GROVER, GEORGE, JR., Saudi Aramco, Dhahran, Saudi Arabia

ABSTRACT: Intrashelf Basins: A Geologic Model for Source-Bed and Reservoir Facies Deposition within Carbonate Shelves

Intrashelf basins (moats, inshore basins, shelf basins, differentiated shelf, and deep-water lagoons of others) are depressions of varying sizes and shapes that occur within tectonically passive and regionally extensive carbonate shelves. Intrashelf basins grade laterally and downdip (seaward) into shallow-water carbonates of the regional shelf, are separated from the open marine basin by the shelf margin, and are largely filled by fine-grained subtidal sediments having attributes of shallow- and deeper water sedimentation. These basins are commonly fringed or overlain by carbonate sands, reefs, or buildups. These facies may mimic those that occur along the regional shelf margin, and they can have trends that are at a high angle to that of the regional shelf. Intrashelf basins are not intracratonic basins, which tend to be much larger in size, record a long history (several hundred m.y.) of basin filling, and may contain regional carbonate shelves. The history of most intrashelf basins is a few million to a few tens of millions of years. Examples of intrashelf basins are known throughout the Phanerozoic; the southern portion of the Holocene Belize shelf is a modern example of an intrashelf basin.

Two types of intrashelf basins are recognized. Coastal basins pass updip into coastal clastics of the craton with the basin primarily filled by fine clastics. Shelf basins occur on the outer part of the shelf, are surrounded by shallow-water carbonate facies, and are filled by peloidal lime mud, pelagics, and argillaceous carbonates.

Intrashelf basins are commonly the site of organic-rich, source-bed deposition, resulting in the close proximity of source beds and reservoir facies that may fringe or overlie the basin. Examples of hydrocarbon-charged reservoirs that were sourced by an intrashelf basin include the Miocene Bombay High field, offshore India; the giant Jurassic (Arab-D) and Cretaceous (Shuaiba) reservoirs of the Arabian Shelf; the Lower Cretaceous Sunniland trend, South Florida basin; and the Permian-Pennsylvanian reservoirs surrounding the Tatum basin in southeastern New Mexico.

AAPG Search and Discovery Article #90990©1993 AAPG International Conference and Exhibition, The Hague, Netherlands, October 17-20, 1993.