Investigation of pore structure and fractal characteristics of organic-rich Yanchang formation shale in central China by nitrogen adsorption/desorption analysis

The Chang-7 of the Upper Triassic Yanchang Formation in the Xiasiwan area, located in the southwest of the Ordos Basin in central China, was investigated for its shale gas potential, which is the continental shale gas reservoir and still under exploration. In this study, sixteen shale samples were collected from four wells in Ordos Basin, and their geochemistry, pore structures and fractal characteristics was investigated by X-ray diffraction (XRD) analysis, total organic carbon (TOC) analysis, low-pressure N2 adsorption/desorption analysis and fractal analysis. The relationships between TOC, mineralogical compositions and pore structure parameters were investigated, and the relationships between TOC, mineralogical compositions and fractal dimensions also were discussed. The results showed that shales have TOC contents ranging from 3.89 to 5.11%, and the major mineralogical compositions of shales are quartz and clay minerals. The clay minerals contents are between 12.37 and 61.55%, the quartz contents are between 17.10 and 72.33%, and there is a negative relationship between TOC and quartz contents in shales. The total pore volume is in the range of 0.001215–0.007495 cm3/g and the specific surface area is between 0.38 and 3.03 m2/g. The total pore volume and specific surface area of shale investigated are much less than those of the marine shales. The organic matter and clay minerals have a positive influence on total pore volume and specific surface area, whereas quartz has an opposite influence. The fractal dimensions are large and the values are between 2.5777 and 2.6280, which increase with the increase of the total pore volume and specific surface area, and the decrease of the average pore size. The fractal dimensions show positive correlations with the contents of TOC and clay minerals, whereas there is negative relationship between the fractal dimensions and quartz contents.