Source Rock Potential of the Icebox Formation, Winnipeg Group (Ordovician), North Dakota

Timothy Nesheim and Stephan Nordeng
North Dakota Geological Survey

The Icebox Formation (middle Winnipeg Group) has been long viewed as a petroleum source rock within the Williston Basin. While several studies have examined the source rock potential of the Icebox Fm. in Saskatchewan's portion of the Williston Basin, limited work has been published in North Dakota. Extending throughout most of North Dakota, the Icebox Formation is typically ~130 ft. thick while reaching burial depths of over 14,500 ft. The Icebox Fm. contains sandy to sandstone intervals and becomes partially to fully oxidized towards the east, but primarily consists of green to grey to black shale deposited in a marine shelf environment. Thermal modeling reveals that the Icebox Fm. may be thermally mature beneath ~22,000 sq. miles of western North Dakota as well as parts of the Montana, South Dakota, and Saskatchewan. Overall, the Icebox Formation averages as a fair quality source rock while containing an excellent quality source rock interval near its base. Based on 249 measurements off of drill cuttings from 18 wells, the Icebox Fm. in North Dakota averages 0.55 wt. % TOC. Individual well averages, however, range from 0.23 to 1.63 wt. % TOC, with the highest well averages located in the central portions of the state where the Icebox Fm. is neither thermally mature nor oxidized. Also, a core sampled from the Taylor Field (southwestern North Dakota) contains a 3-4 ft. black shale interval near the base of the Icebox section that averages 3.5 wt. % TOC and an S2 of 18.9 mg/g (type I kerogen). Mapping the sonic-resistivity log cross-plot signature of this shale interval, it appears to be regionally extensive and reach thicknesses of 8-10 feet.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62^nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013