LATE ORDOVICIAN OIL SHALE, VININI FORMATION, CENTRAL NEVADA

An organic-rich (20-25 percent TOC), 9 meter thick, dark gray to black mudstone and shale unit with hydrogen-rich (HI=400-600) kerogen occurs in the uppermost part of the Late Ordovician Vinini Formation exposed in the Roberts Mountains, central Nevada. The unit has been quarried for its petroleum potential. It consists of 1 to 25 cm thick beds of blocky-weathering, brittle mudstones that alternate with 2-13 cm thick partings of dark gray to black, fetid mud shale. Some of these partings are composed of 60-80 percent graptolites. Phosphatic nodules are common in many of the layers. This organic-rich unit is interpreted to have accumulated on the upper part of a continental slope, a setting that, today, is a position at which many oceanic oxygen minimum zones develop. The unit's vanadium concentrations of 7000-7200 ppm are not only similar to those found in known petroleum source rocks, but also they are consistent with accumulation under anoxic conditions as discussed in the Quinby Hunt-Wilde thermodynamic model for depositional conditions under which black, organic-rich shales accumulated. A modern analogue for the depositional environment of the Vinini mudstones may be the upwelling system in the Gulf of California, where high seasonal surface productivity results in depletion of oxygen above the water-sediment interface and development of anoxia in the sediments. Seasonal differences in upwelling and related productivity in the Gulf of California result in alternations of relatively more organic-rich with relatively less organic-rich laminae. The many alternations of graptolite-rich partings with relatively graptolite-poor mudstones in the Vinini unit may, similarly, reflect seasonal differences in upwelling and related organic productivity.