The Osage oil field is one of the two simple monoclinal areas in the Rocky Mountain region that produce relatively large amounts of oil. Small amounts of oil are produced in shallow wells bottomed in openings in the marine Belle Fourche shale member of the Graneros shale, of Upper Cretaceous age, and larger amounts from the lensing, erratic, continental, and marine, or brackish-water, Newcastle sandstone member of the Graneros. The field is part of a relatively gentle, terraced homocline dipping westward from the Black Hills uplift into the Powder River Basin. The oil wells range between 100 and 3,323 feet in depth. The initial production of oil per well per day ranges from a few gallons to about 700 barrels, and of gas up to several million cubic feet. The gravity of the green oil is about 40°, and in 1939 the field produced about 248,000 barrels. The oil has probably accumulated in the Newcastle sandstone as the result of the saturation of locally thick, discontinuous sandy bodies enclosed in marine shale. A map shows the boundary of the productive area as of January 1, 1940, the attitude of the Newcastle sandstone by structure contours, and the outcrops of formations and the members thereof.