The pore structure and its complexity and heterogeneity control the occurrence states and fluidity of shale oil. The multifractal theory effectively characterizes the complexity and heterogeneity of the shale pore structure. In this study, serial technologies were applied to detect the pore systems of shale obtained from Dongying Sag, Bohai Bay Basin in China. The multifractal characteristics of the pore structure of the shale were analyzed based on its nuclear magnetic resonance (NMR) T 2 spectrum. The analysis results show that shale oil reservoirs can be classified into four types based on their T 2 spectra. Type I shales T 2 spectra show large p2 (1−20 ms), moderate p3 (>20 ms), but little p1 (<1 ms), characterized by large NMR and connected porosity, the lowest BET specific surface area (SSA), and the largest average pore throat diameter and S 1 contents. Large p2, moderate p1, and tiny p3 are the main distinctions of type II shales with the largest NMR porosity, large connected porosity, and BET SSA. The T 2 spectra of type III shales have large p1, moderate p2, and little p3, corresponding to large NMR porosity and BET SSA and the largest total organic content (TOC) and S 1 contents but lower connected porosity. Type IV shales have the most significant contents of micropores with the relatively largest p1 in the T 2 spectra characterized by the lowest NMR and connected porosity, the largest BET SSA, the lowest TOC, and S 1 contents but the largest clay mineral contents. Both types III and IV shales are unfavorable shale oil reservoirs. D q decreases monotonically as q increases, indicating the multifractal nature of shale pore structures. D 0 varies from 0.88 to 1.00 (mean: 0.95), and Δα ranges from 1.24 to 2.82 (mean: 1.79), suggesting complex and heterogeneous pore structures. Types I and II shales have lower D 0 values than types III and IV shales. Thus, type I organic-bearing massive felsic and type II organic-rich layered calcareous shales are favorable for shale oil reservoirs with large pores and large porosity. They have the least complex pore structures among the four shale types considered.

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Heat treatment plays a significant role in determining the petrophysical properties of shale reservoirs; however, the existing studies on the evolution of pore structures are still insufficient. This study conducts a series of tests, including Rock–Eval, low-temperature nitrogen adsorption–desorption, nuclear magnetic resonance (NMR) T 2, and T 1–T 2 tests on samples from Shahejie Formation, Dongying Sag, Bohai Bay Basin. The tests aim to determine the changes in the shale pore structures under increasing heat treatments (ranging from 110 to 500 °C) and identify the factors that control pore structures. The results show that the gradual decomposition of organic matter leads to an eventual decrease in the total organic carbon (TOC) content. The decrease in TOC is more prominent when the temperature exceeds 300 °C. For shales with lower TOC contents (<2%), the Brunauer–Emmett–Teller specific surface area (BET SSA) first decreases, then increases, but eventually decreases again. However, the average pore diameter demonstrates an opposite trend when the temperature increases. In contrast, for organic-rich shales (TOC > 2%), the BET SSA increases at temperatures above 200 °C. The similarity between the D 1 values implies that the complexity and heterogeneity of shale pore surface only undergo minor changes during heat treatment. Porosity shows an increasing trend, and the higher the contents of clay minerals and organic matter in shales are, the greater the change in porosity is. The NMR T 2 spectra suggest that micropores (<0.1 μm) in shales first decrease and then increase, whereas the contents of meso- (0.1–1 μm) and macropores (>1 μm) increase, corresponding to the increase in free shale oil. Moreover, shale pore structures are primarily controlled by clay minerals and organic matter contents during heat treatments, with higher contents resulting in better pore structures. Overall, this study contributes to detailing the shale pore structure characteristics during the in situ conversion process (ICP).

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