Petroleum Generation, Retention, And Expulsion In Lacustrine Shales Using An Artificial Thermal Maturation Approach: Implications For The In-Situ Conversion Of Shale Oil

Organic-rich lacustrine shales with medium-low maturity in China have considerable potential to produce oil with the aid of in-situ underground conversion technology. Two shale samples from the Qingshankou Formation in the Songliao Basin and the Chang 7 Member in the Ordos Basin were selected for semi-closed pyrolysis, and the pyrolysis products and solid residues were subjected to detailed organic geochemical analysis and focused ion beam scanning electron microscopy (FIB-SEM) analyses to elucidate the evolution of organic matter; to quantitatively and qualitatively characterize the generation, retention, and expulsion of hydrocarbons; and to explore the required optimal temperature ranges for the two shales during in situ heating. The results showed that (a) the labile organic matter in the samples was progressively converted into hydrocarbons and (b) the cumulative yield of the retained oil continuously decreased as the temperature increased, the expelled oil reached its peak value at 400-450 degrees C, and oil cracking into gas occurred above 350 degrees C. The characteristics of the oil compositions indicated that hydrocarbons are directly decomposed from kerogen in the studied lacustrine shale samples, instead of being formed by bitumen cracking. Oil retention in the artificially mature samples is associated with organic richness, and large amounts of oil retained in samples can lead to a strong charging effect in FIB-SEM images, which is expressed as strong brightness. Mass balance calculations demonstrated that the maximum values of the oil expulsion efficiency are 82.59% for the Chang 7 shale sample and 96.27% for the Qingshankou shale sample at 350 degrees C. Meanwhile, substantial light hydrocarbons are produced and expelled in the pyrolysis of the latter sample. During an in-situ conversion process, the optimal temperature ranges are suggested to be 400-450 degrees C for the Chang 7 shale and 350-425 degrees C for the Qjngshankou shale. In these ranges, most labile organic matter can be converted into hydrocarbons, hydrocarbon expulsion efficiency is higher, and oil is characterized by appropriate mobility as well as a relatively lower secondary cracking degree.