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The Bakken Petroleum System of the Williston Basin, from Unconventional Oil Resource Play

Sonnenberg, Steve 1; Sarg, J. F.1
1 Department of Geology, Colorado School of Mines, Golden, CO.

The Bakken Petroleum System of the Williston Basin is an unconventional oil resource play characterized by low porosity and permeability reservoirs, organic rich source rocks, and regional hydrocarbon charge. The play is the current focus of exploration and development activity by many operators. Estimates of oil generated from the system range from 10 to 400 billion barrels. The Bakken Petroleum System has reservoirs in the Bakken, lower Lodgepole, and upper Three Forks formations. The source beds for the system are in the Bakken.

The Bakken consists of three members: (1) lower shale member; (2) middle dolomitic siltstone/sandstone member; (3) upper shale member. The main target of horizontal drilling to date has been the middle member of the Bakken. The middle member is complex lithologically and petrophysically. Recent giant oil discoveries in the Elm Coulee and Parshall field areas suggest enormous potential for future oil discoveries in the Bakken middle member.

The Bakken shales (upper and lower members) are relatively thin, and contain largely amorphous kerogen characterized by high TOC contents (averaging 11%). The shales have unique petrophysical properties that can be used to distinguish maturity areas and organic richness in the Bakken shales. The upper and lower members are lithologically similar throughout much of the basin. The shales are dark-gray to black, hard, siliceous, slightly calcerous, pyritic, and massive to fissile. The shales consist largely of dark organic material and silt-sized quartz and feldspar, minor illitic clay, and some calcite and dolomite. In the deeper parts of the basin, the shale is kerogen rich and the organic material is distributed evenly throughout. The upper and lower shale’s are interpreted to have been deposited in an offshore marine anoxic environment.

The middle Bakken is a silty dolostone or dolomitic siltstone and several distinct facies can be correlated through much of the basin. Some sandstone units are also locally present. The unit is interpreted to be a marine unit deposited under aerobic to dysaerobic conditions.

Although regional maturity and hydrocarbon charge exist in the Bakken interval, matrix permeability and fracturing will play key roles in controlling Bakken sweet spots. Identifying these key elements with remote sensing or on geophysical logs and seismic will aid in future Bakken exploration.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009