Thermal maturity assessment in the Lower Huron using Raman micro-spectroscopy

Abstract

Raman micro-spectroscopy was utilized to confirm thermal maturity assessments in the “LHRN 7” lithologic unit in three cores which were part of a larger study on production mechanisms in the Lower Huron shale. Traditional methods of determining thermal maturity provided conflicting results. These methods included optical vitrinite reflectance, programmed pyrolysis parameters TMAX and Hydrogen Index (HI), and organic porosity development (i.e. apparent transformation ratio (ATR), derived from scanning electron image analysis).

Raman micro-spectroscopy provides a spectrum of Raman shifts that are indicative of various bond types and their relative concentrations in a sample (e.g. carbon-hydrogen single bonds carbon-oxygen double bonds, etc.). The peak at 1500-1650 cm-1 is indicative of the vibration mode of carbon-carbon double bonds. Baselined, intensity-normalized G-band full-width half-maximum (ΓG) narrows as concentration of carbon-carbon double bonds increases.

ΓG results from LHRN7 samples consistently agreed with associated HI and ATR measurements, as well as with thermal maturity derived from regional maps. These results suggest that this study's optical vitrinite reflectance and programmed pyrolysis TMAX data are inaccurate.

The benefits of Raman micro-spectroscopy are that it is relatively low cost, rapid, non-destructive, in situ, spatially high-resolution, repeatable, does not require sample preparation, and the results are quantitative and objective. Raman micro-spectroscopy is therefore ideal for situations where traditional bulk analysis or destructive analysis is not feasible, not practical, is undesirable, has provided suspect or unusable results, or is subjective.

Limitations to Raman micro-spectroscopy are that the resulting spectra and the parameters associated, like ΓG, are excitation wavelength specific, possibly instrument configuration specific (e.g. grating size), and are subject to noise from fluorescence common with low maturity samples. Additionally, the power of the excitation laser must be kept low to prevent thermal degradation of the sample. Finally, various techniques of spectral deconvolution are often used in the literature (e.g. Lorentzian and Gaussian curve fitting) to obtain parameters similar to ΓG. For these reasons, comparison of data between different Raman studies can be challenging.

AAPG Datapages/Search and Discovery Article #90258 © 2016 AAPG Eastern Section Meeting, Lexington, Kentucky, September 25-27, 2016