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Petroceltic plc, Orchard House, Clonskeagh Square, Dublin 14, Ireland
Studies of regional seismic data tied to exploration wells demonstrate that the North Celtic Sea Basin consists of distinct Upper Jurassic/Lower Cretaceous depocentres or sub-basins. The most southwesterly and least drilled of these depocentres is situated 70 km south of Mizen Head in Quadrants 55 and 56 and is called the Mizen Basin.
The Mizen Basin is separated from the Fastnet Basin to the south by a significant NE plunging basement ridge, the Fastnet High. It is also bounded to the west and north by pre-Mesozoic Variscan basement. Variscan basement folding and faulting controlled early basin development. Early Mesozoic (Triassic to Middle Jurassic) deposits are associated with significant NNW-SSE trending transfer faults. This faulting became dormant in Middle Jurassic times, coincident with an angular unconformity prevalent at the basin margins, and igneous activity. Upper Jurassic to Cretaceous sediments are deposited in the hangingwalls of ENE-WSW trending fault systems which downthrow to the northwest, creating a half-graben depositional pattern with a feather edge to the northwest. This trend continues to the northeast, at least as far as the Kinsale Head gas field. A significant unconformity is also observed at the base of the Aptian and marks the end of a phase of Early Cretaceous faulting.
The only well to be drilled in the central part of the basin is 56/12-1, which reached a total depth in sediments of Valanginian age at 8773 ft. However, extrapolation of surrounding well data indicates that the Mizen Basin contains a full sequence of Triassic, Jurassic and Cretaceous sediments.
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