This study aims to reveal the differential oil enrichment mechanisms of tight(low-permeability)sandstone reservoirs in the Paleogene Shahejie Formation in the Linnan Sub-sag, Bohai Bay Basin. Initially, we categorize the source rock-reservoir assemblages in the Linnan Sub-sag based on their spatial distribution and lithologic combination. The analysis on oil-bearing properties, hydrocarbon supply, reservoir storage spaces, conduit system, and migration and accumulation dynamics of various source rock-reservoir assemblages is carried out using an integration of data of logging, well tests, production tests and core analysis and tests. Accordingly, the differential oil enrichment model is established for tight (low-permeability) sandstone reservoirs. The results indicate that the source rock-reservoir assemblages in the study area can be categorized into three types, which can be further divided into six subtypes: the source-reservoir coexistence type, including interbedded and intercalated sub-types; the source-reservoir adjoining type, comprising three distinct subtypes with reservoirs located above, between, or below source rocks; and the sourcereservoir separation type, including a subtype with reservoirs located below source rocks. These types correspond to three oil enrichment model of the tight(low-permeability) sandstone reservoirs. The source-reservoir coexistence type exhibits an oil enrichment model featuring “a strong hydrocarbon supply, strong migration and accumulation dynamics,efficient charging, and reservoir-controlled oil enrichment.” Specifically, this type boasts the optimal hydrocarbon supply conditions, the strongest migration and accumulation dynamics, and efficient hydrocarbon charging via pores and fractures, all of which contribute to the most favorable oil-bearing properties. Compared to that of the intercalated subtype, the hydrocarbon enrichment scale of the interbedded subtype is restricted by sand-body thicknesses. The source-reservoir adjoining type manifests an oil enrichment model characterized by a comparatively strong hydrocarbon supply, differential migration and accumulation dynamics, combined conduit systems, and multiple factor-controlled oil enrichment. In detail, this type features a comparatively favorable hydrocarbon supply, significant changes in the migration and accumulation dynamics, and combined conduit systems consisting of pores, fractures, faults, and sand bodies, with hydrocarbon preferentially charging reservoirs with favorable physical properties and pore structures. This type of source rock-reservoir assemblage exhibits comparatively favorable oil-bearing properties. Among others, its subtype with reservoirs located between source rocks outperforms the other two subtypes in terms of both hydrocarbon supply and migration and accumulation dynamics, thus demonstrating the optimum oil-bearing properties. The sourcereservoir separation type displays a pattern characterized by a weak hydrocarbon supply, weak migration and accumulation dynamics, conduit systems including faults and sand bodies, and oil enrichment under the control of conduits and reservoirs. Due to the weak hydrocarbon supply and migration and accumulation dynamics, effective transport pathways composed of faults and sand bodies, along with the presence of high-quality reservoirs, are crucial to hydrocarbon enrichment. This type of source rock-reservoir assemblage generally exhibits inferior oil-bearing properties.

The organic-rich mudstones and dolostones of the Permian Fengcheng Formation (Fm.) are typically alkaline lacustrine source rocks, which are typified by impressively abundant β-carotane. Abundant β-carotane has been well acknowledged as an effective indicator of biological sources or depositional environments. However, the specific biological sources of β-carotane and the coupling control of biological sources and environmental factors on the enrichment of β-carotane in the Fengcheng Fm. remains obscure. Based on a comprehensive investigation of the bulk, molecular geochemistry, and organic petrology of sedimentary rocks and the biochemistry of phytoplankton in modern alkaline lakes, we proposed a new understanding of the biological precursors of β-carotane and elucidated the enrichment mechanism of β-carotane in the Fengcheng Fm. The results show that the biological precursors crucially control the enrichment of β-carotane in the Fengcheng Fm. The haloalkaliphilic cyanobacteria are the primary biological sources of β-carotane, which is suggested by a good positive correlation between the 2-methylhopane index, 7- + 8- methyl heptadecanes/C_max, C₂₉%, and β-carotane/C_max in sedimentary rocks and the predominance of cyanobacteria with abundant β-carotene in modern alkaline lakes. The enrichment of β-carotane requires the reducing condition, and the paleoredox state that affects the enrichment of β-carotane appears to have a threshold. The paleoclimate conditions do not considerably impact the enrichment of β-carotane, but they have some influence on the water’s paleosalinity by affecting evaporation and precipitation. While it does not directly affect the enrichment of β-carotane in the Fengcheng Fm., paleosalinity does have an impact on the cyanobacterial precursor supply and the preservation conditions.