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Petrography and Provenance of Sandstone, Sunnyside Oil-Impregnated Deposit, Uinta Basin, Utah

M. Dane Picard, Elizabeth Y. Banks

The Sunnyside oil-impregnated arkosic sandstone deposit (Eocene) is located on the southwestern edge of the Uinta basin, northeastern Utah. It is a giant deposit with more than 4.5 billion bbl of in-place bitumen. Thin section, microprobe, and heavy-mineral analyses indicate a mixed provenance of crystalline and sedimentary sources. Primary crystalline detritus was derived from southwestern Colorado several hundred miles distant, sedimentary detritus from locally adjacent Colorado Plateau uplifts.

The sandstone was deposited in meandering fluvial settings. Lacustrine rocks of the Green River Formation overlie and underlie and are occasionally interbedded with the fluvial rocks. The average paleocurrent direction is N45°E, indicating a source to the southwest. The incongruent relationship between the average paleocurrent direction and postulated source areas is probably related to the presence of the San Rafael swell upwarp, and to the influence on the drainage pattern by streams that drained the orogenic highlands of western Utah and joined the major north-flowing drainage at the northern end of the San Rafael swell.

Migration of bitumen into the rocks at Sunnyside from lacustrine sources occurred after authigenic development of spar-size dolomite rhombs, syntaxial albite and quartz overgrowths, and hematite. Development of calcite cement may be related to the introduction of bitumen and associated pore fluids into the sandstone. Correlation of bitumen content with textural data is weak. Incomplete saturation of rocks by bitumen prevents a determination of the effect of mean grain size and percent matrix on saturation.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.