Assessing the Relative Importance of Compaction Processes and Cementation to Reduction of Porosity in Sandstones

Abstract

At the depositional surface, well-sorted sand has approximately 40% porosity. During burial diagenesis, that porosity is reduced by mechanical compaction, intergranular pressure solution, and cementation. Mechanical compaction and intergranular pressure solution can both be considered compactional processes because they irreversibly reduce the intergranular volume of sand. In contrast, cementation occludes, but does not reduce, intergranular volume. The relative importance of compactional processes and cementation to porosity reduction can be quantified using a graph of intergranular volume vs. cement. This diagram can be used to evaluate which diagenetic processes have been most influential to intergranular porosity reduction and to determine why some sandstones retain better reservoir quality than others. The diagram can also be used to reconstruct pathways taken by sandstones during burial diagenesis. Results of applying this technique to data from the Nugget Sandstone and Bromide sandstone (Simpson Group) indicate that mechanical compaction and intergranular pressure solution were much more important than cementation in determining ultimate porosity. Moreover, the best porosity is preserved in samples that have undergone the least intergranular pressure solution. These conclusions emphasize the importance of integrating an evaluation of these compactional processes into analyses of reservoir sandstones and into models of burial diagenesis.