The Williston Basin’s Bakken and Three Forks formations within Montana and North Dakota contain substantial natural gas and oil reserves yet to be discovered, but extensive drilling since 2013 has reduced the overall amount, according to the U.S. Geological Survey (USGS).

Researchers in the latest report, as in the one published in 2013, assessed the undiscovered, technically recoverable and continuous, i.e. unconventional, resources in the Upper Devonian to Lower Mississippian Bakken and Upper Devonian Three Forks.

According to the review, the formations within the two states hold estimated, undiscovered unconventional reserves totaling 4.3 billion bbl of oil and 4.9 Tcf of natural gas. By comparison, the 2013 assessment found that the undiscovered reserves totaled 7.4...