Quantitative Analysis of the Geometry of Submarine External Levees; Exponential or Power-Law?

Nakajima, Takeshi and Kneller, Benjamin C.
[email protected]

Although typically containing rather low proportions of sand, submarine levees may be very large, and chiefly consist of laterally continuous beds, and thus may form significant and well-connected hydrocarbon reservoirs. Characterization of the reservoir depends, amongst other things, on good quantitative models for the variation in the deposit thickness (levee thickness). To develop a quantitative model for the geometry of submarine external levees, the shape of submarine external channel levees has been assessed in a variety of systems, over a range of slope gradients, measuring both their thickness decay away from the parent channel, and also the maximum gradient on the back-slope of the levee. The mathematical description of the shapes of the levees has been based on normalized data, using as characteristic length scales the distance from channel axis to the levee crest, and the thickness of the entire levee (or levee package) at the levee crest. The variation in levee thickness perpendicular to the channel shows a clear pattern of power-law decay on steeper slopes (generally >0.6°) and either exponential or logarithmic decay on gentler slopes. The levee shape is sensitive to local variations in slope, and may change over only a few hundreds of meters in the flow direction. The threshold gradient between these two styles shows some variation, and may be dependent on grain-size. The maximum gradient on the back-slope of the levee shows a weak correlation with slope gradient, whose slope varies from one system to another; this may also be a function of grain-size. The variations in behaviour can be explained in part by differing rates of entrainment of ambient sea-water into the currents as they flow over the levees, these rates being dependent upon the slope. Negligible entrainment on low gradients helps to explain the tendency of wider levees on low gradient basin plain as well as the persistence of channelized flows on basin plain with little dissipation over extreme distances.

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013