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The AAPG/Datapages Combined Publications Database

Rocky Mountain Association of Geologists

Abstract


The Bakken-Three Forks Petroleum System in the Williston Basin, 2011
Pages 102-126

Chapter 4: Mineralogic Analysis of the Middle Bakken Member, Parshall Field Area, Mountrail County, North Dakota

Becky L. Kowalski, Stephen A. Sonnenberg

Abstract

The Bakken Formation of the Williston Basin has become a very important unconventional oil play in the Rocky Mountain region. Exploration and development of the middle Bakken began at Elm Coulee Field in Montana in 2001 and expanded into North Dakota with the discovery of Parshall field in 2006. The US Geological Survey (2008) has estimated that there is 3.65 billion barrels of undiscovered technically recoverable oil remaining within the Bakken Formation.

The Bakken Formation is composed of three units: upper and lower organic-rich shales, and a lithologically variable middle member which is comprised of interbedded siltstones, carbonates, and sandstones. The middle member serves as the primary reservoir but has low matrix porosity and permeability. Porosity is 2-12 percent and permeability is 0.0001-7.0 md.

Quantitative mineralogy shows that the middle Bakken member in the Parshall Field area consists of silt- to sand-sized grains of 14.9-65.6 percent quartz, 2.8-49.1 percent dolomite, 0.9-76.0 percent limestone, and 1.9-10.1 percent feldspar with minor amounts of clay. This indicates a marine depositional environment with water depths ranging from shallow (within wave base and tidal influence) to deep neritic.

Analysis of the quantitative mineralogy shows that there is a relationship between mineralogy and porosity. Porosity is high in areas where there is a high percentage of quartz and dolomite grains and where the mineralogy is homogeneous in distribution and grain size. Porosity is low in areas where authigenic cements and clays are occluding the pore space. Fracturing is present along planes of weakness such as in zones with a high clay percentage. Two facies (E and F) contain the most real (natural or natural-open) fractures. This is due to these two facies containing the largest percentage of dolomite.


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