Recently, oil shale has generated significant interest although this was discovered many decades ago. Incidentally, years ago, Ralph H. McKee of Columbia University reported on his petrochemical research of heating oil shale to generate gasoline, kerosene, and other types of lubricating oils. The title of McKee's presentation was "Future Sources of Gasoline". Now more than three decades later, it was reported that oil shale deposits in Northwestern Colorado probably contained the highest concentration of undeveloped energy in the world.
To this day, oil shale continues to generate significant interest as a viable source for fuel and other petrochemical products. This technical brief discusses the application of Raman spectroscopy to differentiate oil shale, specifically black shale, which is composed of a fossilized organic matter, kerogen. Here, Raman spectroscopy proves quite handy for characterizing shales with varied amounts of kerogen to inorganic mineral, differentiating various kerogen structures, and characterizing polymorphs of inorganic oxides that occur naturally.
Definition of Oil Shale
Oil shale is referred to the sedimentary rock from which oil is obtained via a high-temperature chemical process. In other words, oil shale is porous rock in which kerogen is trapped. Understanding and improving the chemical processes to produce petrochemical products from sedimentary shale infused with kerogen presents a potential area of chemical research.
Black Shale Composed of Calcite and Kerogen
Figure 1. Raman spectrum of black shale. The Raman bands at 1354 and 1603 cm-1 are attributable to kerogen.
Figure 2. Raman spectrum of black shale with considerably more calcium carbonate relative to kerogen than that observed in figure 1.
Kerogen and Amorphous Carbon
Kerogen is mostly composed of carbon and hence its spectrum is similar to that of carbon. The above figure displays a typical Raman spectrum of black shale that consists of calcite and kerogen. The sharp and narrow band at 1603 cm-1 is referred to as the "G band," where G represents graphite. The wide band centered at 1354 cm-1 is designated as the "D band," where D represents disorder. To this end, there is not a special spectrum of kerogen, as in the case for different forms of amorphous carbon.
In effect, scientists Kelemen and Fang did extensive research on kerogens and coals from various sources and their results were published relating the band separations, bandwidths, and relative integrated intensities of the G and D bands of Green River and Bakken kerogens to vitrinite reflectance measurements, which are mostly utilized as a measure of organic metamorphism. This study highlights the efforts made to establish a link between the maturation of kerogen and Raman spectral characteristics. With the help of Raman spectroscopy, the researchers correlated the changes in solid state structure and chemical bonding to the extent of maturation or to the degree of catagenesis and metagenesis that took place in the natural state of the organic material.
Oil shale is composed of kerogen, which is trapped in the sedimentary rock. This rock includes mainly calcite and silicates. The presence of calcite is more visible in the Raman spectrum of figure 2 in which the kerogen bands along with the much sharper bands at 156, 282, 714, and 1088 cm-1 is seen. The sharper bands reveal the presence of calcite. In this case, we assume that there is more calcite and relatively less kerogen in the focal volume of the laser beam when compared to the sample from figure 1. As a result, the calcite contributes more Raman scattering to the spectrum, while kerogen’s signal-to-noise ratio scattering is relatively less. Theoretically, one can utilize the relative intensities of the kerogen and calcite bands as a substitute for the relative amount of kerogen to calcite from sample to sample or within a sample for different locations. Figure 3 displays Raman spectra achieved from black shale at different locations.
Figure 3. Raman spectra from different areas of black shale indicating the varying amounts of calcium carbonate relative.
These spectra show the inconsistency of the calcite-to-kerogen Raman strengths that can be viewed in black shale. One can utilize the relative Raman strengths to carry out quantitative analysis of the black shale by producing reference samples of known amounts of calcite and kerogen.
Figure 4. Raman spectrum of black shale consisting of contributions from kerogen and the anatase form of TiO2
Although, technically, one could quantitatively determine the relative amounts of kerogen and calcite, a precise assay would demand that the solid-state structure and chemical bonding of the kerogen reference must correlate with that of the sample being examined. Therefore, an accurate qualitative analysis of the kerogen spectra is a prerequisite before attempting to utilize Raman spectroscopy for quantitative analysis of oil shale. Furthermore, besides calcite, oil shale rock also contains kerogen and anatase form of titanium dioxide, as illustrated in figure 4.
This information has been sourced, reviewed and adapted from materials provided by HORIBA Scientific.
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