The most important elements used in their geologic model were "(1) the geographic extent of the Bakken Formation oil generation window; (2) the occurrence and distribution of vertical and horizontal fractures; and (3) the matrix porosity within the middle sandstone member." Thus, it appears that the area where the Bakken has generated oil was divided into five separate assessment areas based on what appears to be somewhat comparable geologic characteristics of mainly the middle Bakken member. That consideration appears to be based on the fracturing and porosity present in those areas.
After challenging someone to show where there were more than one billion of recoverables in ND for months and getting no response, I feel pretty foolish for throwing out a 20 billion bbl number last night for the entire Bakken.
From the Dickinson Press:
Mike Armstrong, president of Dickinson-based Armstrong Corp., an oil and gas prospecting firm, said he was excited to hear the news but gave a skeptical reaction to the report. “How about this: Don’t over react,” Armstrong said. “A study is just that, it’s a study. There’s no guarantees.” Armstrong, who’s been in the oil business since 1975, said he thinks the report’s figure is an over-estimate. “I think they’re stretching it – a lot,” Armstrong said.
USGS Press Release
USGS Fact Sheet
USGS Interview (Audio)
Dickinson Press Article
New York Times Article