USING MULTIVARIATE ANALYSIS OF RAMAN SPECTRA TO IMPROVE THERMAL MATURITY ASSESSMENT OF SHALES

Assessment of the thermal maturity of shales is typically accomplished using vitrinite reflectance and chemical analysis. Kerogen, composed of diverse organic materials such as vitrinite, is found as macerals in sedimentary rocks including coal and shale, and in some instances is the major component. It is classified into four categories based on the palynological material it originated from. Accurate vitrinite reflectance of shale is impeded by maceral abundance and size, inclusions, textural features, and the presence of other organic macerals. Raman Spectroscopy has been used to overcome some of the inherent challenges of traditional thermal maturity methods.

Raman spectroscopy (RS) is a rapid, non-destructive analytical tool that has shown promise for characterizing organic material in coal and shale. RS uses monochromatic light from lasers to probe the sample. Scattered radiation generated from this interaction produces a chemical fingerprint that is unique to the chemical bonds in the analyte. Interpretation of the spectra is traditionally completed via a peak fitting technique.

The lack of a standard method for applying Raman peak fitting routines often causes ambiguities in the assignment of thermal maturity of shale. RJLG has applied multivariate analysis of Raman data from a series of standard reference coal samples to create a robust model that correlates spectral data to thermal maturity and other characteristics. Analysis of shale samples originating from several formations in southern Ohio and eastern Kentucky by both vitrinite reflectance and Raman spectroscopy demonstrates the applicability of the RJLG Raman technique for improved assessment of thermal maturity.