Sedimentary and Hydrodynamic Controls on Shale Oil Sweet Spots: A New Storm Deposition Model for the Gulong Sag, Songliao Basin

The First Member of the Cretaceous Qingshankou Formation (K2qn1) in the Gulong Sag, Songliao Basin, contains vast shale oil resources conventionally interpreted as deposits of suspension settling in a quiescent, anoxic deep-lacustrine environment. However, this static “deep-lake” model fails to account for the strong lithofacies heterogeneity and high-energy sedimentary records observed in recently acquired core data. This study reconstructs the sedimentary dynamics of the K2qn1 shale through high-resolution core description, thin-section petrography, and flow-loop hydrodynamic simulations. We identify abundant sedimentary structures diagnostic of high-energy combined flows, including Hummocky Cross-Stratification (HCS), Swaley Cross-Stratification (SCS), erosional scour surfaces, and large-scale tabular intraclasts (up to 40 mm). Hydrodynamic simulations, utilizing an “equivalent substitution” method, demonstrate that the Minimum Vertical Suspension Velocity (Vmf) required to transport these large intraclasts exceeds 1.0 m/s. This threshold is 1 to 5 orders of magnitude higher than theoretical values derived from classical settling equations, confirming that the paleolake bottom was frequently perturbed by high-velocity storm-driven currents. Consequently, we propose an “Intermittent High-Energy Deposition Model,” wherein background suspension settling was punctuated by episodic storm events. We argue that these high-energy events facilitated organic matter enrichment through a “Transport-Burial Pump” mechanism, which operated in concert with the chemical stratification associated with the Oceanic Anoxic Event 2 (OAE2) to enable rapid physical burial and sealing of organic matter. These findings challenge the traditional fine-grained sedimentological paradigm and suggest that storm-reworked intervals—characterized by enhanced brittleness and hydrodynamic winnowing—constitute the primary “sweet spots” for lacustrine shale oil exploration.