Sandstone porosity and permeability tend to decrease with depth. Controlling factors are complex and there is no accurate method of computing the depth to which commercially interesting porosities might extend.

The optimistic view is that quartz is an exceedingly strong material and that short-time crushing experiments indicate that porosities persist to depths roughly double those drilled to date. Other experiments involving saline waters, elevated temperatures and pressures, and times measured in days, show that quartz sand is very much weaker than its theoretical strength and that failure and compaction is progressive over the longest times investigated. As in natural sandstones, experimental consolidation of quartz sands involves two distinct processes, compaction and cementation. Both are accelerated by high temperatures, moving water solutions, and large "overburden" pressures.

Highest porosity might be expected for pure, well sorted and rounded sands of the type examined experimentally. Conditions resulting in porosity reduction in these sands to some minimum value,--say 15 per cent,--should produce similar or greater reduction in most oil sands. Assuming sands are water-bearing and depth is constant, then temperature is the most important variable affecting pore reduction. Experiments indicate the effects of time and temperature are interchangeable, the log of time being a linear function of absolute temperature.

Compaction curves for dry quartz sand at room temperature and for saline water-saturated sands at pressures and temperatures simulating burial, are roughly parallel with trends of maximum porosities in natural sands. These trends seem to be temperature-dependent. If published temperature gradients for the Gulf Coast are accepted, then rough extrapolations indicate that pure quartz sands would be reduced to 15 per cent maximum porosity at depths less than 20,000 feet in the Galveston area and somewhat deeper, perhaps 25,000-27,000 feet, on the Mississippi Delta. These figures are for young Cenozoic sediments. Similar porosities should be attained at lesser depths within older formations.

Scarcity of reliable temperature data sharply limits the accuracy of this type of analysis. Good temperature logs of a few deep wells within a region would be of great value in estimating deeper drilling possibilities.

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