For many years petroleum was assumed to be largely of marine origin. However, it is now clear that significant amounts of petroleum of nonmarine origin are present in the Rocky Mountains, Europe, Africa, China, and elsewhere.

Petroleum in the Green River Formation (Eocene) is indigenous to continental beds, whose lacustrine facies were largely responsible for their formation. Green River source rocks were deposited in fresh to brackish water environments in stratified lakes. Large amounts of organic material accumulated over widespread areas in stable, reducing, low-energy settings. Migration distances from source beds to reservoir beds were small. Similar types of nonmarine marginal basins occur in west Africa.

In the southern San Juan basin, oil is trapped in eolian beds of the Entrada Sandstone (Jurassic). Overlying lacustrine limestone and anhydrite of the Jurassic Todilto Formation provide a seal. Oil was formed within the Todilto and migrated into the Entrada.

Eolian sandstone beds commonly are good reservoir beds in other situations. In the Overthrust belt of the Rockies, eolian parts of the Nugget Sandstone (Jurassic) contain large amounts of petroleum that migrated into the sandstone from marine Cretaceous sources. The best reservoirs in the Permian Rotliegendes of the southern North Sea consist of eolian dune sandstone. Exceptional porosities of 30% are found in some intervals that produce gas.

Gas has formed in several early Tertiary fluvial beds of the Rocky Mountain area and migrated short distances into fluvial sandstone reservoirs. Fluvial sandstone sequences also contain oil generated in nonmarine source beds.

The examples of petroleum occurrences in nonmarine rocks given in this program may serve to reawaken the interest of petroleum geologists. Certainly, the petroleum potential of nonmarine beds is large.

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