Patterns of sedimentation from polydispersed turbidity currents

Particle-driven gravity currents, as exemplified by either turbidity currents in the ocean or ignimbrite flows in the atmosphere, are buoyancy-driven flows due to the suspension of dense particles in an ambient fluid. They are formed naturally from sediment-laden outflows from rivers into coastal waters, from submarine landslides along coastal shelves or as the result of volcanic eruptions. The porous rock and sand of both consolidated and unconsolidated oil-reservoirs are often derived from the sediment deposited from turbidity currents over geological time. A knowledge of the genesis of these reservoirs may provide better methods to estimate their porosity and permeability distribution, which would improve evaluation and management of these valuable resources. This paper presents a theoretical model for the dynamics and deposition of a two-dimensional particle-driven gravity current composed of a polydispersed suspension of dense particles and compares the theoretical predictions against data obtained from laboratory experiments. After developing a scaling analysis of the governing equations, we propose a simple algebraic method to compute the areal density of deposit, or mass deposited per unit area, and the distribution of particle-sizes within deposits arising from either two-dimensional or axisymmetric currents. The resulting formulae suggest an inverse method to estimate the density of deposit and the distribution of particle sizes as a function of position in a reservoir from a limited number of cores.