From a 15 slide pdf presentation by Tom Richmond on Montana's Bakken oil.
Nextbigfuture has already provided detailed coverage of Bakken oil in North Dakota and Saskatchewan
Oil companies in Montana
A breakdown by share of production in 2005. The major companies are Continental Resources, Headington Oil, Burlington Resources, Enerplus Resources USA and Encore Operating LP. Montana's Bakken oil is in Richland and Fallon counties.
Some history of Continental Resources' Montana Bakken production.
In 2006, Elm Coulee was producing about 53,000 barrels of oil per day from more than 350 wells. Ultimate production is expected to exceed 270 million barrels, with some estimates as high as 500 million barrels. Production at Elm Coulee has more than doubled the oil output of the state of Montana
An excellent presentation by Bill Walker and others from Headington Oil.
Montana oil well information is available online.
Scanning through the Montana state oil information it appears that 2007 and 2008 have been relatively flat or slightly declining from the 55,000 bpd level of 2006 for Elm coulee.
24 to 30 million barrels of oil per month for the state. About 80,000 to 100,000 bpd.
Montana's oil and gas board.
A list of links at the Montana oil and gas site for other oil and gas information across the USA
Montana's high fossil fuel scenario.
Great Falls Tribune article from April 2008 discussing the Bakken oil field, Elm Coulee and companies drilling there.