--> --> Lithology and mineralogy of the Upper Cretaceous Niobrara Formation examined by hyperspectral core imaging

AAPG Rocky Mountain Section Meeting

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Lithology and mineralogy of the Upper Cretaceous Niobrara Formation examined by hyperspectral core imaging

Abstract

Organic-rich mudrocks are often similar in appearance but have substantial compositional differences in terms of mineralogy and organic matter content. Extensive bulk sampling and a range of geochemical analyses are required to assess chemostratigraphic variability in dark-colored, finegrained source rock strata in order to obtain even a cursory understanding of heterogeneity in these rocks. As more research and exploration geoscientists recognize the limitations of traditional sampling and analytical approaches to characterizing continuous source rock intervals, spectroscopic methods capable of determining mineralogical, textural, and elemental properties at high-resolution have become more widely utilized to complement traditional characterization methods. Hyperspectral core imaging (HSI) can provide mineralogical and lithological information for source rock strata at submillimeter resolution, representing an intermediate scale between microscopic and bulk measurements. Combining HSI with other continuous (geophysical logging) or discrete measurements (thin section petrography, geochemical analyses) can lead to improved evaluation and interpretation of complex and Rocky Mountain Section – AAPG: 2019 Annual Meeting 50 variable lithofacies in mudrock core. Upper Cretaceous strata in Colorado, Wyoming, and Kansas have been examined by HSI in several cores archived at the U.S. Geological Survey’s Core Research Center in Denver, CO. The HSI system combines high-resolution photography (50 μm), 3D laser profiling (20 μm), and short-wave + near-visible infrared imaging spectroscopy (wavelengths from 0.45 to 2.5 μm, 500 μm pixel size). The imaging measurements provide spectral and textural data facilitating creation of continuous mineral and lithology class maps, 0.5-ft resolution mineralogical and lithological logs, and delineation of grain size and compositional features for carbonate and clay minerals. The Niobrara Formation was examined in a set of cores, including the heavily studied Amoco Rebecca Bounds and USGS Portland 1. The HSI results are combined with the extensive bulk geochemical data available for these cores along with results from interpretative studies of orbital forcing on stratigraphic cyclicity and oceanic anoxic events to expand our understanding of these mudrocks at resolutions not previously possible.