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. ~ ~ 00 C) . . GEOLOGICAL SETTI,G ~~ DEPOSITIO~AL HISTORY The vast Uinta-Piceance Basin, divided by the Douglas Creek arch, ~as the depositional area for oil shale. It is bou~ded on the west in Utah by the Wasatch Plateau, on the no~th by the Uinta. Mountains, and on the east by the Elk Mountains and White Rive~ uplift in Colo~ado. The southe~n limit is not as clearly deEined and the more gentle slopes of this boundary only vaguely indicate the edges of ancient Lake Uinta. Generally, the Book Cliffs, the La Sal ~!Ounteins end the Uncompahgre uplift comprise the southe~n edge of this basin. While most oil reserves of the wo~ld we~e deposited unde~ marine conditions, deposition of the Green River oil shales occurred in a lacustrine (lake) environment. It is the quiescent lake situation which created the flexible, very thinly laminated deposits. The special thermal and chemical characteristics of the lakes in which the Green River Formation (Lake Uinta in Colorado and Utah, Lake Gosiute in Wyoming, and a third, unnamed lake in Wyoming's Fossil syncline) was formed, allowed the deposition of thin-bedded oil shales and a numbe~ of relatively rare carbonate ~inerals, which occur throughout the formation (Weichnan, 1973). The three ancient lakes in which the G~een River Formation was deposited were stable, two-layer lakes which did not overturn annually as most lakes do, probably due to their great depth (Fig. C.J). Lack of circulation and the~al stability led to concentt~tion of the car- bonate minerals, particularly nahcolite. Lack or annual turnover caused the water in the bottom layer to become a strop.~ basic sodium carbonate solution (Oil and Gas Journal, 1964) charged with HZS, RC03, and C03 (Weichman, 1973). Water in the top layer re~ained relatively fresh, and organic matter--mainly blue-green algae--thrived. The anaerobic bacteria that thrived in the lower zone helped process organic material from above into a uniform gel that was deposited in even layers. The basic nature of the lake helped preserve the hydrogen in the organic layers (Oil and Gas Journal, 1964). The depoce~ter for the organic material, at the deepest part of the lake, ~as the Piceance Creek Basin where the largest oil shale resources are fou~d. Various changes occurring in the lakes altered the emphasis on deposition of the organic materials. Possibly the lakQs began a regression period, or evaporation substantially increased over inflow. Salinity increased and deposition of organicsdec~eased) leaving layers of kerogen-lean material that contained large ~ounts of precipitated che~ical salts ia nodules Dr beds (Weic~an, 19i3). The chemistry of oil shale and associated depositional minerals is extremely.coffiplex, as can be seen in Table C.2 (authigenic minerals). Only the most important of these minerals will be discussed here. In Colorado, nahcolite, a sodil~ bicarbonate material, is abundant. The nahcolite could be used as a source of soca ash or it could find a C-6