William D. McCaffrey¹, Rufus Brunt¹, Benjamin C. Kneller²

(1) Leeds University, Leeds, United Kingdom
(2) University of California, Santa Barbara, CA

ABSTRACT: Predicting spatial trends in grainsize distribution within deep marine clastic systems

Spatial variations in the mean grainsize and sorting characteristics of turbidite sandstones directly influence spatial trends in reservoir-significant properties such as porosity and permeability. In natural systems, systematic variations in these properties can be linked to a first order proximality control. However, second order bathymetric controls may impose a local overprint on grainsize trends. We describe two sets of experiments measuring the thickness and the grain-size distribution of the deposits of scaled, mixed grain-size, turbidity currents. In the first set, unponded currents interacted with basin floor topography, whose height was systematically varied from <10% to >150% of the flow head height. Although there was an increasing tendency towards flow deflection with increasing topographic height, local grainsize sorting effects were most pronounced within sediments deposited around low amplitude topography. In the second set of experiments, flow between two minibasins was modeled, in which as upper basin filled, progressively more overspill was directed into the lower basin. In the lower part of the upper basin fill, a vertical trend of increasing grainsize was observed.

In each set of experiments, the parental flows were stratified in terms of density, grain concentration and grainsize. In each case, the measured spatial variations in grainsize distribution may be interpreted in terms of a vertical stratification effect. Where estimates of the scale and type of sea floor bathymetry can be measured or inferred, it may be possible to predict the spatial variation in turbidite grainsize, and sorting, and thus infer trends in the related rock properties.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado