Upper Carboniferous Deep Water Sediments, Western Ireland: Analogues for Passive Margin Turbidite Plays
Ole J. Martinsen, Trond Lien, Roger G. Walker, 2000. "Upper Carboniferous Deep Water Sediments, Western Ireland: Analogues for Passive Margin Turbidite Plays", Deep-Water Reservoirs of the World, Paul Weimer
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The Upper Carboniferous Shannon Basin of western Ireland is an extensional basin with a 930 m thick, sandstone-bearing, deep water succession comparable to several passive margin basins around the world. A range of possible play types can be illustrated, deposited either in a sand-rich basin floor setting, or in a more mud-rich slopebasin floor transitional setting.
The sandstones mainly belong to the 380 m thick basin floor succession of the Ross Formation. This formation is a sand-rich submarine fan system which was axially supplied, and which sidelapped the basin margins. Several reservoir-type sandstones occur, including randomly organized tabular bed packages, channel sandstones, and sandier-upward packages capped by giant flutes.
In contrast, the overlying, 550 m thick Gull Island Formation is a mud-rich turbidite system and can be divided into two parts. (1) The lower 420 m represent interfingering between axially-transported, sidelapping basin floor turbidites and muddy slump and slide deposits from the lateral, probably degrading northern basin slope. This part is composed of three laterally variable facies associations: turbidite sandstones which thin and disappear upwards, slump and slide sheets, and thin hemipelagic draping mudstones and shales. The main reservoir type sandstones occur in this part of the Gull Island Formation and include mud diapir-bounded channel sequences, regular channel-fill sequences, growth-fault controlled packages, thickening sandier-upward inter-channel packages, and tabular-lenticular packages. (2) The upper 130 m, together with the overlying frontal deltaic deposits of the Tullig Cyclothem, represent the prograding, northern basin slope which advanced laterally across the basin floor deposits, and contains deep water reservoir type sandstones of only one type: slope channel-fill sandstones.
The stratigraphy and sedimentology of the Ross and Gull Island formations form an important analogue for turbidite systems in other passive margin basins such as the North Sea Basin, the eastern North Atlantic margin, the West African margin, and in some ways with the Cenozoic Gulf of Mexico Basin. The observed change in the Shannon Basin from sidelapping sandstone-rich to sandstone-poor basin floor systems to a downlapping mudstone-rich slope system is important to understand for prediction in these prolific basins.