Evolution of porosity in kerogen type I during hydrous and anhydrous pyrolysis: Experimental study, mechanistic understanding, and model development

Abstract

In this study, to clarify the role of water during thermal maturation on organic porosity evolution, pore structure characteristics of the Qingshankou shale (type I kerogen) were studied during anhydrous and hydrous pyrolysis (AHP and HP) over wide a range of temperature (300–450 °C). Following each step of thermal maturation, geochemical properties, and pore structure were analyzed with Rock Eval and low-pressure N2 adsorption, respectively. Furthermore, the deconvolution technique was applied to the pore size distribution curves to investigate different pore families and their complexity was assessed using fractal dimension analysis. Next, the volume of N2 adsorbed/desorbed was modeled using several intelligent models. It was found that organic matter displayed Type IV isotherm with hysteresis loops and mesopores stood out as the prevailing type of pores within the samples. Moreover, total surface area and pore volume were found less for AHP samples than the HP samples at all temperatures, which revealed the role of water in the evolution of organic porosity. Pore diameter in AHP samples was larger compared to HP ones, while the diameter of pores increased with the progress of thermal advance in both scenarios. The original sample exhibited five mesopore and two macropore families, while with rising temperature pore families with smaller average pore widths were decreased, and pore families with larger average pore widths were created and expanded. Furthermore, heterogeneity of the pores with maturation was more evident in AHP samples due to the decrease in the number of pores with smaller pore widths. The role of water in the creation of fractures within pyrobitumen was remarkable since it caused a substantial increase in the total pore volume of the organic matter during the gas window. Finally, modeling outcomes showed that cascade forward neural network (CFNN) outperformed other models in predictions of N2 volume adsorbed/desorbed with an average absolute percent relative error (AAPRE) of 2.48 %. It was understood that relative pressure followed by pyrolysis method (AHP or HP) were the most important variables in the estimation of N2 adsorbed/desorbed volumes based on sensitivity analysis. Collectively, the role of water on organic porosity evolution was confirmed based on distinct discrepancies in the two groups of samples pore structure details.