Scanning confocal Raman microspectroscopy was used to analyze thin sections for inorganic mineral content and for evaluation of organic material. Thin sections were selected from a stratigraphic test well core in the Powder River Basin, Wyoming under the Wyoming CarbonSAFE project. The well was analyzed with a variety of rock and fluid characterization techniques to determine the feasibility of a commercial-scale carbon capture and storage/utilization and storage (CCS/CCUS) site. Raman microspectroscopy complements other analyses, including traditional petrography, SEM, porosity and permeability, and XRD. For this aspect of the study, Raman microspectroscopy is especially important in the evaluation of organic material in sealing lithologies. At prospective geologic CCS/CCUS sites, it is imperative to assess the unconventional oil and gas potential of seals to ensure that any future development would not compromise the integrity of the seals.
Whole-slide mineralogical surveys were performed on thin sections from various shale and sandstone formations. Survey spectra were analyzed with Direct Classical Least Squares to identify and quantify minerals. The location and concentration of minerals was color-coded and overlaid on optical images for visualization of the distribution of minerals. Dense hyperspectral Raman mapping of organic material was performed on five thin sections. Eleven spectral parameters diagnostic of organic type and thermal maturity were used to train a Partial Least Squares (PLS) calibration against a set of artificially matured samples spanning the pre- to mid-oil window. The PLS was applied to the study set and a post-mature set. Additionally, the PLS was applied to each point in hyperspectral maps for visualization of trends in maturity across sample sets and discrimination of organic matter types within a given map.
In inorganic surveys on thin sections, a total of 14 unique inorganic minerals were identified in Raman spectra including quartz, dolomite, calcite, hematite, and anhydrite. Shale thin sections tended to be dominated by organic material, which was often observed mixed with inorganic minerals. Sand- and mudstones were dominated by inorganic minerals. The PLS calibration extrapolated the post-mature set to reflectances >1.2%. The study set ranged from very immature to postmature in the median of map fit-peak parameters. However, point maturity maps indicate that matrix organic material in all study samples is immature and that discreet organic particles selected for mapping, which may be inertinites, bias medians towards more-mature.
The present work demonstrates the capabilities of Raman microspectroscopy to perform both whole-slide mineralogy and organic material analysis with applications to formation evaluation in oil & gas, CCS/CCUS and mining. Here, analysis of sealing formations in the well indicates high levels of immature organic material that would not be a viable target for future oil production that could compromise the CCS/CCUS site. This work also brings together diverse disciplines from geology to analytical chemistry, big data, and microscopy.
