Visualizing organic matter content in organic-rich shales through Terahertz imaging

Abstract

Fine-scale visualization and quantification of the distribution of organic matter within shaly petroleum source rocks like oil shales, offers valuable insights into their suitability for energy applications, such as unconventional energy sources. Although methods exist for imaging organic matter content in organic-rich shales, such as X-ray microtomography and acoustic wave imaging, there is currently no non-ionizing, contactless, non-destructive method capable of imaging samples of several square centimetres and sensitive to nano-micrometre organic matter content. This possibility would provide new and complementary information to obtain a better characterization of the reservoir. Thus, to address this gap, we propose a novel method that uses measurements of electromagnetic waves within the Terahertz (THz) band, acquired through a Terahertz time-domain spectrometer operating in transmission mode. It uses information about the dielectric permittivity anisotropy of the material to estimate, for the first time, the average total organic carbon (TOC) per pixel across the thickness of the rock. We therefore present, first THz TOC images of organic-rich shale samples, specifically from the Vaca Muerta Formation (Neuquen Basin, Argentina). The images show a TOC pixel variation ranging from 3 to 6% with a relative pixel error of less than 6%, i.e. for a TOC value of 4%, the absolute error would be ±0.24%. The method have been validated with combined results obtained by Rock-Eval®analysis, Raman and infrared spectroscopy and X-ray microtomography. The reliability of this approach lies primarily in the consideration that the main source of dielectric permittivity anisotropy must be caused by the varying organic matter content of the different rock strata.