Assays of oil shale cuttings and cores, P. R. Spring and Hill Creek oil-impregnated sandstone deposits

In 1973 the Utah Geological and Mineral Survey drilled sixteen core holes in the P.R. Spring and Hill Creek oil-impregnated sandstone deposits in the southeast Uinta Basin, Uintah county, Utah (figures 1 and 2). The holes were drilled in cooperation the U.S. Bureau of Mines under a grant from that agency. The purpose of the coring program was to obtain information on oil-impregnation in the sandstones of the upper Douglas Creek Member of the Green River Formation (figure 3). All of the core holes penetrated oil-impregnated sandstone. Because of the stratigraphic position of the oil-impregnated sandstone in the Douglas Creek Member (immediately beneath the Mahogany Zone of the Parachute Creek Member), 9 of the 13 core holes in the P.R. Spring deposit penetrated the Mahogany oil shale zone in the course of reaching the oil-impregnated sandstone beneath. The other four holes were either very close to or already in the oil sand when drilling and coring commenced and so did not penetrate significant amounts of oil shale. The three core holes in the Hill Creek deposit penetrated oil shales in the Parachute Creek Member, the entire Mahogany oil shale zone, and other oil shales in the Douglas Creek Member. Coring the oil shale gave faster rates of penetration than drilling in some cases, and in most cases coring was started early in the operation so that the top of the oil-impregnated sandstone was cored rather than and the maximum amount of information obtained. Coring was continued below the main zones of oil-impregnation (sandstone) for 100 feet or more and in so doing some added oil shale zones were also cored. These occur in the upper Douglas Creek member of the Green River Formation and had not been cored elsewhere in the P.R. Spring deposit and, as previously mentioned, at Hill Creek. The extensive cores from the sixteen core holes are described in detail by Peterson and Ritzma (1974) and Peterson (1975). In sorting through the cores and drill cuttings, those intervals containing oil shale were separated out and representative samples sent to the Laramie Energy Research Center, former U.S. Bureau of Mines, now Energy Research and Development Administration. These oil shale samples were assayed by the modified Fischer retort method.