Pre-Mississippian strata in the Permian Basin area consist chiefly of carbonate deposits of Ordovician, Silurian, and Devonian age which were produced in a marine environment probably characterized by clear, shallow seas covering a broad southward-sloping shelf. Oil accumulated in reservoirs in the carbonates, and in sandstone units of Cambrian and middle Ordovician age, the possible external sources being marine shale associated with the sandstone, a shale formation overlying the highest carbonate, and Pennsylvanian and Permian strata which lie unconformably on the older beds, as well as lower Paleozoic strata in a clastic basin south of the carbonate shelf. Mississippian strata are chiefly shale and limestone, containing probable source beds but generally poor reservoir strata.

Near the end of Mississippian time the tectonic environment changed from one of broad structures of gentle relief on a flat cratonic platform to an almost closed basin surrounded by mountain areas of high relief, the greatest of which was the Ouachita-Marathon complex.

Pennsylvanian strata, consisting of interbedded and intergrading marine shale, sandstone, and limestone, reflect the changed environment. Oil, which the author believes to be indigenous to the system, accumulated in structural, stratigraphic, and porosity traps in Pennsylvanian reservoirs.

The Permian Period was characterized by deep but areally restricted marine basins of clastic deposition, stagnant at depth and surrounded by shoal platforms on which thick masses of carbonates accumulated and shallow lagoons extended to the shorelines. Burial of earlier strata by evaporites and red beds brought the period to a close. The principal sites of oil generation probably were in the basins; reservoirs included basinal sandstone and platform carbonate and sandstone rocks. The Guadalupe Series, which is stratigraphically the youngest and highest important producing unit and was therefore among the first to be exploited, contained more than half of all the oil which to date has been found in the Permian Basin.

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