The Kraft-Prusa oil field in northeastern Barton County, Kansas, is unusual because of its development ratio of one dry hole for every three oil wells. The drilling of 666 tests to January 1, 1947, has resulted in 509 oil wells and 157 dry holes.

As elsewhere in the Central Kansas Uplift area, Pennsylvanian beds rest unconformably on truncated Cambro-Ordovician and pre-Cambrian rocks; the porosity of the pre-Pennsylvanian oil reservoirs is genetically related to this unconformity surface.

The oil reservoirs may be grouped into four types whose relative importance is nearly in direct ratio to the number of wells producing from each. (1) Reservoirs in the arched and truncated Cambro-Ordovician Arbuckle dolomites at their unconformable contact with Pennsylvanian beds at an average depth of 3,350 feet. Porosity is due to solution. The average thickness of producing beds is 23 feet. None of the 390 wells flowed. Reservoir energy is due to water drive. It is estimated that 85 per cent of the field's cumulative production has been derived from Arbuckle dolomite wells. (2) Reservoirs in Pennsylvanian limestones, producing in 53 wells. The best porosity is in beds of hollow oolites, 3-10 feet; or more in thickness in the Lansing-Kansas City (Missouri series) limestones below 3, 50 feet. Some oil occurrences are due to porosity variations, but the most important reservoirs are anticlinal domes above the summits of three buried pre-Cambrian hills where Pennsylvanian limestones rest directly on pre-Cambrian. The largest dome has 1° dips and a closure of 50 feet. Structurally high tests flow initially. Reservoir energy is due to gas-cap expansion followed by a dissolved gas drive and the encroachment of edge water. (3) Reservoirs in sandstones associated with the unconformable contact of Pennsylvanian on Cambro-Ordovician and pre-Cambrian. These sand bodies, ranging to 45 feet in thickness, fringe three pre-Cambrian hills. They form stratigraphic traps with water drive from which 65 wells produce at a depth of 3,350 feet. (4) Fractured pre-Cambrian quartzite at the summit of a buried hill, known to be producing in only one well at a depth of 3,315 feet.

Many dry holes in the Kraft-Prusa field encountered "fossil" sinkholes in the Arbuckle dolomites filled with slumped non-porous residual clays and cherts.

Development is continuing. The cumulative production of the field, discovered in 1937, is about 25 million barrels to January 1, 1947, of which 5 million barrels were produced during 1946 from about 15,000 acres.