Quantitative estimates of undiscovered resources of oil and gas beneath land areas have been prepared by many organizations and individuals on the basis of exploratory test wells and the presumed similarity of undrilled sedimentary basins to well-known ones. These estimates span a wide range of reliability. Undiscovered oil and gas resources beneath continental shelves probably are larger per unit area than those beneath the land, because the shelves are primarily marine depositional features, but the estimates of quantities beneath the shelves are even less reliable owing to fewer drillholes per unit area than on the land.
Least known of all potential oil and gas environments are the deep-water areas that include deep marginal basins, continental slopes, continental rises, and the deep ocean floor. Some analogs with the marginal basins are provided by similar basins that have been filled with sediments to form part of the con-tinental shelf or even the adjacent land, but uplifted and accessible unmetamorphosed examples of continental slopes, continental rises, and abyssal plains are rare, if present at all. Marine geophysical data are available for these deep-ocean sedimentary environments, but drillhole samples do not exist other than those from the JOIDES Deep Sea Drilling Project, which intentionally avoided sites of oil potential to avoid possible pollution of the ocean. In addition, few of these drillholes penetrated as deep as 1,000 m into the bottom. Lack of suitable drillhole data means that quantitative estimates of undiscovered oil and gas resources of the deep ocean floor are meaningless—and, therefore, so are those for the whole world.