Petroleum Exploration of Winnipegosis Formation in North-Central North Dakota (Williston Basin): ABSTRACT

Abstract

The Winnipegosis Formation (Middle Devonian) in north-central Dakota has the greatest potential for large oil reserves in the Williston basin. The Winnipegosis carbonate (50 to 325 ft thick) was deposited in the southeast end of the Elk Point restricted sea. During Winnipegosis deposition, the Williston basin could be divided into two distinct environments: (1) a deep starved basin with accompanying pinnacle reefs separated by interreef, laminated limestone and (2) a surrounding carbonate shelf. Within the carbonate shelf are patch reefs, banks, and tidal flats. Overlying the Winnipegosis carbonate is the Prairie Formation, which has a basal anhydrite (0 to 70 ft thick) and an overlying salt (0 to 650 ft thick). These were deposited in a regressive phase of the Elk Point sea and act as seals for Winnipegosis oil entrapment. Currently, oil production from the Winnipegosis in the Williston basin is from stratigraphic traps and from small structures on the carbonate shelf. The most significant accumulation to date is Temple field, in which 11 wells produce from +/- 20 ft of Winnipegosis dolomite. The pinnacle reef environment has potential for significant oil reserves from 250-ft thick reefs covering 160 ac or less. Two pinnacle reefs have had free-oil recoveriesmore » from thin pay zones. The Rainbow/Zama fields in northwest Alberta have an ultimate reserve of more than 1 billion bbl of oil from Keg River reefs, which are correlative and similar to the Winnipegosis reefs in North Dakota. The strong seismic reflection that originates from the Winnipegosis-Prairie evaporite interface provides an excellent means of detecting Winnipegosis reefs. Amplitude of the Winnipegosis reflection is reduced dramatically over the reefs. The resulting dim spot is one criteria used in identifying reefs.« less