ABSTRACT

The elemental chemostratigraphy of the Upper Cretaceous Niobrara Member of the Mancos Shale shows that six chemostratigraphic zones can be identified in the Piceance Basin, Colorado, based on geochemical data. Chemostratigraphic correlations of nine wells spaced 20 mi (∼32 km) apart closely match lithostratigraphic correlations made using gamma-ray and deep-resistivity wireline logs. Lithologic interpretations made from wireline logs indicate that the Niobrara Member and equivalent strata consist primarily of interbedded calcareous shale and shaley limestone facies that increase in thickness to the northwest in the basin. The geochemical data suggest that during deposition of the Niobrara Member, anoxia and calcium enrichment increased to the east of the basin, whereas terrestrial input and clay enrichment increased to the northwest. Element crossplots suggest that a large part of the silicon is detrital and that the Niobrara Member becomes an increasingly more clastic than carbonate system to the west and northwest.

The Δlog R–derived total organic carbon (TOC) calculated using a sonic-resistivity overlay analysis technique shows that the Niobrara Member comprises organic-rich and organic-poor deposits. Average TOC values range between 1 wt. % (in organic-poor deposits) and 2.37 wt. % (in organic-rich deposits), with higher TOC values recorded in the southern and eastern parts of the basin. Relative-rock brittleness estimates from element and TOC data show the stratigraphic variability of alternating ductile (TOC rich, Ca and Si/Al poor) and brittle (TOC poor, Ca and Si/Al rich) intervals for the Niobrara Member.

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