INNOVATIONS IN ORGANIC PETROGRAPHY AS INFLUENCED BY AND ADAPTED TO THE ‘SHALE REVOLUTION’ (Invited Presentation)

The global ‘shale revolution’ in energy resources has refocused organic petrography techniques and applications to source-rock reservoir petroleum systems containing microscale (<10 μm) fragments of organic matter dispersed in mineral-rich matrices. Obtaining spatially resolved information from organic matter at these scales is challenging but has fundamentally changed our understanding of petroleum generation, expulsion, migration and storage processes. Innovations in sample preparation for organic petrography include broad ion beam (BIB) and focused ion beam (FIB) milling approaches which have enabled discovery and characterization of organic nano-porosity. FIB combined with scanning electron microscopy (FIB-SEM) has engendered digital rock physics models and improved predictions of fluid flow in heterogenous shale matrices. Raman spectroscopy-based thermal proxies have been developed and now rival the accuracy of conventional organic reflectance measurements. Atomic force microscopy (AFM) has enabled prediction of mechanical properties for downstream applications (hydraulic fracturing and production), and, when combined with infrared spectroscopy (AFM-IR), can probe geochemical characteristics at unprecedented resolutions of <100 nm, revealing organic matter chemical heterogeneity and migration fractionation during expulsion. Correlative light and electron microscopy (CLEM) and integration of imaging and spectroscopic capabilities into a single instrument can enable simultaneous identification of organic matter types alongside collection of compositional or mechanical (elasticity) data, all at nano-scale. The future of organic petrography will see increasing use of correlative approaches, e.g., simultaneous IR and Raman; continuous improvements in laboratory-based instrument resolutions, e.g., in situ nano-scale X-ray photoelectron spectroscopy of organic matter; and nano-scale investigations of organic carbon speciation in three dimensions, e.g., as made possible via Raman scattering of synchrotron-based hard X-rays. This presentation will discuss these and other innovative organic petrography approaches as applied by the U.S. Geological Survey Thermal Indices project in support of assessment of fossil fuel resources.