The key point is that the Middle Member--the shaly dolomite/limestone/sandstone--is productive locally, presumably in discrete traps. The billions and billions of oil estimates principally come from extrapolating the shale thickness across the whole basin. Again, the two key questions are to what extent the shale members are commercially productive across a large region and what the recovery factor is going to be.

http://www.contres.com/index.cfm?id=50

The Bakken formation is widespread and relatively uniform in development throughout the Montana and North Dakota portions of the Williston Basin. The Bakken formation consists of three lithologic members—the upper shale, middle member and locally a lower shale. The shales are highly organic, thermally mature and overpressured and act as both a source and reservoir for the oil. The middle member is also productive locally and varies in composition from a silty dolomite, to shalely limestone or sand across the Williston Basin. Horizontal drilling and advanced fracture stimulation technologies have enabled commercial recovery from this historically non-commercial reservoir. Generally, the Bakken formation is drilled horizontally on 1,280-acre units to vertical depths ranging from 9,000 to 10,500 feet with opposing horizontal laterals each extending approximately 4,500 feet, for a total drilled footage of approximately 18,000 to 21,000 feet. The wells are typically fracture stimulated to maximize recovery and economic returns.