Evolution of fractal characteristics in shales with increasing thermal maturity: Evidence from neutron scattering, N2 physisorption, and FE-SEM imaging

Abstract

Fractal dimension analysis has become an essential tool to assess the pore structure of various porous geomaterials including shale rock. In this regard, changes of fractal dimension during the thermal maturation of organic rich shale could represent the evolution of its pore structure. To this end, complementary tests were conducted on six organic rich shale samples that are in a wide range of thermal maturity (equivalent Ro values of 0.52–3.53 %). Fractal dimensions were obtained from small-angle neutron scattering (SANS), N2 physisorption, and field emission-scanning electron microscopy (FE-SEM) data. Herein, surface fractal dimensions were calculated from the N2 adsorption isotherms using the Frenkel–Halsey–Hill method, which was found between 2.46 and 2.71. Power-law exponents (E) of the SANS profiles varied from 2.75 to 3.48, indicating that both surface fractals and mass fractals would characterize the shale samples. As thermal maturity progressed, the fractal dimension shifted from surface to mass fractal while further analyses suggested this was caused by changes in the number density of pores (NDP) of various pore-size ranges. Ultimately, the pore size of the samples during such transition decreased with the increase of Ro.