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In this work, Huadian oil shale was extracted by subcritical water at 300 °C over different time periods to better characterize the underground mining of oil shale in situ. The results revealed that the kerogen in the oil shale mostly transformed into bitumen through extraction over a long time period by subcritical water at 300 °C; however, a portion of the bitumen remained in the shale matrix. The yields of both bitumen 1 (the bitumen extracted by subcritical water) and bitumen 2 (the bitumen remaining in the shale matrix) reached maximums at approximately 250 h. It should take a long time for bitumen 2 to be released because the ability of the transporting substrates of subcritical water was insufficient, the solubility of bitumen 2 was poor, and tiny fractures were generated in the oil shale matrix at such a low temperature. The gas chromatography–mass spectrometry analysis showed that the major components of bitumen 1 and bitumen 2 were similar and consisted of n-alkanes, n-alkanoic acids, n-alk-2-ones, and isoprenoid alkane. Initially, kerogen decomposition produced a large number of n-alkanes with low molecular weights; however, as the reaction continued, comparatively higher-molecular-weight n-alkanes were obtained more. In addition, the bitumen underwent a secondary cracking in the subcritical water, resulting in its decreased yield over time, whereas the contents of gaseous C2–C6 hydrocarbons increased. The organic matter dissolved in the spent aqueous solution consisted of mainly paraffins, isoparaffins, cyclohexanone derivatives, and phenolic derivatives. The analysis of the oil shale residue showed that minerals were less reactive in the subcritical water except for feldspar and calcite and that mesopores developed in the oil shale with prolongated extraction time, as more bitumen diffused out and into the subcritical water at 300 °C.

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