Role of Secondary Porosity and Permeability in Predrill Prediction of Total Porosity and Permeability in Sandstones

Abstract

The subjective nature of some petrographic criteria used to define and quantify secondary porosity and the inadequacy of the mechanisms invoked to explain its generation make it difficult to construct realistic retrospective models of secondary porosity generation. Without such models, secondary porosity prediction, separate from total porosity prediction, is reduced to the realm of speculation. Fortunately, data from a number of sandstone reservoirs and targets suggest that empirical predictions of total porosity are possible even if secondary enhanced porosity is volumetrically important. This is particularly true of sandstones in which secondary porosity is formed predominantly by dissolution of framework grains. Empirical predictions are based on the correlation between porosity and permeability (dependent variables) and a limited number of parameters (independent variables) obtained from calibration data sets or estimated from appropriate geological models. It appears that accurate empirical predictions of reservoir quality in many sandstones affected by leaching are possible because the extent of secondary porosity and permeability generation is to a large extent a function of primary porosity and permeability. Whereas the empirical approach appears to be successful in predicting reservoir quality in sandstones affected by leaching of framework grains, its predictive accuracy in sandstones whose reservoir quality is controlledmore » by dissolution of authigenic cements remains to be determined. However, in many reservoirs the effect of dissolution of authigenic cements on reservoir quality may have been greatly overestimated.« less