Stratigraphic geochemical variations in the Late Cretaceous Mancos Shale, southwest Piceance Basin, Colorado

Abstract

A detailed geochemical analysis (calibrated x-ray fluorescence) of the Late Cretaceous Mancos Shale was conducted on 176 cuttings samples (total interval = 4,730 ft.) collected from the Fees Federal 2-6-8-101 well (API 05-045-07432). Major elements (>2% on an oxide basis)evaluated included Si, Al, Ca, Mg, Fe, and K and minor elements (<2% on an oxide basis) examined include Ti, Mn, Na, S, and P. Trace elements (ppm concentration) investigated include Se, Ni, Co, V, Cr, and Mo. The samples were also analyzed by calcimeter for total carbonate and by ignition loss at 400 degrees C for total organic carbon and water. Numerous vertical trends and discontinuities are present in the data. Some significant observations are: silicon (as SiO₂ is about 65% in the lower Mancos (Mowry through Monument Valley members), drops to 40.6% at the base of the Niobrara (Ft. Hays), then generally increases to about 75% through the Niobrara, Prairie Canyon, Sharon Springs, Castlegate, and Buck Tongue members. Calcium, as CaO, is less than 4% in the lower Mancos, increases rapidly to a high of 18.5% in the basil Niobrara (Ft. Hays), then decreases irregularly to the top of the formation. Aluminum (as Al₂O₃), the other major cation, stays between 11% and 15% throughout the Mancos, suggesting that there was a fairly constant influx of clay minerals during deposition. The ‘MINLITH’ program was used to calculate a normative mineralogy for the cuttings samples. These values allow the identification of zones that are less clay rich and hence more amenable to potential hydraulic fracturing. Another interesting result involves selenium, which creates water-quality issues when the Mancos is weathered. The data show that selenium is primarily concentrated in the lower half of the Mancos. The redox indicators, Ni/Co, V/Cr, V/(V+Ni), and Mo/Al, suggest that a 200-foot-thick interval in the Niobrara was deposited under more reducing conditions than the rest of the formation.

AAPG Datapages/Search and Discovery Article #90266 © 2016 AAPG Pacific Section and Rocky Mountain Section Joint Meeting, Las Vegas, Nevada, October 2-5, 2016