A rough general assessment of the possibilities of petroleum accumulations in deep ocean regions beyond the continental shelf has been undertaken, for convenience, under three major heads--the central ocean region, the continental margin region, and the small ocean basins region. The information used comes largely from marine geophysical surveys and from 417 holes of the Deep Sea Drilling Project.

Evaluation is based principally on thickness of sediments, organic carbon content, reservoir rocks, probability of accumulation traps, probable geologic history, and known occurrences of oil and gas. The distinction is made between environment-accordant sediments (formed under essentially the same geomorphic conditions as at their sites today) and environment-discordant sediments (formed under markedly different conditions).

The central ocean region is considered with respect to the sedimentary rock developments of each of its major geomorphic units. In general, the prospects for petroleum accumulations appear unfavorable because of thinness of sediments, low organic carbon content, scarcity of reservoirs, paucity of shows, and generally horizontal attitude militating against trapping conditions; however, there are some exceptional areas.

The continental margin region includes (1) marginal geosynclines, (2) outer-margin aprons, (3) marginal-plateau blocks, (4) marginal-trench fillings, (5) continent-related deep water fans, and (6) transmarginal ridges. The region as a whole has favorable aspects as regards thickness of sediments, source rocks, reservoirs, sealing rocks, petroleum shows, geologic history, and probability of traps.

In the small ocean basins, thick sedimentary sections, abundant terrestrial and marine organic matter, reducing bottom conditions, well-developed reservoirs, mobile tectonic environment, and commonly an abundance of evaporites and diapiric structures combine to give a very favorable rating, supported by prolific production from the borders of many of these basins.

With respect to production prospects, the deep offshore has obvious handicaps of unfavorable environment and costly operations, but where economic prospects are sufficiently good, the technological challenge can be met.

There are large, almost untouched areas of deep offshore, sufficiently prospective to justify exploratory drilling, that urgently need evaluation. The United States alone has more than 1.5 million sq mi (3.9 million sq km) of combined shelf and deep-water offshore over which it can rightfully claim jurisdiction, of which only about 2% has ever been leased for development.