Abstract

The Parshall Field of the Williston Basin was discovered in 2006 by EOG Resources and is located in Mountrail County, North Dakota. The Devonian-Mississippian Bakken Formation resource play covers some 40 townships in North Dakota and is still expanding. The development of horizontal drilling and modern fracturing techniques has made this play possible.

The Bakken Formation in the field consists of three members: (1) upper shale, (2) middle dolomitic sandstone/siltstone, and (3) lower shale. The total Bakken interval ranges in thickness from 70 to 100 ft. The upper shale is dark-brown to black organic rich mudstone and averages 18 ft in thickness. The middle member ranges in thickness from 30 to 50 ft. The middle member is composed of a range in lithologies from bioturbated, argillaceous, calcareous very fine grained sandstones to low angle planar laminated fine to very fine-grained sandstone to very fine-grained sandstone and siltstones with centimeter and millimeter scale shale laminations. The lower shale member is also a dark-brown to black organic rich mudstone and is approximately 30 ft thick in Parshall Field.

The main hydrocarbon productive reservoir in the Parshall Field is the middle member which has low matrix porosity and permeability and is found at depths of 9,000 to 10,500 ft. The middle Bakken porosities range from 1 to 11 percent and permeabilities average 0.0042 mD, based on core analyses from eight cores in the field. Some key factors that contribute to the success of this play include good stratigraphic trapping conditions, generation of hydrocarbons from the organic rich mudstones, and fractures.

A network of nine regional stratigraphic cross sections illustrate that the Bakken members onlap and thin to the east onto the underlying Three Forks Formation.

A petrophysical model was developed to characterize the reservoir rocks at Parshall Field. Water saturation calculations were done using an Archie saturation equation and permeability was computed using a generalized Timur equation. These were then used to determine net pay in Parshall Field which averages 25 ft in thickness.