Discovery of the Ackman field, Red Willow County, Nebraska, in late 1959 encouraged exploration in that area to the extent that numerous other fields were found. Most of the production from these fields is derived from the Lansing-Kansas City Group, but basal Pennsylvanian sands have also contributed important reserves, particularly in the Sleepy Hollow field approximately 4 mi NE. of Ackman. The Ackman field was discovered as a result of seismic and subsurface work. Structurally, the field is related to the Cambridge arch, the axis of which is about 10 mi NE. The Cambridge arch has undergone several important periods of diastrophic movement during its geological history, the most significant of which occurred at the end of Mississippian time. Ackman field structure is low-relief with some evidence of basement adjustment on the E. side. Sediments within the field area range from Tertiary to Pennsylvanian; Triassic rocks are absent either by truncation or non-deposition. Production at Ackman is confined to beds of Pennsylvanian age. Six limestone zones within the Lansing-Kansas City Group and one limestone zone in the Shawnee Group are productive. There is no basal Pennsylvanian sand developed at Ackman. The thinly interbedded limestones and shales of the Lansing-Kansas City Group in southwestern Nebraska were apparently laid down by rapidly fluctuating shallow seas. Organic micro-build-ups over localized micro-structural-topographic features along with post-depositional modifications have resulted in the formation of competent reservoirs. The age of the productive basal sand in the Sleepy Hollow field is controversial because of its stratigraphic position. Indirect evidence favors Pennsylvanian age. A bar-complex origin is indicated for the sand body. Cumulative production of the Ackman field on Nov. 30, 1961, was 961,230 barrels of oil with a variable water-cut and a negligible gas content. The field apparently does not have a water drive. An area of 2040 acres has been proved productive on a 40-acre spacing program. Fifty-two wells have been drilled in the field; 50 are currently producing.