The evolution of pore structure heterogeneity during thermal maturation in lacustrine shale pyrolysis

Abstract

Although pore structure variations during thermal maturation have been extensively discussed using natural or artificially matured shale samples, related investigations into pore structure heterogeneity and its evolution remain scarce, posing challenges in fine reservoir evaluation and exploration target prioritization. The variation of pore structure heterogeneity during thermal maturation was determined in this study by integrating multifractal theory with nitrogen adsorption-desorption tests using the naturally immature sample and the artificially matured samples, and the governing factors were ascertained. The increasing thermal maturity results in organic matter transformation, hydrocarbon generation, retention and expulsion, and various types of pore formation, driving the variation of pore structure heterogeneity. Pore structure heterogeneity, quantified by the breadth of singular spectra, was chiefly impacted by the pore volume difference between diverse types of pores. Macropores make a main contribution to pore structure heterogeneity due to their higher percentage of total pore volume compared to mesopores and micropores. Through a comparison of pore structure heterogeneity in shale samples with different thermal maturity, the impact of thermal maturity on shales was clarified. At immature to low-mature stages, nonthermal maturity factors make a main contribution to the inherent pore structure of different shales and cause an ambiguous relationship between pore structure heterogeneity and thermal maturity. At mature to high-mature stages, pore structure becomes more heterogeneous with increasing thermal maturity, resulting from newly formed organic pores or dissolution pores and the progressive generation and expulsion of liquid and gaseous hydrocarbons. At the over-mature stage, pore structure heterogeneity decreases and then fluctuates on a small scale, related to the collapse of mesopores and macropores caused by compaction and the predominance of newly formed micropores.