Characteristic Geometries of Distal Fine-Grained Fans Based on Mississippi Fan, Jackfork, and South Africa Studies

Arnold H. Bouma¹, Daniel J. Rozman², Donald E. Rehmer³, and Samuel K. Huisman^4
1 Department of Geology and Geophysics, Texas A&M University, College Station, Texas 77843-3115
² British Petroleum, 501 Westlake Park Blvd., Houston, Texas 77035
³ Southern Illinois University, Carbondale, Illinois 62901
⁴ ConocoPhillips Norge AS, Box 220, Tananger, N-4098, Norway

Coarse-grained and fine-grained deepwater fans are very different. The majority of the fans fall in between the end members, and are composed of a variety of characteristics. Coarse-grained fans are common for active margins with canyon-fed deposits exhibiting high sand/shale ratios. Deposition is often parallel to the coast with grain-size fining in a downdip direction.

Fine-grained fans are typical for passive margins, and basically occur during lowering and low sea level. They are delta fed with density currents that tend to move downdip away from the coast. The significant amount of fine-grained sand becomes deposited at the outer fan, and exhibits gradual distal thinning. The outer fan sheet sands are oblong and onlapping onto each other, thus producing massive sand units. Distal deposits are oblong, and transition into digitized morphology and terminate rapidly. Over 80% of the input sand may travel to the outer fan. Paleocurrent data display various directions, which indicate a cyclical stacking pattern of depositional lobes that is likely below seismic resolution.

The turbidity flows gradually move part of their very fine-grained sediment above the mass of sand in transport. That upper, much lighter current moves slower than the main current, and travels farther into the basin, past the sand deposits. Subsequent deposits of very fine-grained silica deposits on the deepwater sands and on part of the basin floor make it very difficult to seismically separate the outer fan from the basin sediment. Major oil deposits may, therefore, become overlooked if outcrop studies are not integrated into seismic interpretation.

AAPG Search and Discover Article #90069©2007 GCAGS 57th Annual Convention, Corpus Christi, Texas