The definition of the Albian ‘A’ Sand reservoir fairway and aspects of associated gas accumulations in the North Celtic Sea Basin

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Abstract
Log and core data collected from exploration, appraisal and development wells, together with a large quantity of seismic data collected over a 21 year period, have allowed the delineation of a fairway of reservoir quality Cretaceous Albian sandstone (‘A’ Sand) in the north central part of the North Celtic Sea Basin. The ‘A’ Sand is the primary reservoir at the Kinsale Head and Ballycotton gas fields. Burrow types and the presence of glauconite indicate that it was deposited in a marine setting, although the precise depositional environment remains controversial. Regional isopach and lithofacies mapping indicates that only limited development of reservoir quality sandstone exists outside the main ‘A’ Sand fairway, such as at the well 48/18-1 discovery approximately 13 miles west of Ballycotton gas field. The sands are believed to have been derived from the Irish Massif to the north and were transported along a shallow-water shelf. Some northward thinning of the sandstone is recognized in the area that was closer to the interpreted palaeoshoreline. Reservoir quality is controlled, in part, by distance from the source area, while thickness, and to some extent coarseness, appears to have been controlled by palaeobathymetry. The thickest ‘A’ Sand is in the Kinsale Head field area, a local depocentre within the passively subsiding North Celtic Sea Basin. Reservoir quality diminishes to the south and southwest of the Kinsale Head gas field. Topseal for the reservoir is provided by the Albian to Cenomanian Gault Claystone and the associated overlying Cenomanian claystones. Untested structural closures within this ‘A’ Sand fairway may require a similar stratigraphy and structural configuration to the existing fields to prove successful. The Kinsale Head and Ballycotton fields are the result of fortunate geological coincidence. Albian reservoir sand accumulation was followed by deposition of deeper-water clay facies for topseal and subsequent burial beneath the Chalk. The Cretaceous stratigraphic evolution of the basin was succeeded by trap-forming basin inversion in the Early Tertiary, with concomitant and subsequent vertical migration of gas from the deeply buried and very mature thick shales within the Liassic. Additional ‘A’ Sand gas accumulations may be restricted to the structural closures in the general area of the existing fields, where there is well developed claystone facies above the reservoir and where fault offsets are less than the associated topseal thickness.

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