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Micro-FTIR Imaging and Confocal Microscopy for Quantitative Characterization of Organic Rich Mudstones


Documenting the relationship between organic content and surrounding rock micro-textures in source rocks is a necessary first step in predicting hydrocarbon productivity. Arrangement of the mineralogical and organic microstructures in mudstones and how they interact during hydrocarbon generation are first order controls on reservoir quality. Quantitative analysis of both organic and inorganic components is fundamental to understanding the bulk rock properties. Standard methods for imaging mudstone micro-textures are either limited by image resolution or by small areas of investigation. Standard methods for evaluating kerogen type and maturity, involve destructive bulk rock measurements on homogenized samples. While these analyses are robust, they are expensive, time consuming, and results are difficult to correlate to mudstone micro-textures. We present two methods for spectral characterization of mineralogy and geochemistry based on non-destructive measurements: Micro-Fourier Infrared (FTIR) Imaging and Laser Scanning Confocal Microscopy (LSCM). These spectroscopic techniques have specific molecular responses from which physical and chemical properties such as kerogen type and maturity can be inferred. These techniques are compared with traditional imaging methods for four mudstones with a range of maturities. LSCM is a fluorescence imaging method that, unlike traditional UV microscopy, allows for enhanced signal-to-noise ratios and supports quantitative measurements at a much higher spatial resolution (~250 nm). Micro-FTIR Imaging uses a focal plane array coupled with an FTIR spectrometer for simultaneous acquisition of multiple (> 4000) spectra in a single measurement. Spectral maps are generated by integrating over chemically significant wave number bands, where pixel color corresponds to peak intensity. Resulting spectral maps provide spatial distribution of chemical information at a high resolution (~500 nm). These maps are used to quantify minerals and specific compositional structures within kerogen, such as aliphatic and aromatic compounds. This joint geochemical and spatial characterization highlights the genetic relationships between organic content and surrounding micro-textures. These relationships are not readily apparent using traditional imaging and bulk rock analysis. Coupling Micro-FTIR Imaging and LSCM supports quantitative, non-destructive, evaluation of the chemical and spatial heterogeneity within mudstone fabrics.