PATTERNS OF SEDIMENTATION FROM POLYDISPERSED TURBIDITY CURRENTS

Particle-driven gravity currents, as exemplified by either turbidity c urrents in the ocean or ignimbrite flows in the atmosphere, are buoyan cy-driven flows due to the suspension of dense particles in an ambient fluid. They are formed naturally from sediment-laden outflows from ri vers into coastal waters, from submarine landslides along coastal shel ves or as the result of volcanic eruptions. The porous rock and sand o f both consolidated and unconsolidated oil-reservoirs are often derive d from the sediment deposited from turbidity currents over geological time. A knowledge of the genesis of these reservoirs may provide bette r methods to estimate their porosity and permeability distribution, wh ich would improve evaluation and management of these valuable resource s. This paper presents a theoretical model for the dynamics and deposi tion of a two-dimensional particle-driven gravity current composed of a polydispersed suspension of dense particles and compares the theoret ical predictions against data obtained from laboratory experiments. Af ter developing a scaling analysis of the governing equations, we propo se a simple algebraic method to Compute the areal density of deposit, or mass deposited per unit area, and the distribution of particle-size s 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 fu nction of position in a reservoir from a limited number of cores.