Petrology of bituminous sandstones in the Green River Formation, Southeastern Uinta Basin, Utah

The bituminous sandstones in the Green River Formation are dominately arkoses. Feldspars constitute from 19 to 60 per cent of the total min­ eral content, and in many thin-sections both fresh and altered feldspar of the same species appear in the same section, indicating that the arkose was tectonically derived. Quartz is the dominant mineral in most samples studied. Using Folk's classification of quartz types, both igneous and metamorphic quartz is present. The presence of well-rounded heavy minerals indicates that r e ­worked sediments are also present. The bituminous sandstones also contain carbonate sediments. All seem to indicate a high-energy environment, probably a near shore or a beach environment. Using Folk's carbonate classification, the following rock types are present: algal-oolitic biosparudite, algal biosparudite, sandy-sorted sparite, oosparudite, oosparite, sandy-oolitic biosparudite, and oolitic intra-sparite. The bituminous sandstones studied are very heterogeneous. The gross textures and lithologies are extremely varied. In a given stratigraphic section, the most common variations are in the size and shape of the grains, in cements and degree of cementing, in bitumen content, in sorting and packing, and in porosity. The abrupt changes in texture vertically, within short stratigraphic intervals, possibly indicate an intermittent turbulence. The effect of variation in texture is demonstrated in the amount of bitumen a sample contains. Eighty per cent of the variation in the bitumen content of the samples studied is related to median diameter and to the percentage of clay- and silt-size material (material less than 62 microns). Analysis of the two independent variables shows that both median diameter of the grains and the percentage of clay- and silt-size material are equally effective in determining the amount of bitumen the sediment can hold. The most saturated sandstones were those whose grain size ranged between 4-phi and 2-phi, and whose percentage clay- and silt-size material was low. The amount of original carbonate cement has also greatly influenced the bitu­men content of the sandstones by reducing the porosity. The source of the sediments comprising the bituminous sandstones was probably to the south and southeast of the area studied, probably from the Uncompahgre uplift and the mountains of western Colorado. This is only a short distance which is suggested by the submature sorting and rounding of the grains.