Diagenesis of the Bakken Formation, Elm Coulee Field, Richland County, Montana

Elm Coulee Field, discovered in 2000 in Richland County, Montana, is the largest oil field in the Williston Basin. This field produces from the middle member of the Devonian-Mississippian Bakken Formation and has an estimated ultimate recovery of 200-250 MMBO. The Bakken’s diagenetic history in combination with horizontal drilling and hydrofracturing practices make it possible to get large recoveries from this low permeability and porosity field.

The Bakken Formation at Elm Coulee Field is composed of three main members that average 40 feet in thickness. The lower and upper Bakken members consist of dark, organic-rich marine shales with silty laminations. The middle Bakken member is predominantly a silty dolostone. It contains six shallow-marine facies that include laminations, abundant burrows, and brachiopod and crinoid fragments.

Production at Elm Coulee would not be possible if the Bakken Formation had not undergone a variety of diagenetic stages that have resulted in a dolomite-rich reservoir rock with enhanced secondary porosity. The diagenetic sequence of the Bakken begins with mechanical compaction, early dolomitization of the original silty lime mud in a shelfal seepage reflux setting, and pyrite formation. Next came a period of dedolomitization, deeper burial-related dolomitization, and formation of secondary mineral cements of anhydrite, sphalerite, and quartz. There are also two stages of fracturing at Elm Coulee. The first, mineralized vertical to subvertical fractures, is likely related to either underlying salt dissolution, the reactivation of basement faults, or pressure release from sediment dewatering, and the second stage, open horizontal fractures, is related to pressure release from the hydrocarbon expulsion process. The beginnings of hydrocarbon generation in the shales also expelled acids into the middle Bakken dissolving parts of the dolomite rhombs forming narrow slot pores and larger dissolution pores which further enhanced the storage space for the subsequently expelled oil.

The results of this study indicate that the Bakken petroleum system within the Williston Basin is a complex system that has huge potential for future discoveries. Understanding the distribution of the facies and the diagenetic stages that have occurred within the middle Bakken reservoir member is the key to determining new drilling targets within Elm Coulee and to the search for similar fields in this basin that may also be good production targets.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California