Shale oil occurrence and mobility mechanisms in brittle mineral-enriched reservoirs: Insights from the Paleogene Kongdian Formation, Bohai Bay Basin

Abstract

Understanding shale oil occurrence and mobility is essential for evaluating resource potential and optimizing exploration in lacustrine shale systems. This study investigates the Paleogene Kongdian Formation in the Bohai Bay Basin, integrating organic geochemistry, mineralogical analysis, scanning electron microscopy, solvent extraction, multi-step Rock-Eval pyrolysis, and 2-D NMR to characterize shale oil occurrence states and mobility mechanisms in brittle mineral-enriched reservoirs. Results indicate that shale oil mainly occurs in free and adsorbed states within interparticle pores, dissolution pores, microfractures, and organic pores, with most retained oil hosted in nanopores smaller than 200 nm. Quantitative analyses show that siliceous and calcareous shales possess higher movable oil ratios than clay-rich counterparts, primarily due to their rigid mineral frameworks that resist compaction and preserve interparticle and intragranular pores. These brittle-rich lithofacies exhibit broader pore size distributions, enhanced connectivity, and reduced adsorption affinity, facilitating more efficient oil accumulation and displacement. In contrast, micropore-dominated, clay-rich shales exhibit strong adsorption and limited pore continuity, which hinder hydrocarbon mobility. Appropriate TOC levels (2.0–4.5 wt.%) favor shale oil mobility, while excessive organic content increases adsorption, reducing the proportion of free oil.

Among various geological factors, brittle mineral content and thermal maturity play the dominant roles in controlling shale oil mobility, as they fundamentally shape pore structure and fluid properties. In combination with organic matter abundance and sedimentary features, these factors jointly govern pore network evolution and hydrocarbon occurrence states, thereby determining shale oil enrichment and extractability. These findings enhance the understanding of shale oil enrichment processes and provide a scientific basis for identifying sweet spots and optimizing development strategies in lacustrine shale reservoirs.