Understanding pore evolution in a lacustrine calcareous shale reservoir in the oil window by pyrolyzing artificial samples in a semi-closed system

Calcareous shale is a favorable target for shale oil exploration, because shale oil moves more easily in calcareous shale than in clay-rich shale and the high carbonate content is beneficial to shale fracturing. Although shale oil exploration has been undertaken successfully in marine carbonate-rich shale (e.g., the Eagle Ford shale) in the USA, no major breakthrough has yet been made considering lacustrine calcareous shale. The lack of understanding pore evolution in lacustrine calcareous shale increases exploration risk. Thus, an integrated analysis (Rock-Eval pyrolysis, X-ray diffraction (XRD1), low-pressure gas adsorption, field emission scanning electron microscopy (FE-SEM2), energy dispersive X-ray spectroscopy (EDS3), and helium pycnometry) of lacustrine calcareous shale from the Lucaogou Formation in the Santanghu Basin and of pyrolyzed shale was undertaken to document the hydrocarbon generation and expulsion, oil bearing capacity, and evolution of pores in the oil window. The thermal stress was calculated as the vitrinite reflectance equivalent (%Re-4) based on the Easy%Ro method. The experimental results showed that the residual oil content and oil saturation index (OSI5) increased with increasing maturity at 0.6-1.29 %Re. The clogging of pores by generated oil and intense compaction caused a decrease in the volume of mesopores and macropores at 0.79-1.2 %Re, while abundant organic matter (OM6) pores formed due to hydrocarbon generation and expulsion at 1.29 %Re. Micron-scale dissolution pores and shrinkage OM pores contributed considerably to the total porosity, which improved the performance of the lacustrine calcareous shale reservoir at 1.0-1.29 %Re. Accordingly, the maturity range of 1.2-1.29 %Re is favorable for shale oil exploration in lacustrine calcareous shale due to the high OSI and the widely developed dissolution pores and shrinkage OM pores.

Automated summary

Calcareous shale is a promising target for shale oil exploration due to its favorable properties for the movement of shale oil and shale fracturing. However, lack of understanding of pore evolution in lacustrine calcareous shale poses exploration risks. Experimental results indicate that the maturity range of 1.2-1.29 %Re is highly favorable for shale oil exploration in lacustrine calcareous shale.