--> Abstract: The Architecture Of Turbidite Slope Channels, by M. Mayall, I. Stewart, and P. Ventris; #90933 (1998).

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Abstract: The Architecture Of Turbidite Slope Channels

Mayall, Mike and Ian Stewart - BP; Paul Ventris - Statoil

Extensive 3D seismic data from around the world has started to show remarkable details of the geometry and facies of Tertiary turbidite slope channels. The slope systems which have been studied are characterized by subtle to complex topography created by salt or shale diaperism or faults. In the upper part of the slope the channels are often relatively narrow (<1km) with fairly straight leveed margins which may or may not contain a moderate-high sinuosity channel axis. Downslope the channels become broader (1-2km), highly sinuous (sinuosity >2) with erosional bases and local levee and crevasse-splay development. In this part of the slope the channels typically show a vertical sequence which comprises an erosional base, a coarse grained lag (by-pass phase), slumps and/or debris flows (locally derived or more distant transport?), high N:G sandy fill of stacked channels which may be straight or sinuous and finally a lower N:G with highly sinuous channels and levees. The relative proportion of each of these facies can vary significantly.

In detail the channels can vary radically over short distances down-stream. This implies that heterogeneity models for channels may only be applicable for short segments of the reservoir. In many cases the sinuous channels appear to show dominantly vertical aggradation with the channel thalweg shifting only slightly with time, in other places however, the channels show very marked lateral migration producing features very similar to fluvial point bars including seismic-scale lateral accretion features. The channel fills may contain medium to coarse sand and even pebbles over 200km from the contemporaneous slope entry points.

In our experience ponded systems i.e. basins in which the channel systems terminate on the slope as a result of slope topography, are not common. The appearance of "ponding" can be a function of the extent of the 3-D data set. In areas of smaller data coverage it is often easy to interpret channel systems terminating in intra-slope basins. However with increasing coverage of 3-D data the channels can usually be seen to have continuous but very convoluted courses which takes them through and beyond complex slope topography. At sharp bends in channels it is common to observe sheet-like seismic facies (although generally thin), extending away from the channels. Topographic constraints within the slope topography may locally fix the course of the channel system for some time while down-slope of the constriction the channels take different, usually compensatory off-setting courses through time.

Associated with the channels are more sheet-like seismic facies. The origin of these features is less clear and quite possibly they have multiple origins. They may be parts of levees of the larger channels, minor splays which were deposited laterally, splays which are an initial part of the channel avulsion process (similar to the HARPs of the Amazon Fan), or by processes unrelated to the major channels.

These Tertiary channel systems have very similar geometries to those observed in many modern fans such as the Amazon and the Zaire and pose many questions regarding the nature of the currents which transported and deposited the sediments.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil