Understanding the Geologic Controls on “Shale Oil Play”: Lessons Learned from the Bakken Formation, Williston Basin, Elm Coulee Field, Montana

The Bakken Formation is a subsurface formation in the Williston Basin that is divided into three informal members: upper, middle and lower. Upper and lower Bakken consist of dark grey to black pyritic and finely laminated mudstones. The middle member of the Bakken is composed of various lithologies including siltstones, sandstones, limestones and dolostones. Total thickness of the Bakken Formation in the Williston Basin ranges from 0 ft – 110 ft with the upper member ranges from 0 ft - 30 ft, the middle ranges from 0 ft - 92 ft, and the lower ranges from 0 ft - 46 ft.

The upper and lower mudstones are world-class petroleum source rocks with TOC range from 8% to 36% wt. with average of 25% to 28% wt. across large areas of the basin, and can be easily identified by their high radioactive signatures (GR > 200 ^oAPI). The Bakken was estimated to have generated 200 – 400 billion barrels, low sulfur (< 0.1%), 40 – 45 ^oAPI gravity oil in place.

The lower Bakken is absent in the Elm Coulee Field. Source of petroleum in the middle Bakken is primarily from the upper Bakken mudstone with TOC varies from 8% - 15% wt. with estimated initial TOC ranges from 12% – 25% wt., increasing towards the basin center (northwest to the Elm Coulee).

The total thickness of the Bakken Formation in the Elm Coulee field 10 ft - 50 ft with the middle Bakken ranges from 5 ft – 45 ft. Low permeability rocks characterize the overall middle Bakken member lithologies. The accommodation space for the Bakken in the Elm Coulee field is believed to be developed by the dissolution of the evaporite Prairie Formation to the north of Elm Coulee or by basement related structures or both.

The upper Bakken mudstone is dark-grey to black, hard, siliceous, slightly calcareous, pyritic, and fissile. The mudstone consists of dark organic kerogen, minor clay, silt-size quartz, some calcite and dolomite. The kerogen consists mainly of amorphous material and the organic material is distributed evenly throughout the mudstone interval. The middle Bakken in the Elm Coulee Field consists of dolostones with 3 identified lithofacies: calcareous dolomudstones, bioturbated dolomudstones and sandy dolostones. Identified lithofacies represent different rock properties of the middle Bakken. Sandy dolostone lithofacies has the greatest porosity and permeability both calculated from core analysis and wireline logs (5% - 10% porosity and 0.05 – 0.2 mD of permeability). The pore network development in the middle Bakken is the function of degree of bioturbation, mineralogical-depositional fabric, and diagenesis. Permeability is highly dependant by matrix porosity from intergranular and intragranular from the dissolution of dolomites, with less of natural fractures involved based on core observation in the field.

Due to the simplicity of structural deformation in the Williston Basin, the oil generated by the upper and middle Bakken remains in the system. Therefore Bakken petroleum system is a self-sourced system. The middle Bakken in this field is oil saturated. Both fractures and pore network are the important factors in controlling petroleum charge access from the upper Bakken source to the middle Bakken tight reservoir.

The middle Bakken fairway boundaries in the Elm Coulee field were defined using porosity from neutron-density logs with cutoff of 5%, and true resistivity above 20 Ohm-m. Net thickness of saturated oil zone within the middle Bakken ranges from 5 ft to 25 ft. The Bakken in the field is slightly overpressured due to petroleum generation. The over-pressuring and middle Bakken reservoir quality are the important factors in understanding the oil production rates in the Elm Coulee Field. The middle Bakken is a brittle dolostone, which is fracture-able for hydraulic fracturing completion. Long-lateral horizontal drilling and multi-stages hydraulic fracturing of the middle Bakken dolostones are keys of success in the Elm Coulee Field development area.