The hydrocarbon distribution in the N sand reservoir in Main Pass Block 35 field is anomalous. In the eastern part of the reservoir, the N sand accumulation contains a gas cap about 120 ft structurally lower than the oil in producing wells in the western part of the reservoir. Geologic and pressure data indicate a common reservoir.

A review of the paleostructural history indicates that after 1,000 ft of burial, the relict N sand structural high was in the eastern part of the field near the area of the present-day gas cap. Isopach maps of younger intervals show that the structural high shifted westward. The presence of an N sand gas cap in the time-persistent, but now low, eastern culmination and the absence of a gas cap at the present apex of the reservoir suggest early migration and accumulation of the gas and perhaps of the oil in the eastern area. Later structural tilting would have resulted in adjustment and remigration of the N sand oil column into the developing western apex. Tilting was not sufficient for remigration of the gas column, and thus the gas remained trapped at the present lower site of the eas ern structural high.