Pre-Cambrian basement rocks are the reservoir from which several scattered wells in central Kansas produce oil. More than a million barrels of oil have been produced from pre-Cambrian quartzite in the Orth field, Rice County, from fifteen wells. Similar quartzite is the reservoir from which oil is produced in four wells in the Kraft-Prusa field, one well in the Eveleigh field, and one well in the Trapp field, all in Barton County. Fresh pink biotite granite is the reservoir rock producing oil in three wells in the Hall-Gurney field and in one well in the Gorham field, both in Russell County. A few other wells, not investigated by the writer, are reported to be producing from pre-Cambrian rocks.

All wells known to be producing from pre-Cambrian rocks are located on the summits of buried pre-Cambrian hills. Porosity consists of a reticulated fracture system. The pre-Cambrian reservoir rocks are unconformably overlapped by Pennsylvanian limestones now draped above the hills in gentle anticlinal folds which trap oil in multiple thin porous zones in the Topeka (Virgil) and Lansing-Kansas City (Missouri) limestones. The Pennsylvanian rocks are considered as the probable source from which oil migrated locally into the cracks in the pre-Cambrian rocks. Truncated Cambro-Ordovician Arbuckle dolomites, themselves an oil reservoir, are present on the flanks of each hill and are a less probable source from which the oil in the pre-Cambrian reservoirs was derived.

Production records for individual wells are difficult to obtain, but available data indicate that recoveries from several quartzite wells in the Orth field exceed 75,000 barrels per well. Two wells, now abandoned, drilled by Slick, Pryor, and Lockhart on the Habinger lease in Sec. 27, T. 18 S., R. 10 W. produced 173,217 barrels of oil or 86,608 barrels per well from a depth of 3,200 feet. Wells on three other leases in the Orth field are estimated to have produced 100,000; 130,000; and 150,000 barrels per well from pre-Cambrian quartzite and are still producing. The Sunray's Culbertson No. 1 in Sec. 26, T. 18 S., R. 10 W. produced 168,000 barrels of oil (gravity 43°) into its own individual tank battery from the pre-Cambrian quartzite encountered at a depth of 3,211 feet from its completion in February, 1941, to July, 1948, and is still producing (January 1, 1950). Data on oil recoveries from wells producing from biotite granites are not available.

It is the writer's opinion that in a great many wells the pre-Cambrian rocks have been inadequately tested for porosity and fluid content. It is suggested that the pre-Cambrian basement rocks, where encountered structurally (or topographically) high in future drilling in central Kansas, are worthy of careful consideration as a potential oil reservoir.

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