Abstract

Mean porosity and permeability of many sandstone intervals can be predicted prior to drilling. The predictive technique involves use of multivariate regression equations derived from calibration data sets. The effectiveness of this technique is illustrated by case studies, including several pre-drill porosity and permeability predictions.

The critical independent variables controlling porosity (dependent variable) are detrital composition, sorting, temperature history, and pressure history. In sandstones with a high content of detrital quartz and chert, grain size may also affect porosity. Permeability can be expressed by the same parameters as porosity. Most importantly, the independent variables, that correlate with porosity and permeability, can be often estimated prior to drilling from facies models and seismic data. The predictive applicability of the empirical approach is, however, constrained by the limits imposed by the calibration data set.

Data from a number of sandstone reservoirs and targets suggest that empirical predictions of total porosity are possible in sandstones with volumetrically important secondary porosity. This is particularly true of sandstones in which secondary porosity is formed predominantly by dissolution of framework grains. Accurate empirical predictions of reservoir quality in many sandstones affected by leaching are possible because preservation of secondary porosity is controlled by the same geological parameters as primary porosity, and is, therefore, implicitly accounted for by the calibration data set that provides the basis for empirical predictions.