Cradle of the American oil industry, the Appalachian basin originated as a stratigraphic trough whose present deep-buried floor was the ancient Precambrian erosion surface first covered by Cambrian seas. It now holds an intricate series of marine and continental sediments whose deposition followed a fairly constant pattern, with primary source at the east, medial thickening, and an outer, western, zone of facies change and shoaling conditions along the east flank of the Cincinnati arch that formed a hinge in the subsidence following each formational deposition. The same country is now a structural trough shaped during orogenesis that finally crumpled the former source area into the familiar Appalachian highlands, and whose shadow or foreland folds were thrown far west even of the center of the trough. Details of local orogenic history show at least two early stages of folding overridden by the final fold pattern with actual or incipient domes at various points of intersection.
The Appalachian sedimentary trough was also a supply basin for petroleum hydrocarbons that moved mainly upward and outward to become concentrated in border zones having suitable reservoir qualities. Such zones have a present basinal position that generally parallels the west margin of the individual horizon, wherein emplacement of oil and gas was controlled primarily by favorable shoreline porosity and secondarily by fold patterns, with major imponding at the intersection of early and late folds. Subsequent differential sorting of the fluid systems of connected reservoirs was a result of gravitational flushing with concentration ultimately determined by relative buoyancy and local reservoir elevation.
Other specific Appalachian entrapment occurs along linear carrier beds, in sedimentary wedgeouts, in fracture systems, in deep relic reservoirs, and in uncompromised islands of favorable porosity within regions otherwise unfavorable. The possibility of locating additional reservoir traps in the deep central basin, along the basement floor, or below unconformable surfaces, is favorably regarded. Attention is paid to the northeast end of the Appalachian basin, particularly east of the Adirondack arch, out of which no production has yet been found but which has been imperfectly tested so far.
Similarities are discussed between such blanket sand deposits as the Tuscarora-Clinton, Oriskany, and Berea horizons. The shallow younger producing sands are treated as another genre, here named casual sands; and the several carbonate reservoirs are considered as a group. The possible future development of older horizons is anticipated.
Finally, with the familiar Appalachian basin as a type, certain stages in general basinal emplacement are reduced to a simple list that may be useful in the similar analysis of other basins of this general type.