Detailed Description of Oil Shale Organic and Mineralogical Heterogeneity via Fourier Transform Infrared Microscopy Kathryn E. Washburn,* ,† Justin E. Birdwell, ‡ Michael Foster, † and Fernando Gutierrez † † Ingrain, Incorporated, 3733 Westheimer Road, Houston, Texas 77027, United States ‡ United States Geological Survey, Denver Federal Center, Box 25046, MS 977, Denver, Colorado 80225, United States ABSTRACT: Mineralogical and geochemical information on reservoir and source rocks is necessary to assess and produce from petroleum systems. The standard methods in the petroleum industry for obtaining these properties are bulk measurements on homogenized, generally crushed, and pulverized rock samples and can take from hours to days to perform. New methods using Fourier transform infrared (FTIR) spectroscopy have been developed to more rapidly obtain information on mineralogy and geochemistry. However, these methods are also typically performed on bulk, homogenized samples. We present a new approach to rock sample characterization incorporating multivariate analysis and FTIR microscopy to provide non-destructive, spatially resolved mineralogy and geochemistry on whole rock samples. We are able to predict bulk mineralogy and organic carbon content within the same margin of error as standard characterization techniques, including X-ray diffraction (XRD) and total organic carbon (TOC) analysis. Validation of the method was performed using two oil shale samples from the Green River Formation in the Piceance Basin with differing sedimentary structures. One sample represents laminated Green River oil shales, and the other is representative of oil shale breccia. The FTIR microscopy results on the oil shales agree with XRD and LECO TOC data from the homogenized samples but also give additional detail regarding sample heterogeneity by providing information on the distribution of mineral phases and organic content. While measurements for this study were performed on oil shales, the method could also be applied to other geological samples, such as other mudrocks, complex carbonates, and soils. 1. INTRODUCTION Petroleum production in the past decade has shifted from a focus on “conventional” reservoirs with high porosity and permeability to “unconventional” or continuous resources. Some of the most successful of these unconventional resource plays are dominated by mudrocks, often described as gas and tight-oil shales by industry, that have low porosity and extremely low permeability. For mudrock reservoirs, much of the petroleum resource is stored in porosity within the organic matter. 1 As such, production from wells is usually positively correlated with organic content. Therefore, geochemical evaluation of the forma- tion is needed to determine the most organic-rich sections for well placement. These reservoirs also tend to be highly hetero- geneous. 2 In addition, because the permeability of the rock matrix is so low, unlike conventional resources, hydraulic fracturing is required for production to increase accessible surface area to the well bore. Mineralogy is important to assess the brittleness and, hence, how difficult it will be to fracture the rock. Carbonate and quartz regions fracture well, while clay-rich, particularly smectite-rich, intervals tend to fracture poorly. The most common method for examining mineralogy is X-ray diffraction (XRD). Monochromatic X-rays are used to irradiate the sample, and crystalline constituents will scatter these X-rays at different characteristic angles that can be measured and used to identify the mineral phases present. To perform clay speciation, the samples need to be treated and then heated to align the clay particles. Amorphous materials, such as organic matter, are not identifiable because they do not scatter the X-rays at distinct wavelengths. 3 The organic matter content is mostly assessed by combustion methods, such as total organic carbon (TOC) analysis or programmed pyrolysis. 4 These are performed by heating small amounts of pulverized rock to high temperatures (300-1000 °C) and observing the products using a flame ionization detector for pyrolysis methods (hydrocarbons) or an infrared cell for com- bustion methods (CO and CO 2 ). Samples often need to be pretreated with hydrochloric acid to remove carbonates from the rock matrix; otherwise, when the sample is heated, the inorganic carbon present will breakdown and lead to overestimation of TOC. Transmission Fourier transform infrared (FTIR) spectroscopy has been used for many years to assess mineralogy. 5-9 While results are frequently quite good, pellet preparation is exacting, time-consuming, and prone to problems, such as cracking or cloudiness. Diffuse reflectance FTIR spectroscopy (DRIFTS) does not require that samples be made into pellets but still requires use of a diluent, such as potassium bromide (KBr), for high-quality reflectance results that conform to Beer’s law. 10 Sample preparation for DRIFTS measurements is still somewhat onerous. To avoid distortion because of grain size effects, the samples need to be pulverized to a uniform size below 5 μm. DRIFTS does have the advantage that, because no physical contact needs to be made with the analyte, the technique can be automated and performed on numerous previously prepared samples. Attenuated total reflectance (ATR)-FTIR is gaining popularity as an alternative infrared spectral acquisition mode. 11,12 ATR- FTIR has been used in a wide range of fields from geology 12-14 to pharmacuticals. 15-17 Unlike transmission and DRIFTS, ATR can Received: April 15, 2015 Revised: May 22, 2015 Article pubs.acs.org/EF © XXXX American Chemical Society A DOI: 10.1021/acs.energyfuels.5b00807 Energy Fuels XXXX, XXX, XXX-XXX