Flow transformation processes recorded in the Eocene early syn-rift deep-lacustrine fine grained sedimentary rock in the Qibei Sub-sag, Bohai Bay Basin, China

Gravity flow is a key sedimentary process in deep-lacustrine environments, with transitional flow deposits commonly occurring in both distal and proximal zones of the turbidite systems. These deposits are crucial to understanding the sedimentary dynamics of fine-grained deep-water sediments. The transitional deposits between turbidity currents and mud-rich debris flows are particularly important for advancing our understanding of fine-grained sedimentation processes and have significant implications for unconventional oil and gas exploration.

The aim of this study is to describe transitional-flow facies, interpret their flow evolution and depositional processes, and assess their impact on the differential accumulation of organic matter in a fresh-water syn-rift deep-lacustrine system. Data were collected from the 111.39-m-thick Eocene  lacustrine oil-prone source rock succession, penetrated by the two wells in the Qibei Sub-sag, Bohai Bay Basin, China. Nine sedimentary facies were identified in the studied fine-grained succession, with various internal sedimentary structures (e.g., ripple cross lamination, low-angle cross lamination, wave lamination, parallel lamination, graded structure, deformed structure, and homogeneous structure) reflecting the dynamics of sedimentary processes in a deep-lacustrine depositional lobe distal environment. Millimeter-scale logging defined 5 bed types based on 2383 measured and recorded beds, with inferred transitional flow deposits exhibiting distinctive stacking patterns, from coarser grained turbidites to fine-grained debrites. A wide range of transitional-flow facies are recognized and can be assigned to turbulence-enhanced transitional flow, lower transitional plug flow, upper transitional plug flow and quasi-laminar plug flow. Despite the predominance of finning upward grain size trends, sedimentary structures in these heterolithic deposits may stack in varying orders, reflecting different flow dynamics.

The vertical facies trends of transitional flow deposit provide insights into the longitudinal flow evolution of flows, which were initially turbulent, but became increasingly laminar through deceleration and fine-grain entrainment. The assimilation of the lake-bottom mud into the density flows likely played a key role in modulating flow turbulence, helping to explain the common occurrence of transitional-flow facies indicated by sedimentological features such as sheared flame structures and deformed mud intrusions, which suggest interaction between the flow and the muddy lake floor.

Lacustrine organic matter was delivered to the lake floor by continuous settling, whereas terrestrial organic matter was transported via sediment density flows. The deep-lacustrine background mudstone is dominated by Type II₁ kerogen, whereas the quasi-laminar plug flow mudstone is dominated by Type II₁ and II₂ kerogen, turbulence-enhanced transitional flow and lower transitional plug flow mudstones are dominated by Type II₂ and III kerogen. These observations challenge the view that mud accumulates only from suspension fallout in distal basin-floor environments. This study suggests that composition, texture, and organic matter types of mud-dominated deep-lacustrine mudstones vary predictably in response to changes in depositional processes. The results have broader applicability to other deep-lacustrine sedimentary systems, highlighting the dynamic nature of transitional flows. Detailed microtextural and compositional analysis, combined with rigorous geochemical parameters, is essential for the understanding of the source-rock potential of basinal mudstones and fine-grained organic-rich sediments more general.