Shale Pore Network Characterization Across Thermal Maturity: A Comparison of He Pycnometry, Gas Adsorption, and Mercury Intrusion Porosimetry

Multiple laboratory techniques are applied among laboratories to quantify the porosity network in shales for the purpose of estimating hydrocarbon reserves or quantifying CO2 storage. Because a wide range of pore sizes exists, there is no single, universal method to assess properties of the pore network, and each of the applied methods offers advantages but also carries shortcomings. Obtained porosity characteristics often differ between methods, suggesting factors not related to the rock sample but resulting from the specifics of the techniques. The porosity network of 39 Silurian shale samples from the Baltic Basin was characterized by helium pycnometry, gas adsorption, and mercury intrusion porosimetry. The samples were selected to represent a wide range of thermal maturity, from immature samples that underwent relatively little overburden compaction, through oil and gas windows, to overmature samples, which underwent substantial mechanical and chemical compaction. Comparing the results between the applied methods reveals the influence of factors unrelated to porosity. One of them is analytical particle size used in different techniques. Crushing opens up isolated pore network patches that represent occluded porosity in larger particles, thus resulting in overestimating pore volume with decreasing particle size. This effect is less pronounced in high-porosity and low-maturity samples and increases in significance with decreasing porosity and increasing thermal maturity. Mercury intrusion porosimetry can lead to sample structural deformation, overestimating porosity, and affecting pore size distribution. The deformation is more likely to occur in lower-maturity samples, with the likelihood of deformation decreasing with increasing mechanical stiffness and thermal maturity.