Geological Controls on Reservoir Characteristics Influencing Shale Oil Occurrence in Lacustrine Mixed Fine-Grained Sediments of the Paleogene Funing Formation, Subei Basin

Abstract

Unconventional lacustrine shale formations exhibit significant heterogeneity, with substantial variations in vertical shale oil content vertically. However, most research focuses on the reservoir characteristics of shales, analyzing the impact of organic matter and pore structure on shale storage while overlooking the material foundation of shale hydrocarbon generation capacity. This study adopts petrophysical and geochemical experiments to systematically analyze the total organic content (TOC), free hydrocarbon (S1), thermal maturity (Ro), OM type, mineral content, and overall pore system of the second member of the Funing Formation in Qintong Sag, the Subei Basin. Additionally, statistical methods were employed to analyze the data correlations. The results indicate that the TOC content, thermal maturity, mineral composition, and pore network structure significantly influence the enrichment of lacustrine shale oil. Fluorescence observations reveal that shale oil primarily occurs within the interparticle pores, and its migration is influenced by microfractures. The maturity level is low (within the oil window), with organic matter significantly reducing the pore volume and specific surface area while enhancing the pore throat size. As the TOC content increases, the concentration of free hydrocarbons (S1) also rises. Oil occurrence is influenced by both the mineral composition and pore network structure. Clay minerals positively impact the pore volume and specific surface area but negatively affect the pore throat size. The critical threshold for clay minerals is 28%. Free oil initially increases with the clay mineral content but decreases when the clay mineral content surpasses the critical threshold. Carbonate minerals undergo alteration due to organic acids formed during hydrocarbon generation. The critical threshold for carbonate minerals is 30%. Furthermore, the predominant mineral combination was identified. Free oil increases with increasing carbonate mineral content and then gradually declines. When the ratio of clay minerals to siliceous minerals to carbonate minerals ranged from 1:1:1 to 1:2:1, free oil increased with a higher siliceous mineral content. Additionally, based on qualitative and quantitative pore analysis, a microscopic pore network structure model of reservoirs was developed. It was elucidated that a network structure comprising brittle minerals and mixed pore types facilitates the storage and migration of shale oil.