Pennsylvanian Stratigraphy and Productive Zones, Paradox Salt Basin

Abstract

The Paradox Salt basin is an elongate northwest-southeast Pennsylvanian depositional basin extending from central Utah through the Four Corners area. The Pennsylvanian system is comprised of the Molas formation, a basal dark red shale unit deposited on the karst topography eroded into Mississippian carbonates, and the Hermosa formation. The Hermosa formation, wherever evaporites and carbonates deposited in a restricted environment are present, is divisible into three members: upper Hermosa, Paradox, and lower Hermosa. The Paradox member is an evaporite megacyclothem composed of discrete sedimentary sequences which are complete or partial cyclothems. The member is gradational and interfingers with open marine sediments of the upper and lower members above and below. Latera ly, at the south, southwest, and west, the relationships with the undifferentiated Hermosa formation are similar, but toward the northeast the Paradox member grades into and interfingers with coarse clastics derived from the Uncompahgre uplift. Since 1954 eleven Pennsylvanian discoveries and several important field extensions have resulted in substantial oil and gas reserves, with at least one field, Aneth, of major importance. All accumulations found to date, with the exception of Mexican Hat, are associated with the evaporite cyclothems of the Paradox member or its normal marine lateral equivalents. Two important productive objectives, the Desert Creek and Bluff zones, are widespread in the Four Corners area. Accumulations in reservoirs having similar stratigraphic position to these zones have been found but, except for the Barker Creek gas field, appear to be of less importance. Rapid lateral and vertical variations in the porosity and permeability of potential and proved reservoir beds make facies studies a necessary too of exploration and production in evaluating structures and field development. Daily production is limited to a small fraction of the potential due to lack of outlets.