ABSTRACT

Most deep-water sands are part of a submarine fan and by far the majority of these deposits have accumulated in the marine realm on continental margins and adjacent basins. The shape and size of these fans, as well as the sand/shale ratio of the sediments and the thickness and distribution of the sands, may be more variable and complex than any other depositional environments. The main influencing factors that control the development and characteristics of submarine fans are: 1) the tectonic setting and activity of both the sediment shedding and the sediment receiving areas, including width of the shelf, seafloor gradients, and the size and shape of the receiving basin, 2) sedimentary factors, especially sand/shale ratio, volume of the sediment involved, and climatic conditions, and 3) relative sea level changes.

Non-compatibility in approaches and scale of resolution has introduced major confusion between the results from outcropping formations, subsurface occurrences, and seismic analyses of submarine fans. Because of subsequent tectonic activity, basically, only active margin settings permit later exposure of deeper marine sedimentary series. Most, if not all, published

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models have been derived from outcrops and modern fans located in active margin settings. This has led to serious misunderstandings by earth scientists dealing with passive margins. In addition, most models are based on criteria that are too small to be recognized seismically.

The Mississippi Fan is the only submarine fan that has been drilled systematically and has been reported on in the open literature. Significant findings have certainly changed our opinions about these types of sediment bodies. These can be summarized as follows: 1) a submarine fan is a non-vertical stacking of individual depositional units (fanlobes), 2) each fanlobe may display variability between fanlobes, but, in general, can be described as a channel-levee-overbank complex, or complexes, if channel switching took place, 3) each fanlobe starts at the upper slope or outer shelf with a canyon that merges downslope into an erosional upper fan channel (conduit), 4) near the base of the slope the erosional characteristics change to depositional ones, 5) the middle fan is aggradational with a sinuous channel (meandering characteristics), probably underlain by debris flow deposits.

Channel axis sands, gravels and fine-grained sediments, form ideal migration paths, while "pointbar"-type deposits have reservoir qualities. Accoustic high-amplitude zones often indicate the location of these channels, where channel shapes may not be apparent on seismic records. On the lower fan, the channel (s) gradually become smaller and terminate, changing the depositional style from channel-overbank to very significant

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sheet sand deposits with high sand/shale ratios.

Although the large volume Mississippi Fan has a low sand/shale ratio, the total amount of sand in the channel fill and in the outer fan sheet sands is very attractive. The major exploration/production question is how far can we utilize the findings and interpretations from the Mississippi Fan working in the intraslope basins of South Louisiana, offshore Louisiana, or part of the Flexure Trend. Extrapolations have to be handled with extreme caution and likely a COST-well-type program will be needed to significantly advance our knowledge.

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