--> --> An Outcrop to Subsurface Stratigraphic Analysis of the Niobrara Formation, Sand Wash Basin, Colorado

AAPG Pacific Section and Rocky Mountain Section Joint Meeting

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An Outcrop to Subsurface Stratigraphic Analysis of the Niobrara Formation, Sand Wash Basin, Colorado

Abstract

The Niobrara Formation constitutes an unconventional oil and gas play in the Rocky Mountain region. In the Sand Wash Basin of northwestern Colorado, historical production from the formation (>26MMBO) has largely occurred at the flanks of the basin from vertical wells that were drilled into swarms of natural fractures associated with Laramide deformation. As oil and gas companies explore further basinward, away from natural fracture swarms, the understanding of lateral and stratigraphic variations in mineralogy and TOC in the Niobrara Formation is critical for successful future development. This study combines information gathered from core descriptions, outcrop descriptions, XRD analysis, XRF analysis, geochemical data, and electric well log data to understand how the composition of the Niobrara Formation varies stratigraphically in the Sand Wash Basin and laterally from a chalkier lithology in the eastern region of deposition to a marlier lithology in the western region of deposition.

Based on the whole rock and well log data analyzed, both peak carbonate deposition and TOC preservation in the Niobrara Formation of the Sand Wash Basin are interpreted to be associated with periods of maximum transgression. These transgressions are represented in the Buck Peak Bench, Tow Creek Bench, and (to a lesser extent) the Wolf Mountain Bench. These periods correlate to a discrete mineral assemblage in the formation (50 wt.% carbonates, 30 wt.% clays, and 20 wt.% quartz). This mineral assemblage represents the optimal conditions that existed for the preservation of TOC in the Western Interior Seaway during the deposition of the Niobrara Formation. Consequently, the most prospective lateral targets in the Niobrara Formation appear to be associated with maximum transgressive events in the Buck Peak Bench and Tow Creek Bench, and possibly in the Wolf Mountain Bench. These intervals are predicted to be both high quality source rocks and reservoirs in the formation. High TOC values associated with these intervals suggest favorable conditions for hydrocarbon generation. Additionally, high carbonate content suggests favorable brittleness characteristics that make the rocks prone to fracturing (both naturally and from hydraulic stimulation).