Sedimentary characteristics and fault-controlled depositional models of gravity flows in the Eocene Shahejie formation, Bohai Bay Basin, China: Insights for hydrocarbon exploration in rifted lacustrine basins

Abstract

Sediment gravity flows are critical sediment transport mechanisms in deepwater environments and play a key role in hydrocarbon accumulation. The well-developed sediment gravity flow deposits of the Shahejie Formation are major oil-bearing units in the Dongying Depression (Bohai Bay Basin, eastern China), yet the types of sediment gravity flow deposits and their controlling factors remain debated. This study introduces a novel step-fault-controlled framework in the lower section of the third member of the Eocene Shahejie Formation (ES3L sub-member) that goes beyond traditional models by highlighting the finer-scale structural influences on sediment distribution. The research discusses sedimentary characteristics, lithofacies types, facies distribution, and depositional models based on core samples, well logs, and seismic data from the relay ramp of the Dongying Depression, offering important insights for hydrocarbon exploration strategies. The results indicate that the sediment gravity flows responsible for the deposition in the study area were mainly sandy debris flows, turbidity currents, and hyperpycnal flows. Debrite sandstone deposits are characterised by numerous floating mud clasts, often associated with soft-sediment deformation structures. Turbidite sandstones are characterised by load casts and normal grading. Hyperpycnites are characterised by couplets of inverse grading and normal grading, periodic changes in grain size, and abundant plant debris. The development of sediment gravity flows within the study area was controlled by step faults. As a result, a step-fault-controlled sedimentary model of sediment gravity flow deposition was developed with the following characteristics: 1) Delta front deposition prevailed on the upper step fault, with minor deposition from gravity flows; 2) hybrid deposition of sandy debris flows and turbidity currents took precedence on the middle step fault; and 3) hyperpycnites dominated on the lower step fault. This study investigates the impact of stair-step faults on the sedimentary facies of gravity flows, providing guidance for oil and gas exploration and development in rifted lacustrine basins worldwide.