Assessing the Spatial Variability of Shale Maturity Estimates From Raman Spectroscopy

Abstract

Studies over the last 20 years have applied Raman spectroscopy to estimate thermal maturity of organic matter (OM) in source rocks as a complement to more traditional approaches such as vitrinite reflectance and programmed pyrolysis [Beyssac et al., 2003]. A maturity proxy based on Raman spectroscopy is especially attractive in samples where vitrinite reflectance and programmed pyrolysis measurements are challenging, such as pre-Devonian formations that predate the evolution of woody plants, and any sample that has low hydrocarbon-generative potential (i.e., organically lean or high maturity source rocks) which typically leads to unreliable T_max estimates [Cheshire et al., 2017; Sauerer et al., 2017]. Furthermore, Raman measurements on OM are rapid, non-destructive, and require only micrometer-size samples. Maturity estimates are typically extracted from the spectral data through iterative peak fitting based on analogy to the well-known Raman spectrum of graphite [Mapelli et al., 1999]. Most publications on Raman spectroscopy of OM indicate that spectra are collected from multiple sample spots within an individual sample, but little attention has been given to the degree of variation in Raman response across a source rock surface, especially in shales with dispersed OM, which may impact the derived thermal maturity estimate.

Here we assess the spatial variation in Raman estimates of thermal maturity from several geochemical reference materials (GRMs), including the Boquillas, Mancos, Marcellus, Niobrara, and Woodford shales. We show that within these GRMs thermal maturity parameters extracted from Raman spectra can vary widely across distances of ≤5 µm. This implies that the Raman response from the OM contained within these materials is influenced by the surrounding matrix, even within the same OM type. For each GRM, fluorescence and scanning electron microscopy were also conducted on the same sample field of view as the Raman experiments allowing for a correlative analysis and careful identification of OM type. These findings highlight the experimental care which must be adopted when making Raman measurements of OM in source rocks, especially for samples which feature highly dispersed, heterogeneous OM.

References:

Beyssac, O., et al. (2003), Spectrochim. Acta - Part A, 59, 2267

Cheshire, S., et al. (2017), Int. J. Coal Geol., 180, 29

Mapelli, C., et al. (1999), J. Mol. Struct., 480–481, 615

Sauerer, B., et al. (2017), Int. J. Coal Geol., 173, 150

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018