APPLICATION OF A PORTABLE RAMAN TOOL FOR THERMAL MATURITY ANALYSIS OF CRUSHED ROCK

The continuing reliance of the United States and other countries on unconventional petroleum systems to meet current energy demands has led to numerous studies leveraging Raman spectroscopy to obtain thermal maturity proxies for organic matter evaluation in shales. In parallel, methods have been established for rapid, wellsite evaluation of sample mineralogy and chemistry, providing rich datasets characterizing the lithology along the entire length of a wellbore. This study presents a simplified method and empirical relationships for rapidly determining organic matter thermal maturity of crushed rock samples using a portable Raman spectrometer equipped with a 785 nm laser that could easily be applied in a field or wellsite setting in tandem with other characterization techniques.

A suite of shale samples from several U.S. basins with variable organic matter thermal maturity, mineralogy, and total organic carbon was used to develop the method and build correlations between Raman band separation (RBS) values and traditional thermal maturity indicators such as programmed temperature pyrolysis (T_max) and vitrinite and/or solid bitumen reflectance (R_o). Measured R_o values of these shale samples range from 0.40 to 4.62% and corresponding RBS values range from ~200 cm^-1 to ~290 cm^-1. Below 3.35% R­_o, RBS and R_o show a strong linear correlation (R²=0.96), whereas above 3.35% R_o, RBS values are invariant representing a possible upper limit of application for this method. The relationship between RBS and T_max was also evaluated using shale samples with R_o < 2% and T_max < 551°C. T_max and RBS values show a strong linear correlation (R²=0.94). Precision on RBS values measured on cuttings samples improved with washing, drying, hand crushing, and sieving samples to <500 mm, although less preparation still yielded reliable results. The high degrees of correlation between whole-rock RBS data and two commonly used thermal maturity indicators (i.e., R_o and T_max) demonstrate the potential utility of this approach for rapidly generating thermal maturity assessments of minimally processed, whole-rock shale samples.