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.
Figures & Tables
The geology of the entire United States, including the continental shelf and slope, was studied by petroleum geologists to determine its petroleum potential. Prospective areas of the 11 regions were assessed qualitatively and, usually, quantitatively.
The prospective basinal area covers approximately 3.2 million sq mi (statute; 8.3 million sq km) and contains approximately 6 million cu mi (25 million cu km) of sedimentary rock above basement or 30,000 ft (9,144 m). Other less prospective areas are, in the aggregate, large.
The prospective area has not been explored adequately. Many high-potential areas are indicated by the geology and extent of exploration, particularly in parts of Alaska, California, Colorado, Louisiana, Mississippi, Montana, New Mexico, North Dakota, Oklahoma, Texas, Utah, and Wyoming, and in parts of the offshore of Alaska, California, Louisiana, and Texas. The prospective Atlantic, Florida, and Alaska continental shelves, and the entire continental slope, barely have been touched by drilling, and other prospective areas and depths on land and the continental shelf remain largely unexplored.
Estimates of potential crude oil reserves of the basinal area only, exclusive of known reserves, range from 227 to 436 billion bbl of original oil in place. The potential probably exceeds the mean of 332 billion bbl. Approximately 32 percent of the oil in place would be recoverable at known rates of recovery. Ultimately, the rate of recovery may reach 60 percent.
Estimates of potential natural gas reserves exclusive of known reserves range from 595 to 1,227 trillion cu ft of recoverable natural gas. The gas potential also probably exceeds the mean of 911 trillion cu ft.
The ultimate petroleum potential of the United States, including known reserves, may exceed 432 billion bbl of crude oil, 1,543 trillion cu ft of natural gas, and 49 billion bbl of natural gas liquids.
Finding and developing the large petroleum potential will require a great amount of drilling because a significant percentage of the visualized undiscovered crude oil and natural gas is in stratigraphic traps, combination stratigraphic and structural traps, reefs, and complex structural situations. Estimates of future domestic demand call for accelerated exploration. To the extent that policies of industry and government militate against accelerated exploration, particularly drilling, a high percentage of the petroleum resources of the United States will not be reduced to possession.