ABSTRACT

Downdip sand/shale ratios, internal architecture, sand body geometries and connectivities are major aspects that differ between fine-grained, mud-rich and coarse-grained, sand-rich submarine fans. Such differences have a direct impact on exploration and development strategies. Fine-grained, mud-rich systems are typical for the Gulf of Mexico, offshore Nigeria, and other "passive" margins.

Fine-grained fans often start with a major delta and a wide shelf. Relative sea-level lowerings cause basinward transport to the shelfbreak area, rapid deposition with high pore pressures and failure. Slumping will carve a major conduit across the slope. Deposition will start at the base-of-slope with a channel complex, followed by channel-levee-overbank systems and ending with sheet sands. These fans prograde rapidly and then tend to switch laterally. The channel complex and the sheet sands have a high sand/shale ratio, the leveed channel area a low one, except for the channel fill.

Coarse-grained, sand-rich submarine fans commonly are formed off mountainous coasts where a high sand/shale sediment input is typical. The narrow shelf is less influenced by sea-level fluctuations to pass sediment across. Gradual progradation of a fan across the basin floor is typical. The interchannel area is sand-rich and a general basinward decrease in sand/shale ratio is common.

Although no model should be applied blindly for any field, it is advisable to start with the most applicable one, and to insert changes with the increase of data obtained during the different phases of field development. The two end members (fine-grained and coarse-grained) will provide a start for any selection.