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High-Resolution Carbon Isotope Chemostratigraphy of the Niobrara Formation, Denver Basin, Colorado

Humphrey, John D.; Stout, Lauren; Canter, Lyn; Nakamura, Kazumi

The late Cretaceous Niobrara Formation is a productive, unconventional resource play covering a large area of the Rocky Mountain region, including the Denver basin. A marine deposit of the Cretaceous Western Interior Seaway, the Niobrara Fm. is a mixed lithology unit comprised principally of pelagic coccolith chalks and marls. Throughout the Denver basin, chalk intervals are generally the producing intervals, while the marls typically represent intraformational source rocks.

Stable carbon isotopes of inorganic carbon derived from marine carbonate components can produce synchronous variations useful for stratigraphic correlations. This can be especially important in the Niobrara Fm., in which lithologic variability is visually subtle and biostratigraphic resolution is poor.

Traditional lithostratigraphic and biostratigraphic approaches have resulted only in a layer-cake understanding of Niobrara stratigraphic variability. This study uses a high-resolution carbon isotope chemostratigraphic approach to correlation of cores within the Denver basin, providing a basis for sequence stratigraphic interpretations. One "type core" was chosen and yielded a highly detailed chemostratigraphic profile from a 6-inch (15 cm) sampling density (678 samples). Beyond our correlations of Niobrara Fm. in the Denver basin, our stratigraphic analysis produced correlations to the Texas Comanche Shelf (Austin Chalk and Eagle Ford Shale), as well as to other global late Cretaceous carbon isotope excursions.


AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013