Coupled Shale Oil and Conventional Oil Systems in a Lacustrine Basin: Molecular Constraints on Hydrocarbon Generation, Migration, and Fractionation in the Boxing Sag

Abstract

This study investigates the molecular geochemistry of the Boxing Sag, a low-maturity lacustrine system in the Dongying Depression, using integrated Rock–Eval pyrolysis and gas chromatography–mass spectrometry (GC–MS) analyses of source rocks, shale oils, and conventional oils. The upper Es4 Member of the Shahejie Formation is characterized by moderate organic richness, Type II kerogen, and oil-window maturity (Ro ∼ 0.65–1.05%), supporting active hydrocarbon generation with limited expulsion. Although shale oils and conventional oils share a common lacustrine source, they exhibit fundamentally different compositional behaviors. Shale oils display relatively uniform molecular compositions and systematic depth-dependent maturity trends, reflecting in situ generation and short-distance micromigration within nanopore networks. In contrast, conventional oils show pronounced compositional variability and no correlation with depth, indicating multistage charging, long-distance migration, and mixing of hydrocarbons with different maturity histories. Apparent biomarker variations, such as increases in G/C30H and Ol/C30H ratios, are primarily driven by selective depletion of C30 hopane rather than changes in depositional conditions. These results suggest that shale oil and conventional oil in the Boxing Sag represent two end-member expressions of a single lacustrine petroleum system governed by varying degrees of retention, redistribution, and expulsion. The resulting retention–migration continuum links shale oil retention, redistribution, expulsion, and conventional oil accumulation within a unified lacustrine petroleum system and provides a conceptual framework for interpreting coupled unconventional–conventional hydrocarbon systems.