Rapid Analysis of Kerogen Hydrogen-to-Carbon Ratios in Shale and Mudrocks by Laser-Induced Breakdown Spectroscopy

Abstract

Evaluation of organic matter quality in petroleum source and reservoir rocks is vital for economic, drilling, and production decisions. However, current methods for assessing kerogen quality in shales and mudrocks are either time-intensive and undesirable from a health, safety, and waste-disposal standpoint or indirect measurements of the factors of greatest interest and utility (e.g., kerogen hydrogen content). We present a new approach using laser-induced breakdown spectroscopy (LIBS) to both directly and quickly measure kerogen hydrogen-to-carbon elemental ratios in shales and mudrocks, providing a rapid means of assessing kerogen quality and thermal maturity with minimal sample processing. Elemental spectral intensities, including hydrogen and carbon, were monitored during LIBS measurements by optical emission spectroscopy, following ablation of a small portion of rock. Changes in the elemental intensities in the spectra over the course of LIBS measurements were then related to H/C ratios determined on isolated kerogens from the rock samples using multivariate analysis. Predicted kerogen H/C ratios from the LIBS measurements of whole rock samples were well-correlated (R2 = 0.99) to values determined for kerogen isolates measured by elemental analysis. These results demonstrate that LIBS is a promising method for examining kerogen quality and thermal maturity in source and reservoir rocks. Because LIBS does not require samples to be pulverized and can be conducted on sub-millimeter scales, this approach will also allow for highly localized, spatially resolved measurements on heterogeneous geologic samples.