Identifying and Quantifying Carbonate Minerals in Quartz–Illite–Muscovite-Dominated Reservoir Rocks With SWIR and LWIR Spectroscopies

Secondary carbonate minerals may influence the porosity of rocks; and, therefore, the quality of hydrocarbon and geothermal reservoirs. These carbonate minerals can be identified using shortwave infrared (SWIR) spectroscopy by their typical absorption features nearly <inline-formula> <tex-math notation="LaTeX">$2.35~\mu \text{m}$ </tex-math></inline-formula>. However, their detection in quartz–illite–muscovite-dominated reservoir rock is hampered by the overlap of diagnostic features of carbonate minerals with the absorption features of illite–muscovite minerals. In this study, we used longwave infrared (LWIR) spectra to overcome this problem. We investigated the 6.50- and 11.30-<inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> carbonate features in the LWIR as well as the 2.50-<inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> feature at the longer wavelength margin of the SWIR to determine whether these features provide a better discrimination of carbonates in illite–muscovite-bearing rocks. We also investigated the possibility to use these features to quantify the carbonate content. We tested this on laboratory SWIR and LWIR spectra of 22 drill core samples from a Dutch reservoir sandstone of Permian age that contain variable amounts of illite–muscovite and carbonate minerals. Spectral parameters, such as the wavelength position and depth of absorption features (or reflectance peaks), were compared with (semi-) quantitative analyses of the mineral composition acquired with quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN). Our results show that the ratio of band depths of the carbonate features at 2.44 and <inline-formula> <tex-math notation="LaTeX">$2.50~\mu \text{m}$ </tex-math></inline-formula> are correlated with the relative carbonate content [carbonates versus illite + muscovite ratio (CIM)] with Spearman correlation coefficient (<inline-formula> <tex-math notation="LaTeX">$\rho$ </tex-math></inline-formula>) of −0.87. The best predictor for the presence and content of carbonate minerals is the 6.50-<inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> feature, with a correlation between the peak height and the absolute total carbonate content of 0.93. Therefore, the 6.50-<inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> peak height can be best used for the quantification of carbonates in the quartz–illite–muscovite-dominated rocks we have investigated. The results of this study are useful for porosity studies of quartz–illite–muscovite rich reservoir rock, where carbonate minerals impact the rock porosity.