Deepwater Fans

Abstract

Technology Today Series articles provide useful summary informationon both classic and emerging concepts in petroleum engineering. Purpose: To provide the general reader with a basic understanding of a significantconcept, technique, or development within a specific area of technology. An increasing share of the world's oil and Las reserves is being found indeepwater fans that contain an accumulation of turbidite deposits. These fansoccur in two major locales: off mountainous, coastal, tectonically active areaswith steep slopes and off major deltas with gentler but unstable slopes (seeFig. 1). They may also occur in lakes, such as Lake Geneva. Most submarine fansare believed to be derived from sediments and debris laid down at or near anincrease in slope. These sediments are unstable. and their movement may betriggered during earthquakes, storms, or floods, causing them to move suddenlyto deeper waters. These dense masses of sediments flow down the basin slope andalong the ocean floors in turbidity currents that cut gullies and channels intoexisting deposits and that subsequently fan out on the gently sloping basinfloor, where they come to rest. Later, as currents in the basin return to thenormal slow fallout of finer particles, increasing amounts of clays are laiddown particles, increasing amounts of clays are laid down over the turbiditesto seal the deposit. Fan deposits can be observed and mapped from (good-quality seismic data. Although individual turbidite flows are usually thinner than can be resolved byseismic reflections, it is generally possible to map the internal reflectionpattern and to possible to map the internal reflection pattern and to definedepositional lobes. Such a detailed analysis may be important in the earlyevaluation of the field and in platform location. Log correlation within thefan complex is difficult: consequently, every effort should be made to obtainhigh-resolution seismic data in a grid of very closely spaced lines to definethe internal geometry of the fan. Three-dimensional surveys may be justifiedbecause of the need to migrate steep depositional dips. Our seismic example (Fig. 2) is the large Frigg field in the north-centralNorth Sea. Gas is in reservoirs of Eocene quartz sands, and the fan is veryquartz-rich. The most probable source of sediments for the fan lies to thewest, many miles away. Lateral continuity of the individual turbidite beds orchannels deposited during a single cycle may range from a few hundred tothousands of feet along the channels and hundreds of feet across their widths. Some fans are composed of many lateral and vertical sequences of turbidite bedsor channels and may cover very large areas. For example, the Mississippi fan inthe Gulf of Mexico covers 300 000 km2 [115,830 sq miles]. Vertical continuity. between beds on a fan is usually extremely poor because of the lenticulargeometry, of the sands and the continuity of the intervening silt/clay beds. The idealized vertical sequence of a single turbidite flow in Fig. 3 showsclean gravel and coarser sand at the base, sand laminated by silt or clay inthe upper part, and a thicker layer of silt or clay at the top. Gamma ray logresponses of sequences of beds are shown in Fig. 4 for the channels and for thedistal edges of the fans. The pay in the channels typically appears as blockyin clean sands or bell-shaped as the shale content increases upward. Individuallaminae of shales are usually too thin to be seen on the logs. P. 402