Shale composition and pore structure variations in the progradation direction: A case study of transitional shales in the Xu-Huai district, southern North China

This study investigated the effects of the composition of gas-bearing shale on pore structures and various properties of Paleozoic-Permian transitional (marine-continental) mudstones and shales from the Xu-Huai district, southern North China, in the direction of progradation. The total organic content (TOC) in the Shanxi formation (SXF), Upper Shihezi formation (USF) and Lower Shihezi formation (LSF) shale samples varied widely, as determined via Rock-Eval pyrolysis. The USF shale TOC values (0.1-3.5%) are markedly lower than those of the SXF (0.3-6.3%) and LSF (0.1-8.6%). The SXF shales exhibit smaller BET (Brunauer-Emmett-Teller) specific surface areas (SSAs; 5.45-16.27 m2/g) compared to those of the LSF (6.36-27.97 m2/g) and USF shales (5.45-28.47 m2/g). The clay and quartz contents exhibit a significant negative correlation. The pore volume and N2-BET SSA increase with increasing clay content in the USF and LSF shales. The average pore size ranges from 2 to 20 nm, generally decreasing with decreasing quartz content. The composition, pore structure, and methane adsorption capacity were investigated by averaging the properties from each sample site to determine whether the variations are influenced by transitions along the direction of progradation. The results reveal that the clay content, TOC, SSA and pore volume increased in the progradation direction, whereas the quartz and average pore size decreased. The correlation between mineral composition and pore properties indicates that quartz has a greater effect on mesopores and macropores than on micropores; in contrast, clay generally controls micropores. The methane sorption capacity is correlated with TOC, exhibiting a prominent linear relationship. The increasing sorption volume in the progradation direction may be attributed to increases in SSA and pore volume related to the mineral composition.