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1 Marathon Oil Company, PO Box 3128, Houston, TX 77253-3128, USA
2 Marathon Oil Company, PO Box 689, Oklahoma City, OK 73101-0689, USA
The Upper Sinemurian sandstone is a primary reservoir target in the North Celtic Sea/St George’s Channel Basins. Based on core examination, analysis of gamma ray logs, and interpretation of possible sandbody geometry from well log correlations and seismic character, a marine shelf depositional setting has been interpreted for the Upper Sinemurian sandstone on the northern margin of the North Celtic Sea/St George’s Channel Basins. Burrows observed in the cored interval are typical for a marine shelf depositional setting. Wavy, low-angle cross-bedding, reactivation surfaces and truncated cross-bedding suggest deposition by storm-driven, surging geostrophic currents in this setting.
Examination of gamma ray logs through the Upper Sinemurian sandstone indicates that it coarsens and cleans upwards and can be divided into at least three units based on changes in curve patterns. Similar log patterns and sedimentary structure sequences in core and outcrop have been observed in elongate single and coalescing shelf ridges in the Ordovician of the eastern Williston Basin in Canada and the USA, in the Cretaceous of Wyoming and Colorado, and the Middle and Upper Jurassic of the North Sea. As will be discussed, the shelf ridge interpretation for some of these examples is controversial, with some workers suggesting an alternative incised shoreface interpretation. These examples range in thickness from 10 to 130m (33 to 425 ft). The 60+m (200+ ft) thickness of the Upper Sinemurian sandstone is well within this range. Such thicknesses suggest that ridge development (sand supply) kept up with basin subsidence. Thus, based on core data, gamma ray log curve examination, and comparisons to other sub-surface and surface examples, the Upper Sinemurian sandstone is interpreted as a series of complex elongate single and coalescing shelf sand ridges.
Interpretation of recently acquired seismic data and sequence stratigraphic analysis of the Lower and Middle Jurassic section indicates that the Upper Sinemurian sandstone is part of a series of stacked sequences. These sequences result from increases in sediment accommodation space due to episodes of basin subsidence and their corresponding relative sea-level rises. Six sequences or cycles can be recognized, with each consisting of a basal onlapping, open marine phase which shoals upwards to shallow marine limestone or sandstone. Onlap occurs at and above a marine flooding surface (or its basinward sediment starvation equivalent) and proceeds towards the northern flexural margin of the basin. The next marine flooding surface is cut into or is in conformable contact with the top of the underlying sequence. The upper part of the sequence has a progradational to slightly aggradational seismic character, which results in an overall parallel seismic signature. The best potential Lower/Middle Jurassic clastic reservoir development in the North Celtic Sea/St George’s Channel Basins, such as the Upper Sinemurian sandstone, corresponds to the last phase of progradation at the top of a sequence. Six such sequences, with their upper boundaries at the tops of the Lower Sinemurian sandstone, Upper Sinemurian sandstone, Pliensbachian limestone, Toarcian marker, Bajocian sandstone/limestone and Bathonian limestone, respectively, can be delineated in the Lower and Middle Jurassic section in the North Celtic Sea/St George’s Channel Basins.
The Lower and Middle Jurassic was largely a time of thermal subsidence in the North Celtic Sea and St George’s Channel Basins. Early thermal subsidence was interrupted by events of post-rift footwall readjustment along the northern margin of both basins. This resulted in the input of clastic sediments into both basins and deposition of Lower and Upper Sinemurian sandstones in rapidly coarsening-upward cycles and sequences. In contrast, Pliensbachian to Bathonian sequences represent deposition in a period dominated by thermal subsidence and thus coarsen upwards more gradually. With the exception of basin margin uplift and basin centre subsidence influenced by intraplate stresses, extrabasinal effects, such as eustasy, are not thought to be a factor in Upper Sinemurian sandstone deposition and Lower and Middle Jurassic sequence development in the North Celtic Sea and St. George’s Channel Basins.
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  S. Johnston, A. G. Dore, and A. M. Spencer The Mesozoic evolution of the southern North Atlantic region and its relationship to basin development in the south Porcupine Basin, offshore Ireland Geological Society, London, Special Publications, 2001; 188: 237 - 263. [Abstract] [PDF]
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