Applications of Organic Petrography and Spectroscopy For Shale Petroleum Systems Analysis: Past, Present, and Future
Classic organic petrology studies of petroleum source rocks date from the 1960s‐1970s when practitioners in Germany and France first applied microscopy techniques developed for coal petrology to Toarcian shales of northern Europe. With the advent of horizontal drilling and hydro‐fracturing technologies, shale source rocks in the U.S. became targets for petroleum production and organic petrology applications developed alongside exploitation of these resources. Literature review shows that 10‐15 peer‐reviewed studies that contain organic petrology techniques and methods applied to the geology of unconventional hydrocarbon resources are published each month. New techniques and applications of organic petrography and spectroscopy have provided a better understanding of shale reservoir properties. For example, ion milling has circumvented pitfalls inherent to mechanical polishing of soft organic matter during sample preparation, allowing observation of shale porosity characteristics at micron to nanometer length 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 and electron microscopy (CLEM) or integration of these imaging techniques in a single instrument (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 (kerogen, solid bitumen) during thermal maturation. Micro‐Fourier transform infrared spectroscopy (micro‐FTIR) reveals loss of heteroatoms, cleavage of aliphatic components and development of aromaticity as oil‐prone kerogen converts to petroleum. Predictable and systematic trends in the shape and position of 1st order Raman spectral bands of aromatic carbon suggest this technique has great promise as a future thermal proxy, especially if new work is developed in areas of fluorescence suppression and standardization. Application of confocal laser scanning microscopy (CLSM) allows for non‐destructive, high resolution 3‐D imaging of in situ sedimentary organic matter which has improved our understanding of compositional and microstructural controls on organic fluorescence. Atomic force microscopy combined with infrared spectroscopy (AFM‐IR) reveals shale geochemical and geomechanical characteristics at unprecedented resolutions of ~100 nm. Time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS) can produce ion maps with sub‐micron spatial resolution, showing oil‐prone kerogen to have a dominant ion species of C2H3O2+ at m/z 59, consistent with results from other in situ analyses. Synchrotron approaches such as scanning transmission X‐ray microscopy (STXM) combined with carbon X‐ray absorption near‐edge structure (C‐XANES) spectroscopy can spatially resolve carbon molecular environments at sub‐micron scales and several researchers have applied this in situ technique to determine carbon speciation in shale organic matter. Applications of these petrographic and spectroscopic techniques will be discussed to highlight areas of new understanding in the evolution of shale organic matter with thermal maturation.
AAPG Datapages/Search and Discovery Article #90349 © 2019 AAPG Hedberg Conference, The Evolution of Petroleum Systems Analysis: Changing of the Guard from Late Mature Experts to Peak Generating Staff, Houston, Texas, March 4-6, 2019