The Bisti field, on the SW. flank of the San Juan basin, produces oil and gas from the Cretaceous Gallup sandstone. The accumulation is in a series of narrow permeability lenses totaling more than 30 mi. in length. The lenses are permeability developments in an epineritic facies. Updip from the epineritic facies is a transition zone of littoral and nearshore environmental deposits. A deltaic facies extends from the transition facies to the outcrops of the formation. Zones of sandstone developments can be traced from the epineritic facies through the transition zone to the deltaic facies. There is no structural closure or nosing in the field. At the W. end of the field, the accumulation forms an oil column more than 365 ft. in height. The height of the oil column is sufficient to develop a capillary pressure of more than 52 lbs. per sq. in. at the up-dip edge of the accumulation. Analyses of the limited cores available indicate that the entry pressures, or threshold pressures, of the more permeable sandstones in the transition facies are in most places less than 22 lbs. per sq. in. The capillary pressure of the column of oil under hydrostatic conditions should be sufficient to cause much of the oil to migrate updip out of the permeability lenses. Favorable hydrodynamics is responsible for retaining the additional oil. A decrease in pressure causes a flow of water down-dip across the permeability barrier to form a hydrodynamic trap sufficient to retain the oil accumulation.