The Corrib gas field, offshore west of Ireland

Abstract

The Corrib Gas Field in the Slyne Basin lies about 70 km west of Co. Mayo, offshore Ireland. The basin is a narrow Triassic/Jurassic half-graben and is one of a series of structurally linked basins that are present along the west coast of Ireland and the British Isles. The field was discovered in 1996 by well 18/20-1, encountering a 61 m gas column in low-porosity Triassic fluvial sandstones at a subsea depth of 3539.6 m. Using extremely poor 2D seismic data, it was originally mapped as a tilted fault block, with an associated rollover structure to the east. The data quality was particularly poor due to the near-surface Palaeogene volcanics that contribute to energy dispersion and extreme multiple generation. A 3D seismic survey was acquired in 1997 and resulted in a major improvement in data quality, allowing improved structural interpretation that showed the Corrib Field to be a faulted anticline. The subsequent appraisal well, 18/20-2z, drilled approximately 1 km from the subsurface location of 18/20-1, penetrated a 185 m gas column in fair to good quality sandstone. It tested dry gas at a stabilized rate of 62.87 × 106SCF/d on a 2″choke. Reprocessing of the seismic data, along with the successful drilling of the following well 18/25-1, confirmed the field to be a relatively simple anticlinal trap, with a complex faulted overburden that is structurally detached from the reservoir by the Mercia Halite. The source rock is assumed to be the Westphalian Coal Measures, similar to that encountered by well 27/5-1. The dry gas is consistent with a Type-III humic source rock. The reservoir is biostratigraphically barren, consisting of fluvial red bed sandstone, and is assumed to be of Triassic Sherwood Sandstone Group equivalent age. The unit is approximately 400 m thick, consisting of a high net-to-gross sequence of low-sinuosity braided fluvial channel sandstones with subordinate sand-flat and playa mudstone deposits. On a macro scale, it is a remarkably uniform sandstone with only subtle facies variations, but on a micro scale there is a significant variation in grainsize and cementation, which affects reservoir productivity. Mineralogically, there are distinct differences from the Sherwood Sandstone Group of the East Irish Sea, indicating that the Slyne Early Triassic system was depositionally similar to, but distinct and separate from, that to the east of Ireland. Dipmeter data indicated that the river system flowed from the SW to NE, essentially along the direction of the present-day Slyne Basin, suggesting that it was largely sourced from the south in the Variscan hinterland, but with evidence of sediment input from local highs such as the Connemara Massif.