancement. Porosity reduction is commonly caused by compaction and cementation, whereas porosity enhancement is primarily caused by dissolution of carbonate minerals. The dissolution of carbonate minerals in the deep subsurface can be attributed to release of carbon dioxide during thermal maturation of kerogen. Clay-carbonate reaction and flushing of undersaturated meteoric water from erosional unconformity are equally important mechanisms in generating secondary porosity at various depths.
Two basic types of primary pores (intergranular and intragranular) and four basic types of secondary pores (grain fractures, rock fractures, intergranular, and intragranular) can be differentiated on the basis of (1) position of pores, (2) timing of origin, and (3) processes of origin. The proposed classification system is useful in inferring reservoir quality.
Various types of secondary porosity are recognized using a comprehensive set of 20 criteria. The various criteria are based on manner of breakage, pore geometry, grain geometry, products of dissolution, and sediment packing. Examples of secondary porosity gathered in part from investigation of 2,000 thin sections will serve as a guide for recognizing secondary porosity in outcrop, in hand specimen, under the scanning electron microscope, and most importantly, under the petrographic microscope.
New evidence suggests that silicate minerals, including quartz, dissolve more commonly than have been reported. The abundant occurrence of secondary porosity in reservoir sandstones emphasizes the importance of secondary porosity in evaluating deep reservoirs.
Since the first recognition of secondary porosity by Nutting in 1934, Chepikov in 1959 and 1961 not only developed the first set of criteria for recognition of secondary porosity but also introduced the concept in which development of secondary porosity is related to arrival of oil.
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