Abstract Shale rocks are highly structurally and chemically heterogeneous, such that the pore structure–transport relationship is complex. Shales typically have porosity over many length-scales from the molecular up to macroscopic fractures. This work utilizes gas overcondensation to probe pore sizes from micropores to very large macropores all in the same experiment without the potential for damage due to high pressures when conducting mercury porosimetry. Indeed, the Bowland Shale samples studied here are generally inaccessible to mercury intrusion. The gas overcondensation method can also be augmented using scanning loops to assess the spatial juxtaposition of very different pore sizes, and this has been used to determine that some large macropores are shielded by pore necks less than 4 nm in size in the Bowland Shale. In addition, the adsorption calorimetry method has been used to assess the accessibility of the void space. It has been found that mass transport is limited by particular ‘hour-glass’-like pore necks that fill at quite low saturation, and thus present a barrier to molecular migration. The shielding of macroporosity by narrow necks was particularly significant for the Above Marine Band sample, with lower shielding observed in the Marine Band and Below Marine Band materials.

ABSTRACT The Monterey Formation and related formations in California have long been the subject of field and laboratory studies on silica diagenesis. Biogenic or amorphous silica (opal-A) alters to a more-ordered opal-CT and eventually to the crystalline end member, quartz, with increasing burial depth and temperature. Low-pressure nitrogen sorption serves as an indicator of silica alteration by detecting the nanometer-scale pore structures associated with opal-CT while excluding contributions from larger pores. To apply this method, calibrations with known compositions are not required, sample preparation and measurements are straightforward, hazardous waste is not generated (as with mercury porosimetry), and subtle changes in silica phase are readily detected. Nitrogen desorption isotherms, collected on mini cores (~0.8 cm diameter × 1 cm) after outgassing at 50 °C and processed using the Barrett-Joyner-Halenda method, provide nanometer-scale pore throat size distributions (nPSD), pore volumes (nPV), and surface areas (nSA). A scatter plot of nPV and nSA reveals two distinct trends. Samples with more nSA per unit volume contain opal-CT, either in transition from opal-A or completely converted. The other nSA trend consists of opal-A and quartz samples in the small nSA and nPV range, whereas samples with small nSA and large nPV also contain opal-CT and are in transition to quartz. These distinct trends are also apparent in the nPSD. Samples with more nSA exhibit a peak between 4 and 10 nm, whereas samples with less nSA have a broad peak between 10 and 100 nm if they contain opal-CT. Images collected via scanning electron microscopy reveal that opal-CT morphologies account for these differences.