The Gothic Shale from Greater Aneth Oil Field, Paradox Basin, Southeastern Utah: Seal for Hydrocarbons and Carbon Dioxide Geologic Sequestration

Chidsey, Jr., Thomas C.¹; Carney, Stephanie M.¹; Heath, Jason ²; Dewers, Thomas ^3
1 Utah Geological Survey, Salt Lake City, UT.
² New Mexico Institute of Mining and Technology, Socorro, NM.
³ Sandia National Laboratories, Albuquerque, NM.

Greater Aneth oil field, Utah’s largest oil producer, was discovered in 1956 and has produced over 440 million BO. Located in the Paradox Basin of southeastern Utah, Greater Aneth is a stratigraphic trap producing from the Pennsylvanian Paradox Formation. Because it represents an archetype oil field of the western U.S., Greater Aneth was selected to demonstrate combined EOR and CO₂ sequestration. The Aneth Unit in the northwestern part of the field has produced 149 million bbls of the estimated 450 million bbls of OOIP - a 33% recovery rate. The large amount of remaining oil made the Aneth Unit ideal to demonstrate both CO₂ storage capacity and EOR by CO₂ flooding.

Within the Paradox Formation, the Gothic shale seals the underlying Desert Creek reservoir zone. The Gothic shale ranges in thickness from 5 to 27 ft, averaging 15 ft. Within the Aneth Unit, it is remarkably uniform consisting of black to gray, laminated to thin-bedded, dolomitic marine shale. The Gothic contains total organic carbon as high as 15% with type III and mixed type II-III kerogen. Natural fractures include horizontal and inclined (30 to 44°) with evidence of shear in the form of slickensides; some mineralization is present. Geomechanical, petrophysical, petrological, and geochemical analyses were conducted to determine (1) the geologic controls on sealing effeciency (using XRD, SEM, and thin sections to interpret mercury injection capillary pressure data), (2) effects of pressure changes on seal efficiency due to CO₂ injection and storage, and (3) chemical interaction between CO₂ and the seal at its contact with the reservoir through time.

"Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U.S. Department of Energy under contract DE-ACOC4-94AL85000."

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009