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Litho- and Chemofacies Variability in a Siliciclastically Dominated Shale/Mudstone System — Heterogeneities of the Developing Mancos Shale Gas Play

Horton, Brendan K.*1; Birgenheier, Lauren P.1; Johnson, Cari 1; Rowe, Harold 2; Kennedy, Angela 1; McLennan, John 3
(1) Geology and Geophysics, University of Utah, Salt Lake City, UT.
(2) Earth & Environmental Sciences, University of Texas at Arlington, Arlington, TX.
(3) Chemical Engineering, University of Utah, Salt Lake City, UT.

The Lower Blue Gate, Juana Lopez, and Mancos B members of the Mancos Shale in the Uinta Basin of southern Utah contain many elements which suggest potential for a shale gas play. Critical to success in the Mancos Shale, and in any shale play, is a thorough understanding the heterogeneities present in the lithology. In this study, six cored intervals of the Mancos Shale, spanning different regions of the Uinta Basin, as well as varying levels of maturity, were examined. A detailed, cm scale, lithological description of the different cored intervals of the Mancos Shale was performed as a critical first step to this research. Geochemical analyses including non-destructive X-ray fluorescence, X-ray diffraction, and Rock Eval Pyrolysis were used to further describe the intervals of study. New techniques, including QEMSCAN analysis, are currently being developed to refine the geochemical character, as well as provide detailed quantitative mineralogical insight into the microfabrics of these lithofacies. Initial findings suggest that chemofacies broadly align with identified lithofacies, but also further subdivide them. Clay content varies throughout all cores, but importantly, the shallowest and least mature cores show no interstratified illite/smectite, which suggests thermal immaturity. Use of QEMSCAN technology can assist in determining chlorite mineralization as a product of illite generation. Using both lithological and geochemical observations, environments of deposition are derived for each lithofacies. To that end, chemofacies and lithofacies work in concert to refine models of sequence stratigraphy in the 6 cored intervals. Herein, we look beyond lithology alone to further explore relationships between geochemical observations and relevant sequence stratigraphic interpretations of a heterogeneous siliciclastically influenced shale system.


AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California