Visible and near infrared reflectance spectroscopy is an accepted method for identification of a wide range of minerals including the phyllosilicates and carbonates. The combination of portable instrumentation and automated analysis methods enables real-time identification and characterization of these minerals at the wellsite. In sedimentary basins, the technique provides clay mineralogy, thermal maturity, and carbonate composition.
The dominant phyllosilicate minerals found in sedimentary systems, including smectites, illites, kaolinite, and chlorite, have sufficiently unique spectral signatures to allow for both their identification and characterization. Kaolinite crystallinity is inferred from the shape of a doublet absorption feature. The progressive conversion of smectite to illite is tracked by monitoring the shape of a Al-OH absorption feature as well as the relative depths of water and Al-OH absorption features. Since oil generation takes place simultaneously with the diagenetic transformation of illite/smectite, substitution of NH4 for K in the interlayer sites of illite is often observed if hydrocarbons are present at the time of illitization. It is possible to determine the presence of ammonium illites since ammonium has unique absorption features near 2120 and 2020 nm. Reflectance spectra of chlorites typically have two hydroxyl absorption features (Fe-OH & Mg-OH) that shift in position with varying Fe/Mg ratios. Chlorites also exhibit a similar change in water to hydroxyl absorption feature depth with varying thermal maturity as do the illites. The Ca/Mg ratio of carbonates is determined from the position and shape of the CO3 absorption. The spectrum of the iron carbonate siderite, has a broad Fe2+ feature centered near 1200 nm and the CO3 is near 2335 nm.
In addition to mineralogy, reflectance spectroscopy is also applicable to characterization of organics in sedimentary rocks. It has been used for analysis of kerogen in oil shales and bitumen abundance in oil or tar sands. And as with many minerals, thermal maturity of hydrocarbons may be inferred from systematic changes in the reflectance spectrum.
