--> --> Multiscale Assessment of Pore Structures in Unconventional Gas Shale Systems

2019 AAPG Eastern Section Meeting:
Energy from the Heartland

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Multiscale Assessment of Pore Structures in Unconventional Gas Shale Systems


Unconventional hydrocarbons occurring in economic abundance require greater than industry-standard levels of technology or investment to exploit. Geological formations that host unconventional oil and gas fields and coal bed methane are extraordinarily heterogeneous and exhibit a wide range of physical and chemical features that can vary over many orders of magnitude in length scale. In the context of subsurface geochemical and mineralogical processes relevant to unconventional hydrocarbon deposits we need to consider both the sedimentologic and diagenetic processes that formed the deposits and the present-day nature of fluid and rock properties and the complex interactions among fluids and between solids, fluids and even microbial communities over broad ranges of temperature, pressure, fluid composition, spatial and temporal scales. Of particular importance is nature of the microstructure and evolution of pore space in rocks, which are critically important factors, controlling fluid flow. The size, distribution and connectivity of these confined geometries dictate how fluids migrate into and through these micro- and nano- environments, wet and react with the solid. This presentation will provide an overview on pore types and their relationship with organic matter in a variety of gas shale formations with special emphasis on the Utica/Pt. Pleasant and Eagle Ford. We will highlight results from polarized light microscopy (PLM), X-ray diffraction (XRD), scanning electron microscopy (SEM) with backscattered electron imaging (BSE), dual-beam focused ion beam scanning electron microscope (FIB-SEM), Mercury intrusion porosimetry (MIP), and small- and ultra-small angle neutron scattering (SANS/USANS) that contribute to two key science question areas: (a) What are the size, distribution, connectedness, and contribution to total porosity of nano- to micropores in representative gas shales? (b) How do these pore features vary with the distributions of clay and carbonate matrix and organic matter? Some key ‘global’ observations include: (1) Total porosity exhibiting bimodality may be typical of mudstones, (2) Connected porosity exhibiting bimodal tendencies may not be uncommon in mudstone and seal rocks (3) Fissile mudstones contain far greater abundance of nanopores, (4) Connected porosity commonly mimics the bimodal total porosity trends with connected nanopores observed below about 400 nm and connected micropores between 50 and 100 microns.