Experimental study on the pore structure characteristics of the Upper Ordovician Wufeng Formation shale in the southwest portion of the Sichuan Basin, China

In this study, the pore structure characteristics of shales from the Upper Ordovician Wufeng Formation in the southwest portion of the Sichuan Basin of China were investigated by X-ray diffraction analysis, total organic carbon (TOC) content analysis, thermal maturity analysis and low-pressure N2 adsorption/desorption analysis. The relationships among the TOC, mineralogical compositions, specific surface area and pore volume were discussed. The results show that the organic matter contents of shales were moderate, ranging from 0.621% to 4.52%; the vitrinite reflectance values were within the range of 2.05%–2.16%. The shales mainly contained quartz and clay minerals. The quartz contents ranged from 48.88% to 72.12%, and the clay mineral contents ranged from 16.74% to 38.58%. The specific surface area was between 6.78 m2/g and 24.59 m2/g, and the total pore volume ranged from 0.03742 to 0.07264 cm3/g. The micropore surface area and micropore volume ranged from 0.0 to 7.85 m2/g and from 0.0 to 0.03801 cm3/g, respectively. The organic matter and clay minerals had a positive influence on the specific surface area and pore volume, especially the micropore structure, whereas the quartz had a negative influence. The DFT-PSDs and BJH-PSDs indicated that there were nanometer-scale pores in the shales ranging from smaller than 1 nm to several hundred nanometers that reveal comprehensively nano-pore characteristics. The shales had a multimodal distribution, as indicated by the DFT-PSDs curves, whereas the BJH-PSDs curves suggest that shales may show unimodal distribution with a peak between 60 and 100 nm. The micropores and fine mesopores (pore size between 2 and 6 nm) of shales have a significant contribution to specific surface area, whereas the large mesopores (pore size between 20 and 50 nm) and macropores of shales have a significant contribution to the pore volume.