Geochemical and petrographic characterization of a shallow drill core through the Pierre Shale and Niobrara Formation in the Denver–Julesburg Basin, Colorado

The U.S. Geological Survey Sugar City 1 core, collected in the southern part of the Denver–Julesburg Basin in 2022, contains thermally immature strata deposited in the Cretaceous Western Interior Seaway. This core serves as a calibration point for original organic matter (OM) content and hydrocarbon-generating potential of source rocks present in the Niobrara Formation and overlying Pierre Shale, as well as the sources of OM and depositional conditions. The 760-foot core penetrates the Niobrara Formation and the lower transition member, Apache Creek Sandstone Member, and Sharon Springs Member of the Pierre Shale. Nearly 400 samples were analyzed for bulk organic geochemical properties, major and trace element content, and mineralogy. A subset of samples was also analyzed for stable organic carbon (δ13Corg) isotopes, thin section petrography, and organic petrography.

The upper Niobrara Formation is highly calcareous (maximum of ~85 wt.% carbonate content) with an average TOC of 2.1 wt.%. The lower transition member has an average TOC content of 2.7 wt.% with decreasing carbonate content transitioning into the Pierre Shale. The Apache Creek Sandstone Member equivalent is dominated by siltstone and mudstone and has an average TOC of 2.0 wt.%. The Sharon Springs Member consists primarily of mudstone and is the most organic-rich unit within the core with an average TOC of 4.2 wt.%. Trends identified from bulk organic geochemistry and elemental results show that intervals with elevated TOC and better-preserved marine OM (as indicated by organic petrography) are most associated with relatively high concentrations of redox-sensitive trace elements and lower concentrations of Mn, which corresponds to anoxic conditions. Maceral characterization reveals that the OM assemblages within the Niobrara Formation, lower transition member, and Sharon Springs Member are dominated by diverse algal forms, comprising ~65–80 vol.% of the OM distribution, and ~15–25% terrigenous fragments, depending on stratigraphic location.