Visible and near‐infrared spectra of carbonate rocks: Reflectance variations related to petrographic texture and impurities

Abstract

Grain size statistics derived from thin section point measurements were used to classify carbonate rock textures and to identify related spectral characteristics in the visible and near‐infrared (0.4–2.5 μm) wavelength range. Although the statistics did not permit all the carbonate textural varieties generally recognized by petrologists to be distinguished, three broad textural classes of pure carbonate material were found that produced distinctive spectra. Further refinement of the statistical technique may permit other textural classes to be identified. However, internal scattering in carbonate rocks is a complex phenomenon involving the grain size distribution, as well as the nature and abundance of inclusions and internal grain defects. To yield an optimum textural classification for spectroscopic studies, all such scattering contributors would have to be quantified. The spectral effects of iron oxide, clay, silica, and organic matter impurities were examined by comparing texturally similar pure and impure carbonate rock samples. Of these impurities, organic matter had the greatest spectral influence, and amounts as low as 0.01 wt % commonly caused marked reductions in sample brightness and absorption band intensity. In contrast, the spectral effects of rock texture were subtle and could only be identified in samples that were nearly free of organic matter impurities. The pronounced organic matter spectral effects imply that the organic matter in most carbonates is very finely disseminated. Samples collected from different regions usually showed different degrees of organic spectral quenching. However, no correlations were evident between sample reflectance and contained kerogen H/C ratios or kerogen amounts. Additional analytical work is needed to examine organic matter compositional variation and associated spectral quenching effects in detail. From the present study, it appears likely that contained organic matter will be a dominant factor in the visible and near‐infrared spectral expression for most carbonate rocks, and perhaps for other sedimentary lithologies. Spectral quenching by organic matter will likely complicate remote sensing and laboratory spectral determinations of carbonate composition. On the other hand, variations in quenching may prove to be useful for distinguishing rocks that have experienced unusual diagenetic, thermal, or other conditions affecting the organic matter contents.