A statistical study of 7,241 reservoir sandstones in various oil-producting regions of the United States was completed by a committee of petroleum geologists. The study shows that 68 per cent of the sandstones are of Tertiary age, although each geologic age from Cambrian to Pleistocene, inclusive, is represented. Most of the sandstones are restricted in physical dimensions; they commonly cover less than 100 square miles of areal extent, and have an average thickness of about 39 feet.

The factors which control petroleum accumulation in the sandstones more commonly result from the structural configuration (56 per cent of reservoirs) than from stratigraphic conditions (10 per cent) or from combinations of structural and stratigraphic features (34 per cent), and it is found that the thicker sandstones tend to be broader and better reservoirs than their relative thickness, alone, would suggest. A closer analysis of the sandstones involved in stratigraphic type accumulations shows that 61 per cent of them were deposited under shoreline or nearshore conditions. It is found that 54 per cent of all the reservoir sandstones studied contain mainly oil, 27 per cent contain gas, and the remainder carry substantial amounts of both oil and gas.