Abstract

The strongly asymmetric Uinta Basin contains 25 tar sand deposits grouped in three geographic areas and distinct geologic settings. The south flank of the basin has simple structure characterized by low dip and minor faulting. Most tar sand deposits here are in situ and are of wide areal extent. Stratigraphy of the host sedimentary formations is of prime importance. About 90% of the Uinta Basin tar sand resource is located on the south flank. (Examples: Argyle Canyon, Sunnyside, Hill Creek, and P.R. Spring deposits.)

The north flank of the basin is structurally and stratigraphically complex. Here oil has been expelled from and has migrated from the deepest part of the basin syncline vertically and horizontally along fractures, faults and unconformities to become concentrated in size and areal extent. (Examples: Daniels Canyon, Tabiona, Whiterocks, Asphalt Ridge, Spring Hollow, Rim Rock and Raven Ridge deposits.)

From east to west across the middle of the basin, oil has migrated up gilsonite veins and associated fractures and impregnates porous sandstones which are in contact. (Examples: Tract Ub, Chapita Wells and Pariette deposits.)

Almost all oil contained in the deposits originated in the fluvial and lacustrine Wasatch and Green River formations (Paleocene-Eocene). The oil is paraffinic, between 4 and 14.5 gravity (API), and averages about 0.45% sulfur (weight %). The contrasting structural pattern and history of the basin flanks has profoundly affected character of oil and configuration of deposits. Factors such as original environment of deposition, depth of burial, geothermal gradient, overburden pressure, and introduction of meteoric water are highly significant.

The Asphalt Ridge, Sunnyside and Thistle deposits have been quarried for paving material for local and regional use. About 10 deposits have supported experimental and pilot mining and extraction operations, some of which continue intermittently and on limited scale. The resource is estimated to total 13.6 billion barrels of oil in measured, estimated and inferred/speculative categories. Most (97%) is in four giant and one large deposits: Asphalt Ridge, 1.1 billion bbls.; Hill Creek, 1.1 billion bbls.; P.R. Spring, 4.4 billion bbls.; Sunnyside, 6.5 billion bbls.; and Whiterocks, 120 million bbls. These deposits, excepting Hill Creek, are considered to contain most of the economically recoverable resource in the basin.