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Raman Spectroscopy of Organic Material in Shales: Identifying Suitable Laser Wavelength for Raman Spectra Excitation, Relationship to Thermal Maturity, Mechanical Properties, and Organic Matter Type Interpreted From SEM Images

Abstract

Vitrinite reflectance and Rock-Eval pyrolysis (Tmax) are the two standard techniques used to determine source rock maturity. However, it is difficult to apply these techniques to small volumes of organic material, high maturity samples due to heterogeneity and bireflectance, and samples where vitrinite is rare to absent (e.g. pre-Silurian rocks which do not have vitrinite macerals). Raman spectroscopy, based on molecular vibration, can provide chemical composition information of samples. In addition, a relationship has been demonstrated between Raman spectra and thermal maturity of coal and kerogen samples. Compared to vitrinite reflectance and Rock-Eval pyrolysis (Tmax), Raman spectroscopy has several advantages. First, the laser focal volume is around 3 μm3 under the Raman microscope. This enables analysis of small volumes of dispersed organic material. Second, as this technique detects the Raman spectrum of target samples based on their chemical composition instead of their optical reflectance, it does not suffer from optical bireflectance and heterogeneity. Third, this technique can also be applied to samples with no vitrinite macerals. We have performed Raman spectroscopy analysis, acoustic microscopy, and SEM imaging on shale kerogen at different thermal maturity levels. Shale kerogens with low thermal maturity typically exhibit a strong fluorescence background that can overwhelm or cover the Raman spectrum for analysis. We have successfully measured the Raman spectrum of low maturity (0.5% Ro) organic material. The Raman spectrum of shale kerogen shows a D band around 1350 cm-1 and a G band around 1590 cm-1, and several bands around are revealed by deconvolution. We are able to pinpoint different organic materials in the field of view under Raman microscope and get the spectra of the target at the points of interest. Different organic matter types show different Raman spectra. Acoustic properties of shale kerogen samples were also measured to correlate with their thermal maturity. Our results will be shown together with SEM imaging of organic material of various types (e.g. bitumen, marine Type II kerogen, and terrestrial OM) so that an integrated understanding of the Raman spectra, mechanical (acoustic) properties, organic matter type, and maturity can be achieved. This study has proved the feasibility of Raman spectroscopy as a thermal maturity indicator for shale kerogen.