The Helvick oil accumulation, Block 49/9, North Celtic Sea Basin

Abstract

Abstract The Helvick oil accumulation in Block 49/9 is situated within the northern margin of the North Celtic Sea Basin. It was discovered by well 49/9-2, drilled in 1983, which tested a cumulative total of 9901 BOPD and 7.44 MMSCFD from four reservoir intervals. These consist of marine limestone of Bathonian age, and an overlying series of Middle to Upper Jurassic age sandstones deposited in a continental/fluviatile environment. The principal ‘Main Sand’ reservoir, which flowed at a rate of 6467 BOPD, lies close to the base of the non-marine sequence and was deposited by a high-energy, braided fluvial system. It is laterally extensive and may be correlated for a minimum east-west distance of 6.5 km. Oil in the Main Sand is 44.06° API with a GOR of 680 SCF/BBL; it is derived from a predominantly herbaceous, terrestrial source which is identified, on regional evidence, as being most likely of Liassic age. The oil-water contact has not been positively established. Calculations based upon RFT and DST pressure measurements give a range of possible oil-water contacts, and hence a corresponding range of oil-in-place values. These range from 8.4 to 12.3 MMBO, with a ‘most likely’ figure of 10.7 MMBO. Reservoir simulation studies indicate potential recoverable reserves in the Main Sand of 2.27–4.42 MMBO. An additional ‘most likely’ figure of c. 3 MMBO oil-in-place may be present within the underlying Bathonian Limestone, of which c. 0.6 MMBO may be recoverable. Further, as yet unquantified, reserves are present in the other Upper Jurassic oil-bearing sandstones.