Accurate porosity and water saturation determination of Cotton Valley sandstone is difficult because typical porosities are below 12%. In this low porosity range, the errors from log-derived porosity and water saturation values can cause significant misinterpretation of productive potential. Such errors can result from the presence of secondary matrix mineralization, parameters selected to calculate porosity and water saturation, and calibration inaccuracies.
The Cotton Valley sands are a complex intermingling of various minerals and an assortment of carbonates. Carbonates in isolated intervals can account for as much as 50% of the rock matrix. The heavy minerals, when unaccounted for, can result in pessimistic log-derived porosity and, hence, overlooked commercial potential. Also, calibration inaccuracies can result in either optimistic or pessimistic porosity analysis, further confounding commercial evaluation. Variations in grain size coupled with the mineral composition complicate permeability determination. These variables account for the difficulty in predicting reserves in Cotton Valley sand exploration.