ABSTRACT
Oil and gas are common in stratigraphic traps in structural basins, both deep down near the bottom, and also along the flanks. In many of these traps initial reservoir pressures were subnormal, indicating a lack of permeable connection to the outcrop. Some maps drawn on the potentiometric (piezometric) surface using pressure data from drill-stem tests show clearly the location of the stratigraphic barriers which have trapped the oil; these maps should be used in prospecting. Many giant gas fields with abnormally low initial reservoir pressures are low on the flanks of structural basins. The geologic factors favoring this type of accumulation are not understood.
The cause of the low pressures may be related to removal of overburden, which has resulted in a dilation of the pore volume in the rocks, and a decrease in reservoir temperature.
