Assessing organic carbon maturity using Laser Raman approaches
Limited work to date (e.g. Quirico et al., 2005; Allwood et al., 2006; Liu et al., 2013) has shown that Laser Raman microscopy may be a viable alternative to current vitrinite reflectance optical microscopy-based assessment methods.
The Kingston University Laser Raman Spectrometer can generate spectra from organic-rich shales and coals that are potentially capable of defining the burial and thermal history of the material being analysed. Liu et al., (2013) have suggested that the Laser Raman spectra from “solid organics” in coals and organic-rich shales can be used quantitatively to define the maturity of carbonized substances that have been deposited and then buried in the range from catagenesis up to metagenesis (ie. from the release of thermally generated hydrocarbons to beyond the final release of thermally generated methane).
As a method of quantifying thermal evolution or “maturity”, this appears to be an appealing and very rapid alternative technique to the traditional Vitrinite Reflectance (vRo%) technique using oil-immersion, reflected light microscopy.
A pilot study, to check the feasibility of this Laser Raman technique, has been carried out on a small number of organic-rich shales from the Bowland Basin of N. Lancashire which were collected and analysed using the Kingston University Laser Raman Spectrometer, which has a very similar specification to the one used by Liu et al., (2013). The organic matter in these Bowland Basin Shale specimens generated Laser Raman spectra that were similar to those illustrated by Liu et al., from Chinese coals at similar states of burial/maturity.
A much more substantial study of the correlation between laser raman signatures and Vitrinite Reflectance would build on our pilot work to date and fully assess the laser, Vitrinite and thermal data generated from a range of different organic materials. As part of this study we would assess the “Laser Raman reflectance” of individual kerogen fragments from coals that have a heterolithic kerogen content and from kerogen separates from organic-rich shales to better test the feasibility for shale gas applications. This analysis should determine whether the original palaeo-botany of the material is immaterial when it comes to generating Laser Raman reflectance spectra or if the state of Laser Raman reflectance is influenced by the original palaeo-botanical compositions of the kerogen.
The goals of the proposed project then would be to:-
1) Investigate the correlation between Laser Raman and conventional vitrinite reflectance data for a defined collection of known substrates at surface.
2) Determine the reproducibility of and validity of the technique across an initial collection of substrates, followed by a wider range including e.g. Bowland Shales.
3) Understand how Raman measurements could be performed as an ‘on-rig’ process applied to cuttings or core samples.
4) Explore the application of Raman spectroscopy downhole for fluid composition and properties.
5) Explore possible protocols for vitrinite measurements in a down-hole environment. Practical issues include understanding how to locate individual organic matter particles, focusing the laser onto selected particles and preventing contamination of the spectra from fluids and particulate material in the ambient environment.
References.
Allwood, A.C., Walter, M.R., Marshall, C.P., 2006. Raman spectroscopy reveals thermal palaeoenvironments of c.3.5 billion-year-old organic matter. Vibrational Spectroscopy, 41, 190-197.
Liu De Hahn, Xiao XianMing, Tian Hui, Min YuShun, Zhou Qin, Cheng Peng, Shen JiaGui, 2013. Sample maturation calculated using Raman spectroscopic parameters for solid organics: Methodology and geological applications. Geochemistry, 58, 1285-1298.