PYRITE FORMATION IN CONDENSED INTERVALS OF THE CRETACEOUS MANCOS SHALE: SIGNIFICANCE OF REACTIVE IRON AND ORGANIC MATTER

This study assesses the role played by limiting factors such as labile organic matter, reactive iron, or dissolved sulfate in the formation of pyrite in two condensed intervals of the Cretaceous Mancos Shale. The two condensed intervals, CI1 (770-820 ft) and CI2 (1162-1209 ft), were identified in the Main Body of the Mancos Shale in cored intervals of the River Gas Utah # 1 well (southern Uinta basin, Carbon County, Utah). The distinctive characteristics of these intervals include lithology (thinly laminated dark gray to black shale), high gamma values (60 API units over baseline), high macrofossil abundance, and elevated total organic carbon (TOC) contents. The average TOC values for CI1 and CI2 are 2.5% and 2.2%, respectively, whereas the other intervals have average TOC values of 1.4%. Whole-rock samples (89) were analyzed for TOC, total sulfur (TS), and total iron (Fe) by elemental analysis and x-ray fluorescence. The chemical composition of kerogen separates obtained from 47 samples was also determined by elemental analysis. Plots of TS versus Fe show that the S and Fe behavior is different in each of the two condensed intervals. In CI1, data plot sub-parallel to and below the "pyrite line" (TS/Fe=1.15), whereas no correlation exists between TS and Fe in CI2. Kerogen separates from CI1 have moderately high H:C ratios (range 1.0 - 1.24, average 1.1), whereas the H:C ratios of kerogen from CI2 is lower (range 0.84-1.33, average 0.99). Of the 24 samples of kerogen isolated from CI2, 71% have H:C ratios lower than 1.0. We propose that pyrite formation in CI1 was limited by the availability of reactive iron, whereas the abundance of reactive organic matter may have limited pyrite formation in CI2.