Microfabrics Related to Porosity Development, Sedimentary and Diagenetic Processes, and Composition of Unconventional Reservoir

Abstract

This presentation illustrates and describes features of shales and mudstones observed at the nano-to micro-meter scale from freshly broken surfaces using standard scanning electronmicroscopy (SEM) and field emission scanning electron microscopy (FESEM) techniques. Microfabric observations at this scale provide insights into not only sedimentary and post-depositional processes, but also offer evidence useful in understanding hydrocarbon storage and primary migration patterns in unconventional shale reservoirs. Examples are provided from a diverse set of shale reservoirs (Barnett, Woodford, Eagle Ford, Fayetteville, Marcellus, La Luna, and Longmaxi). Microfabric and pore features illustrated include open-network floccules, clay-aligned fabric, and features associated with non-clay minerals. Types of organic matter include those produced by zooplankton and algae (e.g. coccolithophores, Tasmanites), and which may bind clay flakes into biosediment aggregates. Organic matter is also common within fecal pellets. Coccolithophores and sponge spicules may contain hollow internal chambers which provide porosity and probably permeability. We conclude that conventional scanning electron microscopy provides a rapid and relatively inexpensive way of evaluating pores and microfabrics in unconventional shale reservoirs.