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Abstract

In recent years deep-water petroleum exploration has been booming. The Gulf of Mexico, some West African regions, and Brazil are leading this growing activity. Current deep-water plays focus on presalt, subsalt, stratigraphic pinch-out and deep-water folded belt targets. Comparisons of conjugated margins across the Atlantic are commonly used by explorationists to extend the prospectivity of known plays. However, what are deep-water plays? Are they only plays presently located in water depths more than 2000 m water depths or do they also include plays developed initially in relatively deep oceanic tectonic settings but are now underneath shallow water? For that matter, what about plays which developed in relatively shallow water but are now located in water depths greater than the continental slope.

Present deep-water tectonic settings are mostly compressional toe-thrust regions of larger massive gravitational collapses related to (A) major deltas (B) allochthonous salt provinces, or (C) subduction-related accretionary wedges. Back-arc extensional basins in deep-water settings are underexplored, except for the Black Sea. Other plays presently in deep waters but from a geologic perspective initially formed in relatively shallow marine water and subsequently subsided are the pre-evaporitic plays of the Campos-Santos basins in Brazil and Angola offshore basins.

Some elements of deep-water petroleum systems are still poorly understood. Although the presence of widespread deep-water source beds on oceanic crust is mostly known through DSDP/ODP wells, the role of tectonics and volcanic activity in the generation and maturation has yet to be adequately evaluated. Classical upwelling models intended to explain marine source beds may need to be refined.

Note that most current deep-water-plays involve siliciclastics. There is no reason to preclude deep-water carbonate plays, the reservoirs of which are analogous to the outer platform, relatively deep-water pelagic carbonates producing in the Bay of Campeche in Mexico.

Traditional seismic stratigraphy views unconformities as being caused by eustatic sea level changes. However, the origin of widespread deep-water unconformities needs to be further elucidated.

An increasing number of deep crustal seismic surveys along West Africa show low-angle extensional detachments similar to those found on the Galicia margin of Spain and Portugal and comparable with tectonic styles of the Basin and Range Province of the western United States or else in the western Mediterranean (e.g., Western Alps or Betic Cordillera). Classical rifting models always need to be updated to be “on target” with new and often surprising seismic observations. In addition to the four principal types of deep-water plays currently being explored, additional plays should be found in deep-water carbonates, volcanic margins including hot spots or volcanic-lineaments, and subsided rifted systems; these may account for significant yet to be explored future plays in many parts of the “deep-water” world.

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