Porosity Reduction in Sandstones Induced by Mechanical Compaction: Cathodoluminescence Evaluation

Abstract

Cathodoluminescence (CL) petrography has been instrumental in documenting the importance of chemical compaction (pressure solution) to porosity reduction in sandstones. Comparatively little work has been focused on the role of mechanical compaction, although this process is probably more pervasive. Brittle failure of quartz grains has been largely overlooked as a compaction mechanism because fractures are commonly healed by quartz cement that is not visible unless CL petrography is utilized. Grain boundary extension fractures form at grain contacts in response to overburden stress and are not related to tectonic stress. They result in extension of detrital quartz fragments into intergranular space, causing reduction of bulk volume, intergranular porosity, and total effective porosity. Samples of quartzose sandstones from sedimentary basins of diverse geologic history have been examined using CL petrography. This survey documents significant reduction of intergranular volumes by mechanical compaction to values as low as 20% in sandstones that have undergone little or no chemical compaction. Factors favoring the incidence of mechanical compaction induced by brittle failure include rapid burial, high temperatures, coarse grain size, and the absence of diagenetically early cements. This reconnaissance work suggests that mechanical compaction associated with brittle failure of quartz grains is more important than commonlymore » thought, and emphasizes the need for CL petrography to be integrated into routine analysis of sandstone reservoir quality.« less