An investigation of the fractal characteristics of the Upper Ordovician Wufeng Formation shale using nitrogen adsorption analysis

Shales from the Upper Ordovician Wufeng Formation in the southwest of the Sichuan Basin of China were investigated in recent years because of its shale gas potential. In this work, eight shale samples were collected from the Wulong County of Chongqing City in the southwest of the Sichuan Basin of China. The samples' geochemistry and pore structure were studied using XRD (X-ray diffraction) analysis, TOC (total organic carbon) analysis and low-pressure N-2 adsorption analysis. Based on the low-pressure N-2 adsorption analysis, the fractal characteristics of the pore structure in the shales were investigated using the fractal FHH (Frenkel, Halsey, Hill) equation from N-2 adsorption data. The relationships between the specific surface area, the pore volume, the average pore size and the fractal dimension were discussed, and the relationships between the TOC, the mineralogical compositions and the fractal dimension were also discussed. The results indicate that the shales have high TOC contents, varying from 1.326% to 3.52%, and the major mineralogical compositions of the shales are quartz and clay minerals; the quartz contents are between 52.88% and 75.53%, and the clay minerals contents are between 12.74% and 34.58%. The specific surface area is between 7.68 m(2)/g and 24.76 m(2)/g, the total pore volume is between 0.04042 cm(3)/g and 0.07546 cm(3)/g and the average pore size is in the range of 11.01-21.05 nm. The fractal dimension D obtained by the expression 'D = 3 + K' provides a more realistic result than the fractal dimension D obtained by the expression 'D = 3 + 3K'. The fractal dimension D is between 2.4982 and 2.6359, indicating that the shales investigated have fractal characteristic. The fractal dimension exhibits positive correlations with the contents of TOC and clay minerals, whereas there is negative relationship between the fractal dimension and the quartz contents. The fractal dimension increases with increasing values of the specific surface area and the total pore volume and with decreasing value of the average pore size. The micropores in the WF shales have a stronger impact on the fractal dimension. (C) 2015 Elsevier B.V. All rights reserved.