Abstract

Eland Field, the most prolific Lower Mississippian Lodgepole mound complex found to date in the Williston Basin, covers an area of about 6 mi² and has produced more than 29 MMBO from 16 wells in the 20 years since the field was discovered. Three of the field’s updip wells have each produced more than 4 MMBO from mounds more than 250 ft thick although generally only the top 20 to 30 ft of the mound is perforated. The lower Lodgepole reservoir rocks are commonly called Waulsortian mounds, but they are Waulsortian in age only and not the micrite-rich mudmounds found in the type area of Waulsort, Belgium. Instead of being mudmounds, they are composed mainly of marine-cemented microbial boundstones and skeletal grainstones with local stromatactis structures. The edges of the mounds dip as steeply as 40 to 60 degrees. Such steep dips would be impossible in a typical micrite-rich Waulsortian mound. In addition to the abundant microbial and marine cementation that lithified the Eland Field mound complex penecontemporaneously, organisms such as stalked crinoids, fenestrate bryozoans, and articulated brachiopods and ostracods thrived on the hard substrate provided by the mounds. However, these organisms were themselves incapable of forming a true reef framework.

Unusual inward dip of the Upper Bakken black shale and significant thickening of the “Extra Bakken Shale” from zero up to about 40 ft immediately beneath the oil-producing lower Lodgepole mounds both support the idea that something structurally significant such as salt dissolution helped localize mound development. We conclude that dissolution of the Lower Devonian Prairie salt created a fracture network during and immediately after deposition of the upper Bakken black shale that allowed compaction-water expulsion-vents and/or warm springs to initiate formation of carbonate “towers” that localized formation of the Lodgepole mounds.

The discovery well for Eland Field, the Knopik #1-11 (NW NW Sec. 11, T139N, R97W), was completed in January 1995 for 2707 BOPD and 1550 MCFGPD with no water. The field was developed over the next few years with up to 16 producing wells and 7 water injection wells. Waterflooding of Eland Field began in 1997 and climbed to over 500,000 barrels per month in just two months. Through September 2015, more than 95 MMBW had been injected into the field, but it has produced over 29.6 MMBO, doubling the 1996 estimated ultimate recovery of 12 to 15 MMBO. The field continues to produce with about a 5.5% oil cut.

In terms of total cumulative oil production in the U.S. portion of the Williston Basin, Eland Field contains 3 of the top 10 wells, and other nearby Lodgepole producing fields contain 4 additional top 10 wells. This excellent oil production leads to the question: Is it possible that the only productive Lodgepole mound reservoirs in the Williston Basin, eight of which have been discovered to date (with all but one discovered in the mid-1990s) are limited to a small atoll-like area about 7 miles in diameter in northern Stark County, North Dakota? Certainly the presence of similar Lodgepole mounds in outcrops in central Montana (e.g., Bridger Range and Big Snowy and Little Belt mountains) and in the subsurface of the western Williston Basin suggests that other Lodgepole mound-type reservoirs could occur across much of the basin. The most promising areas in which to search for other Lodgepole mound reservoirs will be those where the Upper Bakken black shale is well developed and thermally mature because it is the source for the oil in the Lodgepole reservoirs in the Eland Field area.