A high-precision computed tomography (CT) experimental analysis system was used to perform a non-invasive CT scanning of oil shales (OS) from Daqing and Yan’an counties, Heilongjiang and Shanxi provinces, respectively, both northeastern China, in order to study changes in their internal structure at high temperatures. The formation and pattern of pores and fractures with increasing temperature was examined. Professional CT analysis software was used to statistically calculate porosities. The experimental results indicated that the thermal decomposition of organic matter at high temperatures was a major factor causing changes in the internal structure of Daqing oil shale. When the temperature exceeded 200 °C, large heterogeneous pores were formed in the originally solid material due to the expulsion of the gas and oil generated by thermal pyrolysis of organic matter at high temperature. With a further increase in temperature, the pores expanded and became interconnected by fractures. The heterogeneous thermal expansion was a major factor causing changes in the internal structure of Yan’an oil shale. After the temperature reached 200 °C, numerous fractures parallel to the primary strata were formed, which propagated and expanded with increasing temperatures. In summary, high temperatures changed the compact structure of oil shale into a porous medium with well-developed pores and fractures. As the temperature rose from 20 to 600 °C, the porosity of Daqing oil shale increased from 2.23 to 31.61% (a factor of 14.2), that of Yan’an oil shale increased from 2.70 to 8.86% (a factor of 3.3).
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