This paper gives a brief account of the geologic and production problemsencountered in the Grass Creek oil field, the methods used in their solution, and the beneficial results obtained from the work. The author wishes toacknowledge the assistance received from J. H. Pearson and Miss Ninetta A.Davis in the collection of the data and the preparation of the cross-sections.Permission for its publication was given by the officials of the MidwestRefining Co. and the Ohio Oil Co. The Grass Creek oil field is located in Ts. 98–99 W., R. 46 N., Hot SpringsCounty, Wyo. The productive area includes about 2? mi. (4 km.) on the apex of agreat anticlinal fold occupying more than 150 sq. mi. Fig. 1 shows thestructural contours of the field. Oil was discovered in the field in June,1914; development proceeded rapidly and production rose to a maximum in 1918.About 350 wells, 800 to 1500 ft. (243 to 457 m.) deep, have been drilled toreach the upper oil sands; two companies, the Ohio Oil Co. and the MidwestRefining Co., operate the entire field. The principal producing sands in the field are members of the FrontierFormation, a series of sands and shales occurring in the upper portion of theColorado Group of the Upper Cretaceous series. These Frontier, or "WallCreek" sands are also productive in the Salt Creek, Big Muddy, and ElkBasin fields and, therefore, are much the most important reservoirs of lightoil in Wyoming. At Grass Creek, the sands in the Formation are more numerousthan in the other fields and much less uniform in thickness and character.Usually, eight distinct beds are encountered and in some wells twelve sandshave been reported. AIME 068–83