Preliminary geologic analysis of the tar sands near Sunnyside, Utah

Sandstones that crop out along the Roan Cliffs near Sunnyside, Utah, are estimated to contain approximately 6 billion barrels of bitumen, making it one of the largest deposits in the United States. Little is known of the geologic aspects, particularly sedimentology, mineralogy (especially clays), and diagenesis, of these sandstones. These aspects of tar sands must be characterized before the feasibility of bitumen recovery can be assessed. Preliminary results of this ongoing study indicate that the sandstones were deposited in fluvial and marginal lacustrine environments. Sandstone geometry is dependent on depositional environment: Fluvial sandstones tend to be more extensive basinward but arc less laterally extensive than the marginal lacustrine sandstones. The sandstones are mainly feldspathic arenites, containing both plagioclase and K-feldspar. Authigenic cements include calcite, dolomite, ankerite, siderite, and analcime. Authigenic clay minerals include kaolinite, illite, and smectite. The percentages of clays and cements vary considerably from sample to sample, particularly between bitumen-bearing sandstones and adjacent lithologies. Some of these authigenic minerals have replaced framework grains. Dissolution has produced secondary porosity, and the bitumen appears to occupy secondary porosity, preferentially in the coarser grained sandstones.