In order to study the coupled effect of the temperature and pressure on pyrolysis characteristics and pore and fracture structures of oil shale, a total of 25 groups of pyrolytic reaction experiments have been conducted on 14 mm long and 7 mm in diameter cylindrical oil shale specimens under different temperature and pressure conditions ranging from 20 to 600 °C and 0.1–15 MPa. Further, both X-ray microcomputed tomography (μCT) and mercury intrusion porosimetry (MIP) have been used to comprehensively investigate the network structure, interconnectivity, and evolution of pore and fractures. The results show that the temperature significantly affects the pyrolysis characteristics of oil shale. With rising temperature, both the mass loss and the porosity increase gradually, the number and the maximum aperture of fractures also increase, and the pyrolytic degree intensifies progressively. The increase is most significant from 300 to 500 °C. The maximum mass loss ratio is 20.84%, the largest porosity is 13.52 times larger than that under the room temperature, and the total number and the maximum aperture of the fractures are 813 and 0.383 mm, respectively. Moreover, the pressure has a significant effect on the pore and fracture structures of oil shale. As the pressure increases, both the pore volume and the fracture distributions first decreased and then increased. With the continuous increase of pressure, the porosity and the total number of fractures reach a maximum at the pressure of 15 MPa. Under the coupled effect of temperature and pressure, with both the temperature and pressure increasing, the pores and fractures in the oil shale specimens developed increasingly. Furthermore, using the μCT scan technology, the distribution laws and the connectivity characteristics of the pores and fractures have been investigated. The connected fractures appear when the temperature reaches 300 °C and further extend along the bedding plane or pass through it at 600 °C.

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Investigation of the porosity variation of oil shale during the pyrolysis process is of great importance in understanding the migration mechanism of pyrolysis products. Using the Fushun oil shale of China as an example and taking into account the fact that the porosity change of oil shale arises from pyrolysis of kerogen at high temperature, in this paper, the thermal curve of oil shale was employed to obtain its pyrolysis rate equation. A quantitative model of oil shale porosity during pyrolysis was constructed. The porosity of oil shale under different pyrolysis conditions was determined using the conventional method, computed tomography (CT) and the mercury intrusion method. The calculated results were in good agreement with the experimental data.