Objectives/ Scope: There is a strong correlation between the ordered and disordered C-C peaks observed in Raman spectra and conventional vitrinite reflectance values that encourage the use of Raman microscopes to characterize micron-sized particles that are not detected with optical methods. The spatial distribution of minerals with respect to pores and organic matter is examined as an explanation to the range of maturity index values that are observed within an individual sample.
Methods/ Procedures/ Process: A Raman-enabled microscope was used to collect large field-of-view images of organic-rich shale at micron resolution with the use of an automated stage. The acquired spectra were processed with standard spectral analysis software (LabSpec6) that resolved the ordered and disordered carbon bands used to estimate maturity. The non-organic phases were identified with peak-match algorithms that used an in-house mineral database. Mineral maps were compiled with a Classical Least Squares (CLS) fitting or a Principal Component Analysis (PCA). These maps were often compared with SEM-based mineral maps acquired over the same region.
Results/ Observations/ Conclusions: The range in maturity values within a single field-of-view is linked to whether the dominant mineralogy that enclosed the particle was carbonate or silicate-rich. The local environment in a fine-grain rock influences the extent of organic-matter diagenesis.
Applications/ Significance/ Novelty: Raman microscope methods to characterize maturity of organic matter adds the capability of measuring smaller particles in an automated process that limits subjective sampling strategies. The ability to characterize the surrounding matrix and pore space for each organic matter particle opens up new ways to study diagenesis in these low-permeability fine-grain reservoir rocks.
Thermal maturity of source rock in petroleum-generating basins is traditionally determined by optical properties of specific organic macerals, primarily vitrinite. This technique was established in coal basins where terrestrial-derived vitrinite particles were abundant and large enough to identify under reflected light in a microscope. In the case of marine depositional environments, the absence of discernible vitrinite amongst the dispersed organic matter (DOM) makes vitrinite reflectance methods less reliable. The vitrinite reflectance method is well established with a published procedure but remains highly subjective between labs as the selection of vitrinite grains from other types of organic matter, e.g. inertinite, is operator dependent. The absence of suitable vitrinite grains in marine-sourced rocks also is a major limitation to this standard technique.
