Most of the oil discovered so far has come from basinal areas of the earth underlain by great thicknesses of sedimentary rocks. Potential reservoirs must be covered with an adequate thickness of impermeable rocks or the oil will leak to the surface and be dissipated. Thus, in regions where the sedimentary covering of the basement rocks is less than 1,000 m thick, oil and gas are missing or present only in small quantities. The sedimentary filling must be unmetamorphosed, for oil is not found in strata that have been affected by regional metamorphism.
Although regional metamorphism generally precludes the accumulation of oil in quantity, deformation of the strata by faulting and folding often forms structural traps in which petroleum accumulates. The continental side of migeoclinal provinces, where sediments have been slightly disturbed by the movements in the geocyncline or by the instability of the cratonic margin, are among the most favoreable places for the accumulation of oil. Examples of states with large production from such accumulations are PA, OK, and WY. Petroleum may be trapped in both flat-lying and folded sedimentary rocks by facies changes, such as the sealing of porous sand lenses by shale or the transition of limestone reefs into shales.
Study of recent sediments shows that they contain only infantesimal amounts of hydrocarbons, which can be flushed out with a great quantity of trapped water into adjacent porous beds as the sediments are compacted. If petroleum is to accumulate in quantity, the sedimentary basin must have a great volume of the source beds (probably dark shales) to supply the necessary hydrocarbons, because hydrocarbons are in such low concentrations.
The first well drilled for oil in the Canadian Arctic Islands was started in the winter of 1961-1962. Since then about 100 wells have been drilled in exploring this tectonic province, one of the last to be tested for petroleum resources. Although the conditions for the accumulation of petroleum appears to be appropriate, so far the results of exploration in the islands have been disappointing.
The Silurian reef belts of the Parry Islands that appeared to be promising reservoirs have been found to be dry in most wells. However, on a small island off the Northwest coast of Bathurst Island wells have encountered significant accumulations of oil in Devonian rocks. So far oil, in quantities large enough to be of commercial interest in such a remote area, has not been discovered in the Sverdrup Basin, although source beds, structural and stratigraphic traps, and appropriate reservoirs are all present. However, drilling has located large accumulations of natural gas, particularly along the southern boundary of the Sverdrup Basin in Melville Island. At present, sufficient gas has been proven to justify planning of a pipeline to bring it to southern markets, but the search continues for an oil field large enough to justify the great costs of transporting the oil to distant markets.
At the inner margin of the Arctic Coastal Plain in the northeastern corner of Alaska, the largest accumulation of petroleum yet found in North America, was discovered in 1968 at Prudhoe Bay. The recoverable reserves of the Prudhoe Bay field have been estimated at 9.6 billion barrels of oil and 26 trillion cubic feet of gas. To tap this vast energy deposit, the Trans-Alaska pipeline has been built across some of the most formidable mountains in the world. The field is underlain by the Barrow Arch, a buried ridge of early Paleozoic rocks deformed in the Elesmerican orogeny.
Late Paleozoic and early Mesozoic beds arch over the crust of the ridge, are truncated by an unconformity, and their edges are sealed by a Cretaceous shale. Petroleum migrating up the flank of the arch has been trapped under the unconformity in beds ranging from Mississippian to Jurassic age.
In the early 1970's, several oil and gas fields were discovered on the Arctic Coastal Plain in the vicinity of the Mackenzie Delta. The fields in this area are in the thick Cretaceous and Cenozoic clastic sequences, and produce from traps associated with the fault systems that traverse the seaward-dipping edge. The successes south of the Beaufort Sea lend encouragement to geologists exploring the similar beds of the Sverdrup Basin.