Definition and Hydrocarbon Potential of the Late Devonian Three Forks Formation, Williston Basin, South Dakota

Abstract

An evaluation of the Three Forks Formation, to determine hydrocarbon potential, was performed in the Williston Basin of South Dakota using well logs from the South Dakota Geological Survey. Basin analysis included identification of upper and lower boundaries, extent, lithologic description, stratigraphic correlation, and hydrocarbon potential. Thickness of Three Forks rocks ranges from 0 to 170 feet, and was thickest in northern Perkins and Corson counties and thinned toward the basin margins. Lithologically, the rocks consisted primarily of interbedded shale and dolomitic limestone. Stratigraphically, Three Forks rocks occurred between the underlying Birdbear Formation and overlying Englewood Formation. In areas where the Englewood was absent, it was overlain by the Lodgepole Formation. In North Dakota, eastern Montana, and Canada, the Three Forks underlies the Bakken Formation, serving as a reservoir for Bakken shale oil. No Bakken rocks were identified in well logs from South Dakota and the Three Forks and other Late Devonian formations have been underexplored. Well log analysis revealed three previously unidentified potential subsurface structures in the Williston Basin of South Dakota. In addition, data have supported proposed southeastern extensions of the Cedar Creek Anticline and Sheep Mountain Syncline. Black shale, indicating areas of restricted water circulation, have been identified in the Three Forks, in limited areas of estimated maturity, that suggest ideal conditions for the preservation of organic matter. These areas were correlated using gamma-ray spikes and estimated TOC values from 1.4 to 5.6 wt. %, at depths Three Forks rocks would be expected to contain mature hydrocarbons. Based on estimated TOC values and associated thicknesses of TOC-bearing intervals, limited potential exists for Three Forks source and reservoir rock in northwestern South Dakota, particularly in northern Perkins and eastern Harding counties.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015