Abstract

The Uinta Basin in northeastern Utah contains abundant energy-rich resources including coal, natural gas, oil, oil shale, tar sands and solid bitumens (gilsonite). One of the problems associated with the identification of the source rocks from which some of these varied hydrocarbon deposits (oils, tars and gilsonites) are derived is a limited understanding of the thermal evolution of the basin. Major questions this report seeks to answer are: (1) in what parts of the basin and at what depths and at what time interval were the source rocks mature enough to have generated conventional oil and gas, and (2) at what level of thermal maturity were the gilsonites and heavy oils in the tar sand deposits generated?

A number of direct and indirect methods were used to measure the thermal maturity of the rocks and oil deposits. These methods include: vitrinite reflectance, Rock-Eval (Tmax), Rock-Eval transformation ratio, atomic hydrogen to carbon (H/C) ratio, light hydrocarbon yield (C₅-C₇), Lopatin time-temperature indices (TTI), and biological marker ratios associated with thermal isomerization and degradation reactions. The geographic and vertical distributions of geothermal gradients, based on temperatures derived from drill-stem test data (DST), were also determined for the basin.

Using one or more of the above criteria for measuring thermal maturity, the gilsonites and the oils of the biodegraded tar-sand deposits appear to have been derived from middle Eocene lacustrine rocks of the Green River Formation in the vitrinite reflectance equivalence (VRE) range of 0.4 to 0.8% (immature to marginally mature). The oil deposits in the Altamonl/Bluebell field cover a wide range of depths and thermal maturities. Non-commercial oils reservoired at depths less than 8,400 (2562 m) ft appear to be derived from immature, bitumen rich middle Eocene-upper Eocene Green River Formation source rocks with vitrinite reflectance equivalents (VRE) less than 0.7%. Commercial oils reservoired in fractured rocks at depths between 8,400 (2562 m) and 14,000 ft (4270 m) appear to have been derived from upper Paleocene and lower Eocene Green River source rocks ranging in maturity from 0.7 to 1.3% R_m or greater. Most commercial oil in the Red Wash field is reservoired in middle Eocene Green River Formation sandstones at depths between 5,000 (1525 m) and 6,000 (1830 m) ft. Green River Formation rocks at these depths have vitrinite reflectance values in the range of 0.40 to 0.55%, yet the oils have thermal maturities equivalent to maturities of source rocks in the VRE range of 0.7 to 0.8%. These thermal maturity differences suggest that the Red Wash oils have migrated from stratigraphically equivalent, but more deeply buried overpressured Paleocene and earliest Eocene Green River source rocks to the northwest.