Talk Title: Elm Coulee Field Middle Bakken Member (Lower Mississippian/Upper Devonian) Richland County, Montana
Authors: Bill Walker, Al Powell, Dick Rollins and Ron Shaffer Headington Oil Co., LP
Talk Date: January 6, 2006
Publication: The Outcrop, January 2006, p. 4-5
Elm Coulee Field, located in the Richland County, MT, portion of the western Williston Basin, has been informally named by the Montana Oil & Gas Commission to cover Bakken (L Miss/U Dev) Formation production in the county that is not located under previously designated field areas. Approximately 230 horizontal development wells have been drilled since the field’s initial horizontal producer was drilled in late 2000 by LYCO Energy. One or two wells are currently drilled in each well’s spacing unit, which range in size from 320 to 1960 acres. Vertical drilling depths range from less than 8,500′ in the NW portion of the field to more than 10,500′ in the SE portion. Up to three horizontal laterals are drilled from each vertical well to a lateral distance of 1,600′ to as much as 10,000′. Horizontal drilling and large fracture treatment stimulations are keys to developing economically viable reserves from this field, with prior Bakken production from vertical wells in the area having been generally very disappointing.
Current data indicate estimated ultimate reserves per horizontal well ranging up to 1,250,000 bbls of oil, averaging approximately 500,000 bbls per well. From wells drilled to date, the field is expected to produce in excess of 200 million bbls of oil from an area of over 450 square miles. Current and planned exploratory drilling activity may expand the limits of the field dramatically.
The field is a stratigraphic trap developed in the Middle Member of the Bakken formation (L Miss/U Dev) with a variable and uncertain contribution of production from the overlying Upper Shale Member of the Bakken. The Middle Bakken Member appears to be the principal reservoir and is primarily a dolomite, variably silty with scattered pyrite and anhydrite, and with increasing argillaceous content toward its base. This lithology is sharply in contrast with the sandstone/siltstone and limestone deposits encountered in the Middle Bakken Member in the northern and eastern portions of the Williston Basin. The reservoir appears to have developed diagenetically within a large and complex carbonate bar/bank accumulation which extends over 130 miles from NW to SE, ranging from 6 to 15 miles in width and up to 35 feet thick (thinner to SE). This complex stretches from northwestern Richland County, MT, to southwestern Dunn County, ND, parallel to and basinward of the southwestern margin/shoreline of the Middle Bakken depositional basin. Structurally, in the 60+ miles across Richland County, the Bakken pay section dips about 2,000 feet from NW to SE, traversing a few small, local structural closures that often produce from deeper Devonian, Silurian and Ordovician reservoirs.
Within the highly productive Richland County portion of the bar/bank complex, the reservoir is slightly overpressured (approx. 0.50 fluid pressure gradient) and consists of 8-14 feet of 8-12% porosity and low permeability (on the order of 0.05 to 0.10 md.). Local variations in these reservoir properties, occasional increased natural fracture development within the reservoir and variations in completion techniques likely collectively play an important role in the range of well productivity within the field.