Evaluation of visible and near-infrared and thermal infrared reflectance spectra for studying thermal alteration of Pierre Shale, Wolcott, Colorado : J Geophys Res V96, NB11, Oct 1991, P18047–18057

Abstract

Analyses of visible and near-infrared (0.4–2.5 μm) and thermal-infrared (2.5–25.0 μm) spectral reflectance of whole rock and kerogen concentrates of dike-intruded Pierre Shale indicate that the degree of thermal alteration of contained organic matter (OM) can be estimated by use of spectral reflectance. With increasing thermal alteration of the OM, whole rock, and kerogen spectra show the following trends: (1) visible and near-infrared (VNIR) and thermal-infrared (TIR) reflectance decreases progressively, with the largest change in the VNIR occurring between 0.90 and 1.50 μm; and (2) the intensities of absorption features in the 3- to 5-μm region, which are related to C-H stretching vibrations, decrease. The VNIR and TIR spectral reflectance trends are highly correlated with H/C ratios and vitrinite reflectance data. The largest changes in all these parameters occur at a distance that is equivalent to about one-half the dike thickness, where the temperature range is estimated to have been 240°–350° C. The VNIR and TIR spectral reflectance variations are related to thermally induced composition and structural changes in the OM. Landsat Thematic Mapper channels 3, 4, and 5 should have excellent potential for remote detection of OM maturity. Analysis of in situ VNIR and TIR measurements may provide a rapid method for estimating thermal maturity in the field. In addition, laboratory measurements of TIR spectral reflectance are a more sensitive and convenient means of measuring the maturity of kerogen than the traditional transmittance method. Further work is necessary to examine the effects of variable OM abundance and type, mineralogic composition, grain size, and weathering on spectral reflectance characteristics.