The Tensleep Sandstone, an eolian and marine deposit, has produced low-gravity oil at South Casper Creek field (Natrona County, Wyoming) since the 1920s. Until recently, the reservoir was considered a relatively homogeneous sandstone body and was modeled as such for secondary recovery operations initiated during the late 1970s and early 1980s. Poor secondary recovery performance led to extensive reevaluation of the reservoir. Characterization studies by the field operator, Union Oil of California (UNOCAL), and through the Reservoir Characterization Project at the Colorado School of Mines produced an entirely revised picture of the Tensleep interval. Significant stratigraphic, diagenetic, and structural heterogeneities were identified, mapped, and correlated against produc ivity patterns under pilot steamflood programs. The results of this integrated geologic and geophysical effort are significant and have implications for secondary recovery operations elsewhere in the Tensleep and its regional correlatives.