Most of the Pennsylvanian Period in Region 5 was characterized by expanding seas between emergent landmasses, some of which had been raised at the beginning of the period and others of which were uplifted during Early and mid-Pennsylvanian time. Foreland positive areas supplied clastic detritus to parts of the intervening basins, and outlying geosynclines received most of the debris from nearby mountain ranges. Carbonate layers were deposited on marginal shelves, and organic limestone reefs were built at various localities within and adjacent to the shelf areas. After a major tectonic episode in earliest Permian time that affected many areas of previous crustal weakness, the deep basins began to fill with fine clastic sediment as carbonate sediment accumulated along margi al platforms. As the seas became progressively more restricted, evaporite deposition encroached over the rest of the Permian basin. A relatively thin veneer of Cretaceous, Tertiary, and Quaternary sediments completed the sequence.

Crude oil and natural gas are found in structural and stratigraphic traps in sandstone and limestone reservoirs, including reefs. Total petroleum resources yet to be discovered in Pennsylvanian and Permian reservoir rocks are estimated to be 41.9 billion bbl of in-place crude oil and 43.8 trillion cu ft of in-place natural gas. Addition of these amounts to the known Pennsylvanian and Permian quantities that remain in place after production to year-end 1967 gives 116 billion bbl of crude oil and 83.2 trillion cu ft of natural gas.

Much of the undiscovered petroleum is expected to be contained in stratigraphic traps in the less densely drilled areas, and may be present as major accumulations. With all phases of exploratory activity declining, it will take more creative thinking and coordinated effort to find the remaining petroleum.

In addition to the oil and gas normally considered potentially recoverable, other petroleum hydrocarbons are believed to be disseminated in sedimentary rocks and dissolved in subsurface waters. Although the volumes of the disseminated hydrocarbons and the dissolved gaseous hydrocarbons are very large, these hydrocarbons are categorized as very speculative and unobtainable from an economic standpoint.