Abstract: Abstract: Distribution and Origin of Sulfur in Colorado Oil-Shale Deposits

John R. Dyni

The sulfur content of about 1,200 samples of oil shale from two core holes located near the depocenter of the Green River oil-shale deposits in northwest Colorado ranges from 0.01 to 4.9 weight percent. In one core hole, the oil shale penetrated is 568 m thick and contains nahcolite and halite. About 875 sulfur values from this sequence have a normal frequency distribution, a mean of 0.66 weight percent, and a standard deviation of -0.40 and +0.57 weight percent, based on an arcsine transformation of the original percentage data. Two depositional phases of sulfur are indicated: a primary, or early phase, in which the sulfur is distributed vertically in nonrandom cycles, followed by a late diagenetic overprint of randomly distributed sulfur. Primary sulfur in amounts as much as 4.9 weight percent is in laterally persistent zones 1 to 3 m thick. These zones commonly are underlain by a bed of na colite.

All of the sulfur is reduced, and it is mostly in disseminated, fine-grained iron sulfide minerals (pyrite/pyrrhotite); small amounts of organic sulfur are in kerogen. The early sulfur is present as disseminated iron sulfide minerals, and the late diagenetic sulfur occurs as rims of iron sulfide minerals around nodules of nahcolite scattered through oil shale. The physical association of nahcolite and iron sulfide minerals and the lack of sulfate minerals suggest the following depositional model.

Sulfur was introduced into ancient Lake Uinta as sulfate by inflowing streams that drained sedimentary rocks exposed in the drainage basin. If we postulate a permanent body of chemically stratified water, the sulfate in the lower anoxic monimolimnion and in the upper lake-bottom sediments was being reduced continuously, perhaps largely by sulfate-reducing bacteria, and precipitated as sulfide with concurrent partial oxidation of abundant organic matter to bicarbonate. The bicarbonate to sulfate ratio of the lake waters increased with time, except during evaporative lake stages when beds of nahcolite precipitated. Owing to increased concentration of sulfate during arid periods, the rate of sulfate reduction increased, which is reflected by the zones of sulfide enrichment at the tops of nahcolite beds. Occasionally, evaporation was insufficient to precipitate nahcolite, and only a sulfide-enriched zone formed. Some sulfur remained mobile in interstitial waters and reacted with iron to form iron sulfide rims around nodules of nahcolite during late-stage diagenesis. The processes described may account for the absence of syngenetic sulfate minerals and the presence of some of the sodium carbonate salts in the Green River Formation.

AAPG Search and Discovery Article #90969©1977 AAPG-SEPM Rocky Mountain Sections Meeting, Denver, Colorado