Dow issued a paper back in 1974 that estimated total reserves to be 10 billion bbls. Since that time the estimates have continued to be revised up (almost exponentially). The estimates for recoverable oil have grown at a similar, if not greater, rate.
http://www.energyandcapital.com/resources/bakken-oil-field
A landmark paper by Dow in 1974 recognized the Bakken Formation as a major source for the oil produced in the Williston Basin and suggested the Bakken was capable of generating 10 billion barrels of oil.
That is not oil in place, that was an estimate of technically recoverable oil.
The oil in place is several hundreds of billions.