Analysis of the multiscale pore structures of clay-rich earth materials such as shale is of critical importance to understanding their role as seals, hosts for gas and oil, and their contribution to global elemental cycles as a consequence of weathering. 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. While a number of techniques have been employed to understand these structures, small angle neutron and X-ray scattering, which can quantify a number of features of the pore structure from the nanometer scale to tens of microns, provide a unique perspective not found in more standard petrophysical approaches. Although large-scale regional studies using these methods are not yet available, over 30 years of work have led to a number of important conclusions. Amongst the results obtained are data suggesting that multiscale pores structures, even of less, fissile shales, are highly asymmetric, but that this is both scale- and maturity-dependent. Pore surfaces commonly exhibit highly rough surface fractals, which often smooth with diagenetic alteration and weathering. Mass-fractal behavior of larger-scale interpore structures has also been observed. In many cases pore structures are bimodal, reflecting interlayer spacing in clays at the smaller scales, and larger spacing between clay packets or between clay folia and detrital grains. Non-clay porosity may also be observed, including significant nanoporosity associated with organic matter. Data obtained as a function of thermal maturity suggest that, while the overall porosity does decrease with increasing maturity, the change is not linear with Ro, but occurs primarily in the early stages of maturation. Pore size or volume distributions obtained from scattering experiments often differ considerably from those obtained from nitrogen absorption or mercury intrusion capillary intrusion porosimetry, which may reflect the overall accessibility of various pore sizes to external fluids. Despite these results, however, a significant amount of work, and a number of basic questions remain to be answered that can be addressed either uniquely by scattering methods, or in a complementary fashion using these and other microscopic imaging approaches.<br/>

Nanometer to centimeter scale analysis and modeling of pore structures in Earth materials

253rd National Meeting (2017)

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