Study Results: Using Raman Microscopy Imaging to Identify Thermal Maturity of Kerogen in Oil Shale

Our last article discussed how Raman imaging can be used to identify thermal maturity of the kerogen in the oil shale, aiding hydrocarbon exploration. This article will focus on the actual analysis done, the methods used and results seen.

Oil shale is a fine grained sedimentary rock with considerable compositional variations that contains a solid mixture of organic chemical compounds called kerogen. Oil shale was formed from the organic debris millions of years ago. When heated by natural geothermal heat these shales produce liquid organic products by thermal decomposition.

We analyzed several oil shale samples from Eagle Ford Shale (Texas) with a Raman imaging microscope, and demonstrated that Raman imaging identified, with high speed and accuracy, the presence of different minerals, including pyrite, calcium carbonate (calcite) and others. Knowledge of the presence or absence of these minerals in oil shale is essential for efficient oil extraction in drilling operations by the new technique of hydraulic fracturing (fracking). We also demonstrated that Raman imaging can be used for fast identifying thermal maturity of the kerogen in the oil shale.

For this particular study, we took thin sections of the oil shale samples. The Raman imaging microscope with a 100x microscope objective (numerical aperture 0.90) was used to collect Raman images for selected regions of the shale samples. A 532 nm laser (with a 1.5 mW of power at the sample) with a full-range grating (900 lines/mm) provided 50-3400 cm-¹ spectral region. A 25 µm confocal pinhole aperture and 1 µm image pixel size were used to provide high spatial resolution Raman imaging. For imaging the thermal maturity of the kerogen, FWHM values of the G-band were used.

Raman imaging analysis confirmed that most of the sample contained as revealed by the characteristic amorphous carbon Raman spectra with strong and broad D and G bands with peak maxima at ~1350 cm-¹ and ~1600 cm-¹, respectively. Raman imaging also revealed, with high spatial resolution, the presence of several inorganic minerals, including pyrite, marcasite, anatase and calcite. Occurrence of calcite shows that the oil and gas can be easily extracted from this deposit by fracking. When the oil shale contains reasonable amount of calcite (or dolomite), it is more prone to Acid Fracturing (Acidizing), in which an acid, such as HCl, is used to “etch” channels in the rock: HCl reacts with the calcite to produce CaCl_², water and CO_². This breakdown promotes fracture formation and the CO_² adds to the pressure of fluid in the fractures to help facilitate the extraction.

Raman thermal maturity images showed that kerogen in the shale samples had largely similar thermal maturity. However, the carbonaceous materials in close proximity to the minerals, especially pyrites and marcasites, exhibited lower maturity than the surrounding regions. This is an interesting finding demonstrating special role of these minerals in kerogen maturity and consistent with a previous study, which demonstrated that pyrite acts as a catalytic agent for hydrocarbon generation from kerogen.

We concluded that Raman imaging, with the Raman imaging microscope, was successfully used for the fast identification of minerals and kerogen in the oil shale samples. Presence of calcite, among other minerals, indicated that the shale was well-suited for fracking. Raman thermal maturity images, plotted by the FWHM values of the G-band of kerogen, showed that although relatively uniform overall, the areas close to the minerals showed lower maturity, indicating the special role of minerals for oil and gas formation.

You can download the application note to see spectra and the Raman images of an oil shale sample analyzed in this study, as well as access a video.