Chemostratigraphic and depositional characterization of the Niobrara Formation, CEMEX Quarry, Lyons, CO

Abstract

The Late Cretaceous (89-82 Ma) Niobrara Formation is one of the primary targets for unconventional horizontal well activity in several Rocky Mountain basins. With such an abundance of horizontal activity, the question of lateral heterogeneity becomes paramount in evaluating individual wells' performance by a purely geologic standpoint, in lieu of an engineering standpoint that considers drilling and completions. This study incorporates detailed core description, high resolution chemostratigraphy, and petrophysical analyses from four near-surface cores taken at the CEMEX Lyons Quarry in order to assess lateral heterogeneity. Cores were originally intended to determine the geochemistry and areal extent of the Fort Hays Limestone Member of the Niobrara Formation, the primary target for cement production. However, due to the cores' proximity to one another (400-2000 ft) and the presence of not only the Fort Hays Limestone but also the petroliferous chalks and marls of the Lower Smoky Hill Member, the dataset can also be used as a proxy to model hypothetical changes along a pseudo-horizontal wellbore. Initial findings suggest lateral bentonite discontinuity, trace element variations, and distinct changes in sedimentary structures. These findings imply small-scale depositional heterogeneity. The root cause for these subtle changes was likely paleobathymetry with slopes as high as 0.44º. Bathymetric fluctuations likely controlled bioturbation patterns, chemocline elevation, redox conditions, current pathways, and resulting sedimentation.