Fractal characteristics of bulk-mudrock, washed, and kerogen samples of Chang 7 member mudrocks from the Ordos Basin, China

Abstract

We performed low-pressure N2 physisorption experiments to investigate and compare the fractal characteristics of bulk-mudrock, washed, and kerogen samples of Chang 7 mudrocks from the Ordos Basin, China. Their fractal dimensions D were obtained from adsorption data using the Frenkel–Halsey–Hill (FHH) method. The D values for the bulk-mudrock samples were slightly lower than those of the washed samples, while the kerogen samples showed lowest values. The results suggest that the surfaces of the washed samples were rougher than the bulk-mudrock samples and that the kerogen samples had the smoothest surfaces. For the bulk-mudrock and washed samples, D decreased with increasing total organic carbon content, suggesting that organic matter has lower fractal dimension than other fractions in mudrocks. D decreased with increasing free oil, indicating that the fractal dimension can be used to characterize the residual oil content in lacustrine mudrocks approximately. For the bulk-mudrock and washed samples, there are positive trends between fractal dimension and surface area and pore volume. However, similar trends cannot been found in kerogen samples, indicating that the fractal dimensions obtained from low-pressure N2 adsorption experiments cannot describe the fractal characteristics of the kerogen samples well. The fractal dimension decreased with the increasing average pore diameter, which can be attributed to that the samples with higher fractal dimensions contained more small pores and had higher surface area.