Pore Properties in Organic-Rich Shales Derived Using Multiple Fractal Determination Models Applied to Two Indian Permian Basins

Abstract

The pore geometry of shale reservoirs plays an important role in the storage and transportation of petroleum in these unconventional resources. Intrinsic surface roughness, indicated by fractal dimensions and contrasting pore size distributions of shales are primary factors that influence shale’s fluid flow and resource volume characteristics. The Frenkel–Halsey–Hill (FHH) derived fractal dimensions are widely applied for evaluating the pore structural complexities of shales. In this work, for a suite of shales samples collected from two Permian basins, India, with distinct maturities, low pressure nitrogen gas adsorption–desorption has been employed to elucidate the pore structural framework. Three alternative fractal calculation methods (FHH, Neimark, and Wang–Li) are compared to provide further insight into the fractal characteristics of the shales. Ambiguities are revealed in the selection of the most appropriate fractal values. Fractal dimensions calculated by the FHH and Wang–Li methods are in relatively close agreement (ranging between 2.5 and 2.8) and the small systematic differences between their values can be explained in terms of fitting errors and assumptions associated with both methods. However, the Neimark fractal values are unreasonable (>3 for all the samples). Thermal maturity is a key controlling feature of the pore structural facets and fractal dimensions of the Raniganj basin samples (Tmax = 431–463 °C) but not for North Karanpura (Tmax = 434–442 °C) samples. Strong correlations exist between calculated fractal dimensions and specific surface area, pore volume, and average pore radius for all samples. However, systematic differences in these values between the samples from the two basins are most likely explained in terms of their mineralogical differences, specifically the higher kaolinite content of the Raniganj samples.