Hydrocarbon prospectivity of the Central Irish Sea Basin with reference to Block 42/12, offshore Ireland

Abstract

The hydrocarbon prospectivity of the offshore Central Irish Sea Basin has been assessed with particular reference to Block 42/12, which was formerly operated by Hydrocarbons Ireland Ltd (British Gas Exploration & Production). Data from the two wells drilled in 42/12, and other wells and boreholes in the area, have been integrated with seismic, gravity and magnetic data. Onshore outcrop and sub-surface information were also utilized. Interpretation of these data shows that the proven basin-fill consists of several megasequences bounded by major unconformities. At least three separate phases of extension are recorded, in the Carboniferous, Triassic and Jurassic. These were followed by post-rift, regional thermal subsidence, but the stratigraphy is further punctuated with evidence of compression and transpression associated with the Variscan, Cimmerian and Alpine inversion events. An assessment of hydrocarbon prospectivity indicates that the primary reservoir interval occurs in the syn-rift Lower Triassic Sherwood Sandstone Group. This is sealed by the overlying Middle-Upper Triassic Mercia Mudstone Group consisting of shales with major halite intervals. Westphalian coal-bearing facies, proven in 42/12-1 and 42/17-1, are potential gas-prone source rocks, which are late to overmature for oil generation to early mature for gas generation. Unproven Namurian marine source rocks may occur in the main basin depocentre. Burial history modelling suggests that source rocks probably generated hydrocarbons by the Early Cretaceous, with possible further generation prior to the main Tertiary inversion. Tilted fault-block and rollover anticline trapping structures were probably formed during Permian to Jurassic rifting, pre-dating the main phases of hydrocarbon expulsion. Exploration to date has centred on the southern margins of the basin. Future exploration of the deeper and potentially prospective parts of the basin in the north should resolve uncertainties concerning the possible presence of Namurian source rocks and Permian reservoir and seal facies, and the nature of a post-Triassic section identified on seismic data. The Central Irish Sea Basin is an under-explored Late Palaeozoic to Mesozoic basin which is stratigraphically analogous to the prolific East Irish Sea hydrocarbon province to the northeast. This paper presents the results and interpretations of past exploration activity and reviews these in terms of their impact on the tectono-stratigraphic evolution and future hydrocarbon prospectivity of the basin.

The geology and hydrocarbon prospectivity of the North Channel Basin

“…2). The seaway is characterized by a major bathymetric depression, the Beaufort's Dyke, which is a NW-SE linear glacial channel feature locally increasing water depths to in excess of 300 m. The underlying North Channel Basin is part of a related set of basins, including the EISB, which are remnants of a more extensive late Palaeozoic--midMesozoic basin complex evolving under the influence of the opening of the North Atlantic and pre-existing Caledonian and Variscan trends (McLean 1978;Jackson & Mulholland 1993;Shelton 1995;Maddox et al 1995).…”

“…In the analogous East Irish Sea Basin to the southeast, numerous shallow hydrocarbon accumulations have been discovered, and in-house British Gas studies suggest that significant hydrocarbon accumulations (e.g. South Morecambe Field) may be reservoired at sub-sea depths as shallow as 600-800 m (Maddox et al 1995). Another potential risk relates to the presence of a 'waste zone' between the top of the Sherwood Sandstone Group and the base of the lowest effective halite seal (cf.…”

“…Another potential risk relates to the presence of a 'waste zone' between the top of the Sherwood Sandstone Group and the base of the lowest effective halite seal (cf. Maddox et al 1995). This is characterized by thin sandstones which may act as 'thief' zones, causing leakage of hydrocarbons.…”

Fault distribution and timing in the Central Irish Sea Basin

“…2). In addition, interpretation of seismic data suggests that Jurassic and Cretaceous sediments could be locally preserved within the basin (Maddox et al 1995). Two deep seismic reflection profiles, WINCH-4 and SWAT-2, acquired across the CISB and St. George's Channel and Cardigan Bay basins (BIRPS & ECORS 1986) indicate the graben geometry of the CISB with a preserved basin-fill consisting dominantly of Permo-Triassic sediments (Tappin et al 1994).…”

“…Results and a synthesis of these wells have been provided in a number of recent studies, which have discussed the hydrocarbon prospectivity of the CISB (Maddox et al 1995 (Duncan et al 1998;Corcoran & Clayton 1999) and the structural evolution of the basin (Maingarm et al 1999). The aim of this paper is to provide an insight into the distribution and timing of faulting in the CISB, as recorded by the preserved Palaeozoic to Cenozoic sediments, and to discuss the implications of this tectonic activity for the hydrocarbon prospectivity of the basin.…”