Analysis of the Pore Structures of Shale Using Neutron and X‐Ray Small Angle Scattering

Analysis of the multi‐scale pore structures of shale is of critical importance to understanding their role as caprocks and oil/gas reservoirs, as well as their contribution to global elemental cycles because of weathering. While several techniques have been employed to understand these structures, small angle neutron and X‐ray scattering, which can quantify several features of the pore structure from the nanometer scale to tens of microns, provides a unique perspective not found in more standard petrophysical approaches. Although large‐scale regional studies using this approach are not yet available, over 30 years of work have led to a number of important conclusions. Among the results obtained are data suggesting that multi‐scale pores structures, even of less fissile shales, are highly asymmetric. However, 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 at larger scales. Non‐clay porosity may also be observed, including significant nanoporosity associated with organic matter (OM). 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 R_o (percent vitrinite reflectance), 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 or carbon dioxide absorption or mercury 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 approaches.