The Uinta Basin in northeastern Utah is one of the principal petroleum provinces of the Rocky Mountains, and interest has been increasingly converging on its large deposits of unconventional hydrocarbon resources. Despite their apparent abundance and shallow depths, however, the bitumens, heavy and extra-heavy oils of the Uinta Basin have hitherto resisted commercial exploitation. They are immobile and technically stranded and will require innovative applications of in-situ thermal recovery methods for commercial production to proceed. Moreover, although some individual oil samples have been intensely investigated, a coherent literature on the physical and chemical properties of the Uinta Basin's unconventional oil resources is lacking.

Nearly all of the exploration, laboratory research, and field trials on the Uinta Basin have concentrated on surface mining and retorting, producing a database that is heavily slanted toward mining methodologies. The occasional pilot studies of in-situ recovery have been very small and inconsequential. The information that does exist on Uinta's immobile oils is scattered across obscure in-house industry or government reports, unpublished core logs in public files at the Utah Geological Survey, and an array of academic articles.

This chapter assembles data from published and unpublished sources to document the variations in the immobile reservoired oils from one part of the basin to another. The focus of this chapter is the geologic setting, the character of the sandstone reservoirs, the properties of the reservoired oils, and the size of the unconventional oil accumulations of the Uinta Basin.

Immobile oils of the Uinta Basin can be subdivided into two main categories. The first is heavy oils and bitumens form the tar-sand deposits that rim the Uinta Basin and are known to occur in shallow parts of conventional oil fields in the basin's interior. They are immobile by virtue of their high viscosity related to oil composition, with 10 to 20 API for heavy oils and less than 10 API for bitumens. The second category is lighter oils with pour points in excess of the ambient reservoir temperature, rendering them immobile. These are found in shallow pools overlying several of the basin's conventional oil fields. Advances in in-situ technologies will be required to recover these currently technically stranded oils.

Three distinct geologic settings for shallow immobile oil accumulations exist in the Uinta Basin. Along the basin's southern flank, oils within the lower members of the Green River Formation are reservoired in lenticular distributary channels and marginal lacustrine sandstones, forming the main reservoirs of the West and East Tavaputs plateaus. East of the Green River, the sandstones are relatively porous and permeable but tend to have both low oil saturations and highly variable oil impregnation, depressing the average volume of original oil in place (OOIP) per unit area (less than 30,000 bbl/ac) in the P.R. Spring–Hill Creek deposit. The Sunnyside deposit beneath the West Tavaputs Plateau is similar to the P.R. Spring–Hill Creek deposit in this respect, except in the small area of the Dry Creek Canyon–Bruin Point–Range Creek. Here stacked fluvial channels in the Douglas Creek Member locally hold as much as an order of magnitude more oil as the rest of the southern flank deposits. Throughout the southern flank, reservoired oils are asphaltene rich, saturate poor, and very viscous; they are either extra-heavy (bitumen) or heavy oils, averaging less than 10 API. Biomarkers indicate that they are immature Green River oils that have been heavily biodegraded.

Along the northern margin of the Uinta Basin, heavy oil is reservoired in Mesozoic sandstones on the upturned hanging wall of the Uinta Basin boundary fault and in fluvial and marginal lacustrine sandstones of the late Eocene strata that unconformably onlap the thrust fault. Asphalt Ridge and Whiterocks are the only major heavy-oil deposits identified, but exploratory drilling in buried parts of the thrust sheet may reveal others. At Asphalt Ridge, the stacked fluvial channels of the Mesaverde Group contain 120 to 190 thousand bbl/ac of OOIP. The sole reservoir at Whiterocks, the eolian Nugget Sandstone (Triassic–Jurassic), is porous, moderately permeable, and relatively homogeneous. As the reservoir is subvertical, the height of the oil column, instead of the thickness of the reservoir, determines the OOIP, which is in the range of 450 to 485 thousand bbl/ac. The heavy oil in both deposits ranges between 10 and 14 API and has the composition of moderately biodegraded conventional Uinta Basin oil. Viscosities are considerably lower than those of the southern flank oils, and the oil is easily upgraded to marketable products.

The shallower central parts of the Uinta Basin reservoir both moderately biodegraded heavy oil and normal oil with a pour point above ambient temperature in the lenticular fluvial channel upper Eocene sandstones. If the Wonsits Valley shallow oil pools are typical of similar accumulations in the central basin, biodegraded oils yield to normal immobile oils downsection and are replaced with normal mobile oils at current production depths below 1220 m (4000 ft). Emission of small amounts of biogenic gas from the shallow accumulations indicates active biodegradation of these immobile oils.