Geologic Model for assessment of Unconventional (Continuous) Reservoirs of the Paradox Formation

Krystal Pearson, Lawrence Anna, Katherine Whidden, Paul Lillis, and Russell Dubiel

The U.S. Geological Survey (USGS) completed a geology-based assessment of the undiscovered, technically recoverable oil and gas resources in the Paradox Basin, Utah, Colorado, Arizona, and New Mexico. The assessment was based on elements that define a total petroleum system (TPS), which include petroleum source rocks (source rock maturation, petroleum generation and migration), reservoirs (presence and quality), and petroleum traps (type, timing of formation, timing of seal deposition). Using this framework, the Paradox TPS was defined, and four continuous assessment units (AUs) were quantitatively assessed within it. Continuous reservoirs are those with diffuse boundaries and lack of obvious traps and seals, and production is typically enhanced or controlled by fractures. The Paradox Basin is an asymmetric basin, with the deepest part along the north margin adjacent to the Uncompahgre uplift in Utah and Colorado. Interbedded salt and black shales of the Pennsylvanian Paradox Formation were deposited near the north basin margin, along with clastics shed from the Uncompahgre uplift. Carbonates were deposited along the gently dipping southwest basin margin and interfingered with the salt and black shales along the ramp margin. The presence of oils sourced by the Paradox Formation delineates the boundaries of the Paradox Formation TPS, and continuous reservoirs produce both oil and gas. The transition from gas-dominated reservoirs in the northeast part of the basin to oil-dominated reservoirs to the south and west takes place southwest of the Paradox fold and fault belt, near Moab, Utah. The four unconventional AUs defined within the Paradox Formation TPS are: 1) Cane Creek Shale Oil AU; 2) Cane Creek Shale Gas AU; 3) Gothic, Chimney Rock, Hovenweep Shale Oil AU; and 4) Gothic, Chimney Rock, Hovenweep Shale Gas AU. A maturation boundary of Ro = 1.1% was used to differentiate between adjacent continuous oil or gas AUs, with the gas AUs in the deeper, more mature part of the basin near the Uncompahgre uplift.

AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012