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Application of Hyperspectral Imaging to Measuring Organic Carbon Content and Quartz/Clay Ratios in Black Shales


Total organic carbon (TOC) content and the quartz/clay ratio are critical elements of shale reservoir characterization. TOC influences the volumes of hydrocarbon generated, the fraction of porosity and the mechanism of hydrocarbon storage (free gas versus adsorbed gas). The quartz/clay ratio influences the mechanical properties of formations and therefore whether they are prone to develop natural or hydraulically induced fractures. Quantitative measurements of these properties are now based on either direct chemical analyses of rock samples or interpretations of well logs. Chemical measurements on core samples are a subsample of the entire formation, and the continuous distribution has to be extrapolated from discrete points. Well logs provide a continuous record but average properties at scales up to one meter. Where chemical properties vary significantly at smaller scales, likely the case in shale formations, these techniques provide an incomplete or blurry record of the rock properties. We have tested a new technique for characterizing shale formations – hyperspectral scanning. This technique relies on the distinctive spectral properties of quartz, clay and organic carbon-bearing substances, specifically key absorption bands in the shortwave and longwave IR spectra. We report on two data sets. Core samples from two wells in the Woodford Shale, Permian Basin, west Texas, were analyzed for major/minor/trace elements, TOC and Rockeval parameters. The strength of absorptions in reflectance spectra correlates effectively with both TOC and SiO2 content (R2=0.87 and 0.93, respectively). The Woodford has very different thermal maturities in two wells, a difference also effectively identified by spectral results. We are now applying and evaluating this approach to an extensively sampled 150 meter core from the Horn River shale in British Columbia, where TOC ranges from 0.12 to 9.38% and Al2O3/SiO2 ranges from 0.04 to 0.37.