The northern Alaska potential petroleum province comprises the Mesozoic Colville geosyncline in the foothills of the Brooks Range and the ancient Arctic platform on the present coast, a total onshore area of about 70,000 sq mi (181,300 sq km). Depths to pre- Mississippian basement rocks range from less than 3,000 ft (914 m) on the platform to more than 20,000 ft (6,096 m) in the southern part of the geosyncline.
The Arctic platform was uplifted in Devonian time, when it was one of the sources of the thick wedge of Upper Devonian clastic beds deposited on the site of the present Brooks Range. The platform persisted info the Mesozoic as a relatively stable source area which was onlapped by Mississippian and Pennsylvanian carbonate beds and Permian to lowest Cretaceous clastic beds from the ancient Arctic Alaska basin on the south. The platform facies probably contains the most favorable Mississippian to Jurassic reservoir rocks; these include an average total of about 500 ft (152 m) of sandstone and 1,000 ft (305 m) of limestone and dolomite at depths less than 15,000 ft (4,572 m) within an onshore area of about 20,000 sq mi (51,800 sq km). This Mississippian to Jurassic platform facies grades southward into basinal source rocks that have an average thickness of about 6,000 ft (1,830 m). The platform "basement" itself locally may include pre-Mississippian dolomite reservoir rocks.
Cretaceous rocks in the Colville geosyncline, derived from a late Mesozoic orogenic belt in the Brooks Range, are mostly nonprospective graywacke and shale in a narrow disturbed belt along the front of the range. Molasse deposits of marine subgraywacke and shale that grade upward and southward into nonmarine rocks fill the rest of the geosyncline. They comprise about 9,000 ft (2,740 m) of shale and sandstone deposited during an Early Cretaceous regressive cycle and as much as 6,000 ft (1,830 m) deposited in a Late Cretaceous regression after an erosional interval. Lower Cretaceous sandstone beds have an average aggregate thickness of 750 ft (229 m) in an area of 46,000 sq mi (119,140 sq km). The best known Lower Cretaceous reservoirs are in the area of predominantly marine sandstone; however, migration of generally northwest-trending shorelines across the entire area during Early Cretaceous time enhances the overall petroleum potential. Sandstone beds in the Upper Cretaceous have an aggregate thickness of about 750 ft (229 m) in an area of 20,000 sq mi (51,800 sq km) in the eastern part of the geosyncline, and are locally very porous and permeable. Black marine shale at the base of the Upper Cretaceous regressive strata is a probable source for oil and gas, both in the overlying sandstone and in the unconformably underlying Lower Cretaceous sandstone.
Poorly known nonmarine Tertiary rocks that cover about 7,500 sq mi (19,425 sq km) in the northeast part of the geosyncline are more than 2,000 ft (610 m) thick and resemble the nonmarine Upper Cretaceous rocks. Both the Tertiary and the Upper Cretaceous probably thicken offshore above unconformities.
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.