The USGS summary indicates that their estimates are "undiscovered, technically recoverable oil and gas resources of the Bakken Formation in the United States." The term "undiscovered" suggests that the currently known reserves have been excluded from their totals. "Technically recoverable" has not been defined in the USGS summary; we will need to wait for the detailed report to find out their definition. In the mean time, one reasonable definition might be "the volume of hydrocarbons that theoretically could be produced if enough wells were drilled to drain the entire known area of Bakken oil accumulation".
We can contrast "technically recoverable" to "economically recoverable," a term which can be taken as the amount of producible reserves that will give a reasonable return on capital invested. The volume of economically recoverable reserves will change as oil price, cost of wells, etc. vary. In reality, some of the areas of poorer reservoir will never be drilled because of poor economics: the cost and risk will not be offset by the expected recovery of oil. It is true that more of these marginal areas will be exploited as the price of oil rises.
Table 1 - Probabalistic estimate of "undiscovered, technically recoverable oil", from the 2008 USGS Bakken Assessment (Note: MMBO=Million Barrels of Oil, BCF=Billion Cubic Feet)
The USGS estimated reserves probabilistically, as indicated by the P95, P50, P5, and mean values on the table above. (P95, for example, indicates that there is a 95 percent chance that the reserves value given will exceed this value.) Using the mean value as a representative number for the distribution, the USGS estimate that there are 3,649 million barrels of technically recoverable oil in the entire US Bakken accumulation. Associated with the oil production are 1,850 billion cubic feet of natural gas and 148 million barrels of NGL's. On an equivalent energy basis, about 90% of the energy value is in the oil, with the remaining percentage in the gas and NGL's.
The Oil Drum | The Bakken Formation: How Much Will It Help?
