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From Grain to Flume Tank: Analyzing the Hydrodynamic Behavior of Carbonate Sediments

Abstract

Particle-transporting subaqueous density flows depositing calciturbidites occur in a variety of deep-water carbonate environments and can form thick sedimentary successions that host important hydrocarbon reservoirs worldwide (e.g. Adriatic Sea, Red Sea, China Sea). To understand the porosity and permeability distribution within calciturbidite reservoirs it is important to capture the hydraulic particle-sorting patterns. In siliciclastic turbidite systems these processes have been extensively studied, but the depositional mechanisms of calciturbidite systems remain underexplored. Carbonate sedimentary systems produce a heterogenic spectrum of skeletal and non-skeletal particles varying in density, size and shape; the latter including three orders of scale: form, roundness and surface texture. To understand the hydrodynamic behavior of carbonate particles feeding carbonate gravity flows, the settling velocities of individual grains, the behavior of sand/mud mixtures during confined settling, and flume-tank experiments were investigated using muds and sands from the Bahamas, Moorea (French Polynesia), and Favignana (Italy). Settling velocity and shape analysis characterizing the hydrodynamic behavior of particles showed that the falling velocity of a settling particle is not only controlled by density and grain size. Particle shape measured in terms of elongation and flatness, e.g. Zingg-shape classes (spheres, rods, discs, and blades) is equally important. Flattened shapes settle relatively slow while rods and equant shapes fall faster. A sequential set of falling regimes could be developed based on particle shape and Reynolds number. The confined grain settling experiments repeatedly showed three texturally different sediment intervals: (A) fairly ungraded grain- to packstones, often inversely graded, (B) normally graded grain- to packstones, and (C) normally graded wackestones to poorly graded floatstones. Grain-size segregation was most efficient in suspensions with a sediment concentration below 20%, with high carbonate-mud content further lowering the critical sediment concentration for suppressed particle-size segregation to occur. The grain-size distribution data obtained during the flume-tank experiments showed a strong link between mud content, sorting trends, and permeability. However, more experiments are needed to fully capture the sorting processes and relate them to the hydrodynamic behavior of individual grains and confined settling conduct of mud/sand mixtures.