The brittleness of sedimentary rocks is a critical aspect of their potential to form hydraulically stimulated fractures, resist embedment after stimulation, and thereby produce hydrocarbons over economically significant periods more efficiently. As such it is important to understand how brittle versus non-brittle rocks are organized within a play to improve the recovery of petroleum reserves by identifying higher potential stimulation intervals. The goal of this study is to document the link between the sequence stratigraphic hierarchy and its control on the evolution of mineralogy by FEI's QEMSCAN system, unconfined compressive rock strength, and dipole sonic derived brittleness for the Niobrara Formation. The Niobrara ultimately consists of a combination of sharply to diffusely bedded rocks composed of chalks to claystones with variable organic matter. It is mainly calcium carbonate rich with changing concentrations of detrital quartz, clay, and organic matter that function as the main variables of the mineral brittleness index. The detrital and organic components are relatively dominant in the transgressive systems tract (TST) in the form of laminated and sharply laminated-gradational millimeter to centimeter to decimeter intervals of organic rich chalky claystones-to-marly chalks; high resolution scanning electron photomicrographs also document that this fine scale bedding continues at the nanometer to micrometer scale. Discretely interbedded chalks and marly chalks are interbedded with marls at the cm to decimeter scale during the highstand systems tract (HST); by HST time the shorelines have been pushed landward the furthest and detrital terrestrial contribution to the marine environment has been reduced. This environment is ultimately more favorable to chalk deposition and an overall more brittle mineral assemblage. Mineral derived brittleness logs of the Niobrara ultimately covary with dipole sonic derived rock mechanics (Young's Modulus and Poisson's Ratio), unconfined compressive rock strength measurements from the Equotip-Piccolo, and gamma ray values from downhole tools which in turn may be tuned to identify hydraulic stimulation targets away from cored wells.

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