That could well be. USGS estimates tend to be conservative, simply because scientific credibility is at stake and putting an overly optimistic estimate would hurt that.
Consider, though the USGS estimates of the Bakken, and later the Bakken/Three Forks reserves have doubled more than once and will likely increase even more as time goes by.
For the Williston Basin (where the Bakken Formation is found), though, this is just a couple of fifteen known oil bearing formations (not to mention the Winnipeg and Deadwood gas sands). Not all are as amenable to large scale horizontal drilling (production is more localized), but there is a lot more oil yet to be produced.
One of the biggest problems we’re having now is not enough drilling rigs, fracing crews and oil haulers. It’s been 2 months since we drilled out last well and still haven’t fraced it. With the depths were going to now allot of the old rigs just can’t take the weight, they’ve collapsed two derricks in the last two months just in this county alone. I used to get my oil hauled within 24 hours of a call in, now it may go as long as 5 days. Several times I’ve had to shut in wells due to my tanks being full.