Regional sites of oil accumulation in the United States are limited to sedimentary shelves. Lateral extent of the shelves may be only 25 miles, as for example, the Frio (Oligocene) of the Texas Gulf Coast, or 400 miles, as in the case of the Cretaceous or Paleozoic beds in the central western United States.

Reservoir rocks on the shelves are neritic in origin. Sandstone bodies are lenticular and ragged. Isolated, shale-enclosed sandstone bodies, scattered over the shelf, contain prolific quantities of oil. Origin of oil is obviously local, having migrated only a few feet from the source shale into the adjacent porous host rock. After entering the porous reservoir the oil migrated up-dip into stratigraphic and anticlinal traps, the movement limited only by the boundaries of the porous stratum.

Accumulation of commercial oil in early Pennsylvania rocks of eastern Oklahoma and eastern Kansas is limited to a sedimentary shelf. The marine shoreline had advanced northwest from the site of the Ouachita trough over the subsiding McAlester Basin and upon the more stable shelf or southeast margin of the craton. Line of juncture of the subsiding basin and the more rigid shelf became a hinge line. Atoka lithology

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in the McAlester Basin is shale, diverging southeast from the hinge line at the rate of 170 feet per mile, across the shelf, 18 feet per mile. Isolated sand bodies (named the Gilcrease and Dutcher by drillers), scattered over the shelf, are productive of oil.

General observations to be drawn are:

1. A lithotope favoring irregular sand deposition existed over the shelf.
2. Migration and accumulation of oil was limited to the shelf.

Similar conditions of oil origin, migration and accumulation prevailed in the succeeding Des Moines (Middle Pennsylvania) time. The shelf area progessively broadened on the northwest as the sea advanced over lowlying land of the craton. Deposition of neritic sands on the shelf were similar to Atoka Sedimentation. Accumulation of oil in the Booch, Bartlesville, Red Fork, Skinner and Prue sands is limited to the moving shale.

Each of these sands, except for the Bartlesville, is shale-enclosed and isolated from the McAlester Basin. Shoestring sands of Greenwood or Anderson Counties, Kansas, have yielded prolific quantities of oil. The localities are 200 miles from the McAlester Basin.

The Bartlesville sand is persistent across the basin and along the eastern margin of the shelf. This reservoir is a body 100 miles long, 50 miles wide and 100 feet thick. Accumulation of oil in this sand is limited to the shelf. Anticlines of ample relief and stratigraphic traps of definite form exist in the basin province. These do not contain oil.

The principle that oil originated, migrated and accumulated on a shelf is thus validated by repeated evidence in the rock section and region discussed.

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