Hydrocarbon Potential and Exploration Methods Within Cretaceous of Utah-Wyoming Overthrust Belt: A New Frontier in a Not-So-Old Area

Allen J. Bertagne, Jerome W. Boettcher, Claude Vuillermoz

A largely untested stratigraphic play exists within the Cretaceous section of the Utah-Wyoming Overthrust belt. Coarse Cretaceous sediments on the western Absaroka plate merge gradationally eastward into a predominantly shale section, which sets up the potential for Cretaceous sands to pinch out within Cretaceous shale in the Overthrust belt. The discovery well in this play is the Sam Gary Jr. 4-6X Lazeart (Sec. 4, T21N, R116W) with a reported initial potential of 87 BOPD from the Upper Cretaceous Frontier Formation.

The anticipated source of hydrocarbons is Cretaceous shale--the documented source of most hydrocarbons found to date in the Utah-Wyoming Overthrust belt. Published research indicates that Cretaceous shales have reached adequate maturation levels under the Darby and Absaroka plates and probably on the back of the Absaroka plate. In addition, hydrocarbons may have moved updip from the generation area to shallower reservoirs as a result of secondary migration. Therefore, the geographic area of the play is extensive.

State-of-the-art seismic data are an essential prerequisite for systematic exploration of the Cretaceous play. After acquiring a regional seismic grid and tying in well control and surface geology, the following procedure should be implemented: (1) use seismic reflections to establish a regional chronostratigraphic framework, (2) interpret "seismic facies" to determine geographic distribution of depositional environments, and (3) perform detailed interpretation and forward modeling to locate specific prospects. A reprocessed seismic line from the Overthrust belt confirms that data quality is high enough to use this procedure.

An aggressive exploration program in this area is likely to result in significant reserve additions; a commitment to the search for the subtle trap in the Overthrust belt is now needed.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.