ABSTRACT: In the Indiana portion of the Illinois Basin, the rocks of late Mississippian age that are assigned to the West Baden Group underlie an area of approximately 6,500 square miles and range from 40 to 260 feet in thickness. The West Baden Group encompasses, in descending order: Cypress Formation, Reelsville Limestone, Sample Formation, Beaver Bend Limestone, and Bethel Formation. The West Baden was mapped for this report with emphasis on the distribution of the sandstones and clarification of the distribution of limestone units. The regional maps show: (1) structure on top of the West Baden, (2) isopach of the West Baden, (3) sandstone thickness in the West Baden, (4) isopach of the Bethel Formation, (5) sandstone thickness in the Bethel Formation, (6) areal distribution of limestone within the West Baden, and (7) oil reservoirs in the West Baden related to sandstone thickness. Sedimentation of the West Baden Group is dominated by a clastic dispersal system whose major outline is defined in the Indiana subsurface by the complete absence of carbonate rocks. The primary southwesterly oriented dispersal system resulted in a similar alignment of elongate sandstone bodies, and it places emphasis on the fluvial and deltaic environments of deposition in addition to shallow marine environments. Sandstones of the West Baden Group have yielded 58 million barrels of oil, mostly within the past 30 years. Production areas are related to sandstone trends, and in turn the entrapment of oil is related to thinning along the margins of the sandstone bodies.