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A comparison of kerogen maturity analysis in shales using laser Raman spectroscopy, vitrinite reflectance and Rock-Eval pyrolysis

Abstract

Laser Raman spectroscopy (LRS) has proven to be a rapid and cheap, non-destructive technique to determine the maturity of kerogene.g.1,2. Here, LRS results are compared with maturity estimates based on vitrinite reflectance (VR) and Rock-Eval pyrolysis. All three methods have advantages and limitations, and having multiple techniques that can be called on to perform maturity analysis, enables geologists to optimize their analysis based on the composition and age of the sample, maturity grade, equipment availability, and time and money constraints. Independent cross checking of results will also reduce risk. We have applied the three maturity methods to study 86 washed rock cuttings from two potential Carboniferous shale gas plays in the UK3,4, in the Midland Valley of Scotland (MVS) and southern Pennine Basin, age equivalents of the US Barnett and Fayetteville shales. LRS analysis was performed on hand-picked coal and shale cuttings. VR data were compiled from previous studies, together with in-house VR analysis to validate previous results and to obtain values for samples where VR data were unavailable. Rock-Eval pyrolysis was performed on splits of the same samples.

Our results show that the G-band full-width at half-maximum (G-FWHM) is the best LRS parameter to determine the maturity of kerogen and has a strong correlation with VR values of 0.6 - 5 %VRo. Correlation with Rock-Eval Tmax is also strong up to 2 %VRo for both G-FWHM and VR, but this breaks down at higher maturities, with an inconsistent decrease in Rock-Eval Tmax values. Samples from the MVS and Pennine Basin generate similar results, despite originating from different basins with contrasting geological histories. The depositional environments and organic matter types differ in the two areas: the MVS has lacustrine Type I/III source rocks, and the Pennine Basin marine Type II/III source rocks. The MVS has been affected by intrusion of the Midland Valley Sill, creating thick thermal aureoles and rapid heating; the Pennine Basin has also been affected by igneous intrusions, but on a much smaller scale. Further experiments with a portable LRS are underway to test the ability to perform maturity analysis on-site during drilling, offering rapid characterization and development of shale gas/oil reservoirs.

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2. Schito, A., Corrado, S. 2018. Geol. Soc. Spec. Pub. 484.

3. Andrews, I.J. 2013. BGS DECC.

4. Monaghan, A.A. 2014. BGS DECC.