The role of subtle stratigraphic architecture on the distribution of resource accumulations can be assessed using sequence stratigraphic-based mapping techniques together with petrophysical analysis. The distal carbonate ramp setting of the Niobrara Formation of the Western Interior Seaway offers insight into how depositional processes and seafloor morphology can impact a petroleum system.
The four third-order transgressive-regressive events of the Upper Cretaceous Niobrara Cyclothem were major controls on the character of the pelagic and hemipelagic sedimentation of the Niobrara Formation of the Denver-Julesburg (D-J) Basin. Correctly mapping these main stratigraphic sequences and their subintervals is critical to understanding architectural trends that are important controls on the Niobrara petroleum system. Rather than the common approach of mapping based on lithostratigraphy, we employ a sequence stratigraphic framework for the Niobrara Formation in the D-J Basin that more accurately reveals the architecture of key depositional packages. Based on correlations using wire-line logs from >2,000 wells, we present a series of time-slice isochore maps and schematic cross sections to illustrate the evolution of subtle seafloor topographic features. To evaluate the vertical and horizontal nature of resource accumulations, we performed petrophysical analysis of the Niobrara section in >200 wire-line logs from wells across the basin. We discuss reconnaissance-scale original oil in place for three chalk intervals in the context of the following stratigraphic observations and interpretations.
Organized stratigraphic thinning represents examples of
subtle submarine channel forms and
sediment draping over seafloor paleohighs.
Stratigraphic thickening trends represent systematic compensational infilling of channel forms by younger sequences as well as areas with long-lived available accommodation space. Our maps and analysis suggest that benthic processes and tectonic/geomorphic features were important controls on the distribution of source-prone and reservoir-prone intervals of the Niobrara Formation. Basin-scale petrophysical evaluation indicates trends in oil saturations associated with seafloor paleohighs, suggesting a link between paleoseafloor morphology and facies, thermal maturity, and hydrocarbon accumulations.