Nanopore structure characterization of Velkerri Shale: SANS investigation of pore accessibility and anisotropy

• Shale of different thermal maturity from the oldest proven source rock. • CM-SANS reveals pore size distribution, anisotropy, and accessibility of shale. • Pore accessibility ranges 0.1–0.7, with pores of 15–50 nm being most accessible. • The study reveals gas confinement effects and procedural artifacts in nanopores. • Organic-rich samples exhibit greater pore anisotropy. Understanding shale pore structure from the nano to micro scale is crucial for optimizing natural gas and geothermal energy recovery, as well as for potential applications in CO 2 sequestration and subsurface hydrogen storage. This study employs (Ultra-) Small-Angle Neutron Scattering ((U)SANS) with contrast matching (CM) techniques using deuterated methane (CD 4 ) to characterize the pore size distribution, anisotropy, and accessibility in Velkerri Formation shales (Northern Territory, Australia). Results indicate that higher thermal maturity and Total Organic Carbon (TOC) content are associated with lower mass fractal dimensions of the less complex pore-rock matrix interface. Accessible pore fractions range from 0.1 to 0.7, with greater accessibility observed in thermally mature samples, while 15 and 50 nm pores exhibit the highest accessibility. Large-Q SANS background is primarily influenced by inherent CD 4 scattering (coherent and incoherent) and potential CM-CD 4 /cell-windows interfacial scattering, with additional contributions from confined CD 4 in smaller nanopores. Estimated CD 4 density in ∼ 2 nm pores of sample 06 V is 1.08–1.17 g/cm 3 . These large values suggest the presence of additional unaccounted factors contributing to the large-Q background. The study reveals significant pore anisotropy, with pores preferentially oriented along the bedding plane. Eccentricity values suggests that pores are approximately 25 % to 50 % longer in bedding compared to the out-of-bedding direction. In post-mature samples, total porosity increases with thermal maturity and positively correlates with TOC content. The majority of pore volume is concentrated in pores smaller than 10 nm. These findings provide valuable insights into the pore structure and fluid transport properties of shale formations.