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Abstract

Porosity prediction in an open system is extremely difficult because sources of pore-filling cements and grain- and cement-dissolving solutions are not readily delineated, nor are the volumes of these solutions easily quantifiable. A good example of the problems encountered is the case of creating secondary porosity by dissolution of carbonate cement. In this case, porosity prediction depends upon the ability to predict the conditions conducive to both the development of the porefilling cement and its subsequent dissolution.

The occurrence of carbonate cement must be considered in terms of timing with respect to other porosity reduction mechanisms, abundance and distribution within the prospective reservoir sandstone, and the various chemical and biological controls on precipitation. Similarly, late-stage dissolution must be evaluated in terms of timing with respect to both structuring and porosity preservation mechanisms, extent and distribution of secondary pores, and the chemical controls on dissolution. In light of these difficulties, prediction of porosity in a reservoir formed by decementation is highly problematic. A prediction of maximum porosity may be a more easily attained, although highly speculative and less meaningful, value for the explorationist.

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