Bisti Oil Field, San Juan County, New Mexico

ABSTRACT The Bisti oil field in central San Juan County, New Mexico, produces oil from the Gallup sandstone formation, Mesaverde group of Upper Cretaceous age. The field is a stratigraphic type trap on the homoclinal south flank of the San Juan basin. Accumulation of oil is independent of local structure. The reservoir is an offshore sandbar and an underlying less permeable silty sandstone. Both updip and downdip from the field, there is a facies change from sandstone to siltstone. Similar lateral gradation probably completes the trap. The Gallup sandstone is predominantly a marine strandline deposit controlled by northeast regression of the sea across the Cretaceous San Juan embayment. Subsurface studies suggest that tectonic movements have caused a reversal in the original structural attitude of the Bisti field. Relatively recent basinal tilting has introduced hydrodynamic factors.