Distribution and origin of sulfur in Colorado oil shale

Abstract

The sulfur content of 1,225 samples of Green River oil shale from two core holes in the Piceance Creek Basin, Colorado, ranges from nearly 0 to 4.9 weight percent. In one core hole, the average sulfur content of a sequence of oil shale 555 m thick, which represents nearly the maximum thickness of oil shale in the basin, is 0.76 weight percent. The vertical distribution of sulfur through the oil shale is cyclic. As many as 25 sulfur cycles have lateral continuity and can be traced between the core holes. Most of the sulfur resides in iron sulfides (pyrite, marcasite, and minor? pyrrhotite), and small amounts are organically bound in kerogen. In general, the concentration of sulfur correlates moderately with shale-oil yield, but the degree of association ranges from quite high in the upper 90 m of the oil -shale sequence to low or none in the leached zone and in illitic oil shale in the lower part of the sequence. Sulfur also correlates moderately with iron in the carbonate oil -shale sequence, but no correlation was found in the illitic samples. Sulfide mineralization is believed to have occurred during early and late stages of diagenesis, and after lithification, during development of the leached zone. Significant amounts of iron found in ankeritic dolomite and in illite probably account for the lack of a strong correlation between sulfur and iron.