In central Kansas about 15,000 wells produce oil from the Arbuckle dolomite of Cambro-Ordovician age at depths from 3,200 feet to 4,400 feet. About 500 of these wells produce gas with the oil, or produce only gas. Reservoir porosity was formed, or improved, by solution in early Pennsylvanian time when the dolomite beds cropped out on a land surface of low relief. Ground water moved downward through joints, then laterally along bedding planes in the soluble dolomite, continuing downdip under Ordovician Simpson shale cover on the flanks of the Central Kansas uplift. The gently tilted, truncated, solution riddled, porous, and permeable Arbuckle beds were a great regional aquifer until buried under middle Pennsylvanian beds which provide an impervious seal. Oil and gas are tr pped in highs at the top of the Arbuckle, whether anticlinal folds, buried hills, or traps formed by barriers such as clay-filled solution valleys. Dips are ½° or less.

The lowest Arbuckle traps, such as the Shady and Zook fields of Pawnee County, produce sour gas from depths near 2,000 feet below sea-level. In the Sweeney, Ash Creek, and Pawnee Rock fields of Pawnee County, gas is produced from the Arbuckle near -1,800 feet, -1,750 feet, and -1,650 feet, respectively, with a thin oil column present in each field. Both oil and gas are produced from the Otis-Albert field of Rush and Barton counties where the gas-oil contact is -1,600 feet. Oil, and only oil, is trapped in the major Arbuckle fields of Rice, Barton, Russell, Ellis, Rooks, and Graham counties at depths of -1,600 feet to -1,400 feet. In the Kraft-Prusa field of Barton County the oil-water contact is -1,465 feet. The critical closure of the Arbuckle anticline also is -1,465 feet, indicatin that the reservoir is exactly filled with oil. In Ellsworth County, a large anticline with porous Arbuckle as high as -1,350 feet contains only salt water.

This pattern of differential entrapment of gas and oil accords closely with the theoretical distribution of hydrocarbons described by Gussow (1954) and provides part of the evidence from which it is concluded that gas and oil migrated out of Oklahoma northward through Arbuckle rocks for more than 100 miles as postulated by Rich (1931a and 1931b), due to tilting and loading. The gas and oil spilled upward under an impervious roof of Ordovician Simpson shale and Pennsylvanian beds until differentially entrapped in central Kansas. Time of migration was mid-Permian. Remigration eastward within the Central Kansas uplift occurred at the time of pre-Cretaceous westward tilting. Oil within the reservoirs readjusted during pre-Pliocene northwestward tilting and post-Pliocene eastward tilting. owever, the principal time of accumulation was mid-Permian as shown by the original reservoir pressures of the differentially entrapped gas and oil in the Arbuckle reservoirs of central Kansas.