Conodonts (marine apatitic microfossils) are of organic origin and have biostratigraphic value. They are used as mineral thermal indices that undergo visible color changes between 50 and 500°C. Using these changes, color alteration index (CAI) maps were compiled for Ordovician through Triassic Systems in Nevada and adjacent parts of Idaho, Utah, and California using more than 5,000 samples. The maps show three thermal intervals which generally correspond to: (a) the thermal window for oil generation (CAI 1 to 2); (b) the upper thermal interval for gas generation (CAI 2 to 4.5); and (c) the thermal cutoff for most hydrocarbon generation (CAI >4.5).

The Great Basin is one of the most difficult areas in which to interpret thermal metamorphism in Paleozoic and Triassic rocks. Original burial metamorphic patterns are disrupted by thrust and normal faulting, masked by post-Triassic sedimentary and igneous rocks, and "overprinted" by post-Triassic thermal events. Nevertheless, maps show broad regional thermal patterns for Paleozoic and Triassic Systems which help delineate prospective areas for continued hydrocarbon and mineral exploration.

In general, Ordovician through Triassic rocks west of 117°30^primeW longitude and most Ordovician through Pennsylvanian rocks north of 41°30^primeN latitude and west of 113°W longitude and in the Oquirrh basin have CAI values >4.5 and appear to be unfavorable targets for hydrocarbon exploration.

Ordovician through Triassic rocks in central Nevada and Millard County, Utah, have regional moderate to low CAI values. This area, which includes the only two producing oil fields in Nevada, should provide a variety of hydrocarbon exploration targets. The Paleozoic and Triassic rocks of the Overthrust belt of southeast Idaho and Utah predictably have low to moderate CAI values.

End_of_Article - Last_Page 719------------