Of the 52,000 sq mi (134,680 sq km) within western Montana and northern Idaho appraised in this report, over 40,000 sq mi (103,600 sq km) has been eroded down to the Precambrian or batholithic surface. In the remaining 12,000 sq mi (31,080 sq km), Paleozoic and Mesozoic rocks are preserved in two types of structural elements--structural basins and overthrust belts.

There are six small structural basins in southwest Montana, three of which appear to have little possibility of containing hydrocarbons. The other three, located in extreme southwest Montana, are interconnected; they form a long narrow syncline with nearly 20,000 ft (6,100 m) of strata. They contain good source and reservoir beds and appear to be the basinal area of southwest Montana most prospective for hydrocarbons. The basins remain essentially untested.

In that part of northwest Montana covered by this report, only the overthrust belt appears capable of producing hydrocarbons. Good structural traps are present but good source beds appear to be absent.

Even though no commercial oil or gas fields have been found in western Montana, it seems probable that undiscovered accumulations are present. Exploration will continue until the area is more completely tested, though the combination of high exploration costs and small potential will tend to keep efforts at a low level.