The unevenness of the occurence of petroleum is due in part to the unevenness of the exploration effort. Exploration is influenced by current geologic thought, economic considerations, and political strategy. The ultimate goal of the petroleum industry is to find oil and gas that are profitable to produce, which also affects petroleum exploration. Profitability in petroleum means the discovery of new pools or the improved development of known ones.
Although a wealth of gold, tin, copper, silver, and other metals is found in deposits or associated with intrusions of Cenozoic rock, the greatest resource of the era has been petroleum. Indeed, most of the world's petroleum has been found in strata that range from 1 to 60 million years of age.
Most of the oil discovered so far has come from basinal areas of the earth underlain by great thickness (more than 1,000 meters) 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.
Most of the known accumulation of oil has been found in Tertiary reservoir rocks. In the United States, about half of all the oil ever discovered has been in Tertiary rocks.
Petroleum has been recovered from Ordovisian and Silurian rocks in Ohio, Oklahoma, and Texas, as well as in western Europe.
Paleocene petroleum reservoirs occur in Libya and beneath the North Sea.
Petroleum trapped in Eocene permeable limestones and sandstones is being pumped in Texas, Louisiana, Iraq, the former Soviet Union, Pakistan, and Australia.
Strata of Oligocene age yield oil in western Europe, Burma, California, and the Gulf Coast states of the United States.
Miocene reservoir sandstones yield oil on every continent except Australia. They are extraordinarily productive in Saudi Arabia, Kuwait, California, Texas, and Louisiana.
Offshore oilfields in the Gulf Coast and California tap oil held in rocks in Pliocene age.
Oil has been pumped from Jurassic and Cretaceous formations in the Bighorn basin of Wyoming.
Commercial quantities of oil and gas are frequently found in Upper Paleozoic strata. Devonian reefs within the Williston basin of Alberta and Montana have been exceptionally productive reservoir rocks for petroleum. Devonian petroleum has also been produced in the Appalachians. Indeed, in 1859, the first U.S. oil well was drilled into a Devonian sandstone. Oil was struck at a depth of only 20 meters. Carboniferous formations of the Rocky Mountains, midcontinent, and Appalachians also contain oil reservoirs. However, wells drilled into reefs and sandstones of the Permian Basin of West Texas have yielded the treatest amount of oil from Upper Paleozoic formations of the western United States. Oil trapped in Upper Paleozoic strata beneath the North Sea is now being produced for use in Europe.
For many reasons, the Devonian rocks of the Catskill clastic wedge (also called the Catskill delta) have excited the interest of geologists for well over a century. This apron of sediments provides an ideal area for the examination of facies from a varied assortment of both marine and nonmarine depositional environments. Catskill facies charateristically exhibit rapid lateral changes from sandstone to shales. Such relationships form traps for petroleum and are the reason for the thousands of wells drilled into Catskill strata.