Microscopic Occurrence Mechanism of Shale Oil in Saline Lacustrine Shale: Insights from NMR and Micro-CT Combined with Saturated Oil, Centrifuged and Solvent Extraction Experiments

Abstract

The occurrence mechanisms of shale oil in saline lacustrine basins are complex, and accurately quantifying the state of occurrence and assessing mobility present significant challenges, impeding deeper insights into the enrichment processes of shale oil. To comprehensively elucidate the occurrence mechanisms of continental saline lacustrine basin shale oil, and to clearly define the advantageous lithofacies and pore types under stratigraphic conditions, a range of experimental techniques including field emission scanning electron microscope, X-ray diffraction, multi-interval stepped pyrolysis, nuclear magnetic resonance, and micro-computer tomography scanning were utilized. These were integrated with saturated oil, centrifugation, and organic extraction experiments to systematically analyze the occurrence mechanisms and their influencing factors in the Lucaogou Formation. The findings were as follows: (1) Influenced by terrestrial inputs, fluctuations in lake levels, and saline water bodies, high-organic laminated feldspathic lithic fine sandstone, high-organic massive arenaceous dolomite, and high-organic laminated dolomitic siltstone form the dominant lithofacies of “sweet spots” in the Lucaogou Formation, primarily featuring inter-crystalline pores–dissolution pores and residual inter-granular pores–dissolution pores types as the main reservoir spaces. (2) Shale oil reservoirs display “full-scale oil-bearing” characteristics, with adsorbed oil predominantly located in micropores and mesopores, peaking at approximately 10 nm, while free oil is chiefly found in mesopores and macropores, with a general peak diameter around 100 nm. (3) Free and adsorbed oils exhibit differing flow properties across various mineral composition pore structures. Isolated and enclosed pores, having accumulated substantial quantities of shale oil, are crucial for future recovery rate improvements. (4) Based on the controlling factors of shale oil occurrence, a mechanism of hydrocarbon generation and accumulation model matching the upper and lower “sweet spots” mineral compositions, reservoir space types and sizes, and thermal maturation hydrocarbon generation effects with shale oil occurrence was established. These results enhance the understanding of the micro-scale occurrence characteristics and control mechanisms of shale oil in saline lacustrine basins, offering new insights into the differential enrichment mechanisms.