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Structural Setting of Turbidite Systems: A Global Comparison


Over the last 25 years exploration and production in deepwater (in waterdepth in excess of 500 m) has increased greatly to the point that a considerable amount of todays industry budget is spent on these activities. Whereas initially little was known about the geological setting of deepwater systems (and few people believed in the presence of sandstone reservoirs beyond the continental shelf edge), we now know that downdip of several Neogene delta's, major turbidite systems occur with appreciable reserves of oil and gas. Oil accumulations in turbidite sandstones are not new, in fact production from these reservoirs has been going on in the Mio-Pliocene Los Angeles and San Joaquin pull-apart basins in California before Kuenen coined the term in 1957. Similarly, sizeable accumulations have been found in Upper Jurassic, Lower Cretaceous and Lower Tertiary turbidite sequences in the North Sea in a rift-sag setting, described by Peter Ziegler in his Geological Atlas of Western and Central Europe. In the mid-eighties exploration moved into deeper water and driven by successes in the Gulf of Mexico, further efforts focused on the Lower Congo Basin, Campos Basin and offshore Nigeria. A common characteristic of these four basins is that the turbidite depocentres overlie a mobile substrate of salt or overpressured shale which provide a high density of traps with a similar and predictable structural/stratigraphic evolution, stacked reservoir/seal pairs, and an easy access to mature source rocks. A regional comparison of these basins suggests that in a general sense four structural play types can be defined: -Immediately downdip from the major deltaic expanders, an area of Inner Folds with large trap closures postdating the emplacement of channelized turbidite reservoirs.(Angola. Nigeria) -An area of Mini-Basins where structuration and sedimentation are more or less coeval leading to the emplacement of stacked confined turbidite sheetsands. (Gulf of Mexico) -In areas of salt withdrawal, inversion of minibasins with turbidite sheet sands leads to the formation of Turtles. (Angola, Brazil. Gulf of Mexico) -An oceanward area of very large Outer Folds which are structurally coupled to deltaic extension updip and where amalgamated unconfined turbidite channels and sheets occur predating trap formation.(Perdido, Gulf of Mexico, possibly deepwater Colombia, Malaysia) Exploration efforts in turbidite basins without a mobile substrate have so far been more limited and concentrated on the Atlantic margins of the United Kingdom and Norway. Typically the density of structural closures is low but individual structures can be very large in areal extent, quite often with a stratigraphic component. Recently, major discoveries in Equatorial Africa have spurred the industry to take a closer look at the Cretaceous turbidite systems on both sides of the African-South American conjugate margin in what is a very lightly explored play setting. Similarly, major gas discoveries in Tertiary turbidites off Eastern Africa and India also have indicated that there are still some major frontier deepwater basins worth looking at where structural plays are less obvious.