Calculation of compactional porosity loss from modal analyses of thin sections is a useful technique for characterizing the diagenesis and porosity evolution of sandstone reservoirs. To obtain meaningful results, however, it is essential to assume an appropriate value for the "original porosity" and to use point-counting categories that clearly distinguish between grains and intergranular materials. Because of the latter requirement, it is commonly not appropriate to make calculations of compactional porosity loss from modal analyses that were not performed for the specific objective of measuring intergranular volume. Suites of modal analyses from deeply buried sandstone units typically show wide variations in the relative importance of compaction versus quartz cementation. This heterogeneity must reflect subtle variations in depositional sand characteristics, in particular the distribution of detrital intergranular clays that tend to promote both mechanical grain rearrangement and stylolitic grain dissolution, as well as locally inhibiting quartz cement growth. Despite wide variation in the mechanism of porosity loss, total (helium) porosity tends to be relatively constant at individual well locations, suggesting a close interdependence between compaction and quartz cementation, with total porosity loss dependent mainly upon thermal exposure (either maximum temperature or time-temperature integral).