Controlling Factors and Evolution of Oil Shale Quality in the Upper Cretaceous, Songliao Basin: Implications from Thermal Simulations

Algae-rich oil shales have great hydrocarbon potential, and the hydrocarbon generation processes of different quality oil shales are important for shale oil resource exploration. This study comparatively analyzed the hydrocarbon generation processes of the general- and fair-quality oil shales from the first member of the Qingshankou formation (K(2)qn(1)) and the first member of the Nenjiang formation (K(2)qn(1)), separately. Organic petrology and X-ray diffraction analyses revealed that the fair-quality oil shale contained more telalginite and lamalginite and had a lower clay mineral content than the general-quality oil shale. Thermal simulations revealed that the fair-quality oil shale underwent an earlier oil generation peak and exhibited higher oil yields than the general-quality oil shale, implying that the hydrocarbon generation processes were sequential. After a thermal maturity (Ro) of 1.05% was reached, the fair-quality oil shale generated plentiful hydrocarbons, whereby the contribution of illite content rapidly increased from the illite/smectite layer. Analysis suggested that fair-quality oil shales can generate large volumes of hydrocarbons and organic acids under the effect of temperature due to the presence of bulk telalginites, and these organic acids in turn promote the transformation of smectite into illite. This greatly increased the catalytic activity of smectite and thus the rate of hydrocarbon generation. Finally, it was determined that the synergistic effects of organic matter and clay minerals contributed to the rapid hydrocarbon generation of the fair-quality oil shale, implying that fair-quality oil shales that are rich in telalginites have value as sweet spot layers for early shale oil exploration. This is important for the development of shale oil resources.