The porosity of sedimentary rocks tends to decrease with increasing age and depth of burial. Shales lose water and compact regularly and readily during burial; sandstones behave much less predictably, particularly at shallow depths. Laboratory determinations of the crushing strength of quartz suggest that porosity might persist to very great depths. Other experiments have shown, however, that the solubility of quartz in water increases as temperature and pressure increase and that the crushing strength decreases greatly with rising temperature in the presence of water solutions. Natural sandstones should therefore compact and lose porosity with increasing depth and rising temperature. Experiments further suggest that loss of porosity is a function of time. With sufficient data it should be possible to predict the maximum porosity to be expected in well sorted, clean, quartz sandstone saturated with formation water at various depths and temperatures. Such sandstones have been investigated experimentally. As in natural sandstones, the experimental consolidation of quartz sands occurs by two distinct processes--compaction and cementation; both processes are accelerated by high temperatures, moving water solutions, and large pressures equivalent to large depths of burial.

Porosity, depth, and temperature data have been assembled for Ordovician, Pennsylvanian, Eocene, Oligocene, Miocene, and Pliocene sandstones which are reservoirs for oil and gas. All, without exception, show reductions in both maximum and average porosity with increasing depth. Rather unexpectedly, the bounding curve of maximum porosity approximates a straight line. In areas with relatively high temperatures (high thermal gradient), sandstone porosities tend to be lower than for similar sandstones in areas with lower thermal gradients. In general, also, the younger sands have higher porosities at equivalent depths, but higher temperatures in younger rocks may reverse this relation.

Theoretical consideration and porosity-depth curves for natural sandstones agree in suggesting that areas of high thermal gradient are generally less favorable for persistence of porosity with depth. The observed curves provide a basis for estimating maximum expectable porosities of untested sandstones in areas where geologic age and thermal gradient can be approximated.