Still waiting for todays release of the updated USGS Bakken study, Here is a 2001 geological assessment.
The 2008 assessment is 3.65 billion barrels of technically recoverable oil.
Diagenesis and Fracture Development in the Bakken Formation, Williston Basin: Implications for Reservoir Quality in the Middle Member, By Janet K. Pitman, Leigh C. Price, and Julie A. LeFever, U.S. Geological Survey Professional Paper 1653
The Upper Devonian–Lower Mississippian Bakken Formation
in the Williston Basin, North Dakota (fig. 1), is a closed, low-permeability petroleum system that generated approximately 200 to 400 billion barrels of oil in place. Most of this generated oil was expelled into very fine grained sand-stones and siltstones within the middle member, which is bounded by organic-rich shales that are both sources and seals. Source-rock thickness, thermal maturity, and total organic carbon (TOC) contents controlled the amount of oil generated and expelled from the shales.
The middle member of the Bakken is an oil reservoir in the thermally mature part of the basin and has been extensively cored; thus, it offers the opportunity to examine the major controls on reservoir quality.
Reservoir porosity of sandstones and siltstones in the middle member is poor, varying from 1 to 16 percent and averaging about 5 percent. Under in situ conditions, porosity is as low as 3 percent. Most porosity in the middle member is associated with open, hydrocarbon-generated fractures; some secondary porosity caused by organic acids is also associated with fractures. Permeability in sandstones and siltstones ranges from about 0 to 20 millidarcies and averages 0.04 millidarcies. Permeability values greater than 0.01 millidarcies are associated with rocks that have high residual oil saturations and a high incidence of hydraulically induced fractures. Most oil in the Bakken petroleum system resides in open, horizontal (bedding-parallel) fractures and in secondary micro-porosity adjacent to fractures, with only small amounts dispersed in matrix pores. Horizontal fractures form a pervasive network in deeply buried reservoir rocks with high residual oil saturations, but they are generally absent in shallowly buried rocks with little to no residual oil. These fractures resulted from superlithostatic pressures that formed in response to increased fluid volumes in the source rocks during hydrocarbon generation. Unlike mineralized fractures that are incapable of transmitting fluids, porous and permeable horizontal fractures serve to focus hydrocarbon fluids and locally enhance the quality of oil reservoirs at depth.
An article about bakken oil drilling activity in South Dakota.
Kansas City.com has a two page summary of the Bekkan formation.
The U.S. Geological Service said it would release its findings this afternoon on the service’s Web site, www.usgs.gov.
“Hundred-dollars-a-barrel helps offset the risk,” said geologist Julie LeFever, who has spent decades researching the Bakken for the North Dakota Geological Survey. “But if the price drops through the floor, most of the drilling would be over. The bottom line always is economics.”
Horizontal drilling and the modern fracturing techniques used to collect the crude cost about $6 million per well — six times the expense of a vertical well. But Ness said a horizontal operation, if successful, can produce many times the oil.
He compared the method to excavating the creme filling of an Oreo cookie from the side rather than by drilling several holes from the top.