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Regional Stratigraphy, Elemental Chemostratigraphy, and Organic Richness of the Niobrara Formation, Piceance Basin, Colorado

Aya El Attar, Marc Connolly, and Matthew J. Pranter

The Upper Cretaceous Niobrara Formation in the Rocky Mountain region is an emerging hydrocarbon play. A subsurface stratigraphic study shows the elemental composition and organic richness variability of the Niobrara interval in the Piceance Basin, Colorado. The Niobrara and equivalent strata consist primarily of interbedded calcareous shale and shaley limestone facies. This study is based on an analysis of open hole wireline logs (primarily gamma-ray and resistivity curves) from ~300 wells across the study area. Based on well-log signatures, a stratigraphic framework was constructed and lithostratigraphic units were defined and correlated throughout the study area. X-ray fluorescence data were collected from cuttings (at 10- and 30-ft intervals) from 10 wells (16430 ft total footage) across the basin. The stratigraphic variation of the elements and their ratios allowed the definition of seven major geochemical zones. Total Organic Carbon (TOC) was calculated for 70 wells using a sonic-resistivity overlay analysis technique (Delta Log R) calibrated to measurements from published TOC data. TOC varies laterally and stratigraphically throughout the study area: average TOC maps highlight areas of high organic content, and TOC profiles identify organic-rich zones as they relate to lithostratigraphic units. Relative changes in rock brittleness within the Niobrara were also interpreted using whole-rock geochemical data coupled with TOC profiles: organic (TOC)- and clay-mineral rich intervals are considered mechanically ductile, while more silicon and calcium enriched rocks are considered mechanically brittle.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013