Organic matter evolution in pyrolysis experiments of oil shale under high pressure: Guidance for in situ conversion of oil shale in the Songliao Basin

•The pyrolysis process of oil shale can be divided into three stages.•Kerogen matures gradually with the increase of temperature and tends to produce low molecular weight hydrocarbons.•The underground heating temperature must be higher than 350 °C.•425-450 °C is the best underground heating temperature for oil shale in-situ mining engineering. To examine the evolution of shale oil and gas in different heating stages during in situ conversion in oil shale under high-pressure and high-temperature conditions and the hydrocarbon generation potential change in each reaction stage for application in theoretical guidance purposes, core samples are collected from an oil shale in situ mining area. Oil shale pyrolysis simulation tests are performed under the in situ bottom hole seepage pressure with a heating and insulation technique, and geochemical tests of the oil shale semi-coke and shale oil and gas products are conducted. The results show that the pyrolysis products form in three stages. At 300 ℃, water is mainly produced. From 300–475 ℃, shale oil and gas are the main products discharged from the oil shale, and the production greatly increases. From 475–520 ℃, oil and water production only slightly increases. Methane is the main hydrocarbon gas produced, and the methane percentage in the samples above 450 ℃ is higher than 90 %. Part of the shale oil produced by oil shale pyrolysis remains in the oil shale pores, while some is discharged from the oil shale pores and fractures. The proportions of the non-hydrocarbon components (nitrogen, sulphur and oxygen (NSO)) and asphaltenes in the shale oil remaining in oil shale exceed those of the saturated and aromatic hydrocarbons and reach a peak at 350 ℃. In the later stage of the kerogen transformation process, shale oil is rapidly generated and discharged, and the saturated hydrocarbon content in the discharged shale oil is the highest. Non-hydrocarbon components and asphaltenes may be intermediate products in the transformation process of kerogen into shale oil and may block oil shale pores at approximately 350 ℃. However, with increasing heating temperature during pyrolysis, pores reopen and expand, generating small-molecule hydrocarbons. Oil shale sample pyrolysis at low temperatures attains a good hydrocarbon generation potential, and 425–450 ℃ represents a turning point. Oil shale pyrolysis at temperatures above 450 ℃ exhibits a low hydrocarbon generation potential. This temperature threshold is very important