Outcrop to subsurface stratigraphic analysis of the Niobrara Formation, Sand Wash Basin, Colorado, An

Abstract

The Niobrara Formation is an unconventional oil and gas play containing continuous petroleum accumulations. In the Sand Wash Basin, historical production from the formation has largely occurred at the flanks of the basin from vertical wells that were drilled into swarms of natural fractures associated with Laramide deformation. However, 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 describe how the Niobrara Formation changes 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, 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 periods occur in the Buck Peak Bench, Tow Creek Bench, and (to a lesser extent) the Wolf Mountain Bench. Peak TOC preservation also correlates to a discrete mineral assemblage in both the Sand Wash and DJ Basins (50 wt.% carbonates, 30 wt.% clays, and 20 wt.% quartz). This mineral assemblage represents the optimal conditions 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 sources and reservoirs in the formation. The high TOC values associated with these intervals suggest favorable conditions for hydrocarbon generation. Additionally, the high carbonate content suggests favorable brittleness characteristics that make the rocks prone to fracturing (both naturally and from hydraulic stimulation).