New techniques and applications of organic petrography and spectroscopy: insights for shale petroleum systems

Abstract

United States oil production has experienced a sharp increase over the past decade due to technological advances and better understanding of shale oil reservoirs. New techniques and applications of organic petrography and spectroscopy, such as ion milling, SEM, CLEM, iCLEM, micro-FTIR, micro-Raman, CLSM and AFM-IR, that have emerged or re-focused on fine-grained sedimentary rocks have assisted in a better understanding of reservoir properties. For sample preparation, ion milling has circumvented pitfalls inherent to mechanical polishing of soft organic matter, allowing observation of shale porosity characteristics at nanometer scales. Scanning electron microscopy (SEM) of these features has provided ultra-high spatial resolution and revealed new insight into hydrocarbon generation, expulsion, migration and storage processes, although SEM imaging suffers from inability to distinguish or identify organic matter types. Therefore, informed modern organic petrography requires correlative light microscopy (CLEM, correlative light and electron microscopy) or integration of these imaging modalities (integrated CLEM or iCLEM). Applications of infrared and Raman micro-spectroscopy illustrate kerogen conversion processes by examination of in situ compositional and micro-structural changes occurring within individual organic matter types during thermal advance. Micro-Fourier transform infrared spectroscopy (micro-FTIR) reveals loss of heteroatoms, cleavage of aliphatic components and development of aromaticity as oil-prone algal kerogen converts to petroleum. Predictable and systematic trends in the shape and position of 1^st order Raman spectral bands of aromatic carbon suggest this technique has great promise as a thermal proxy, especially with new work in areas of fluorescence suppression and standardization. Application of confocal laser scanning microscopy (CLSM) has allowed non-destructive, high resolution 3-D imaging of in situ sedimentary organic matter and improved our understanding of compositional and microstructural controls on organic fluorescence. Last, atomic force microscopy combined with infrared spectroscopy (AFM-IR) has revealed shale geochemical and geomechanical characteristics at unprecedented resolutions of ~100 nm. Application of these techniques will be discussed in the context of shale petroleum system processes, highlighting areas of new understanding in the evolution of shale organic matter with thermal maturation.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019