The Laramie Energy Technology Center, Department of Energy, completed three in situ oil recovery field experiments, two combustion and one steam flood, in tar sand at Northwest Asphalt Ridge, Utah. Inadequate resource and site characterization prior to the field experiments contributed to process design and operation problems. The 10-acre field site is part of the Sohio Shale Oil Co. abstract located west of Vernal, Uintah County. The target zone, the middle portion of the Cretaceous Rim Rock Sandstone of the Mesaverde Group, varied from 300-500 feet deep. From petrographic analysis of the target zone, this portion is classified as a moderately sorted litharenite with an average visible porosity of 18%. Dominant constituents include quartz, rock fragments, chert, feldspars and clay minerals. X-ray analysis of selected core samples from the Rim Rock and Asphalt Ridge Sandstones indicate the presence of quartz, calcite, dolomite, ankerite, microcline, orthoclase, anorthite, kaolinite and muscovite. Carbonate mineral species were present only in the lower Rim Rock and Asphalt Ridge Sandstones. Reservoir characteristics of the target zone which adversely affected the field experiments include faulting at all three experiment areas, lateral and verticaI heterogeneities of permeability and porosity, inadequate target zone confinement, rough surface texture of clastic grains, and oil-wet grains. Favorable target zone characteristics include high quartz content; absence of carbonates; lack of clay minerals bridging and cementing pore spaces; and sufficient porosity, initial oil saturation and overburden. Recommended geologic evaluation methods to aid in the identification of potentially suitable resources and sites for in situ oil recovery from tar sands include seismic surveys; well logging; coring and core analysis; petrographic, binocular, and scanning electron microscopy; and X-ray analysis.