The geology and hydrocarbon prospectivity of the North Channel Basin

The geology and hydrocarbon prospectivity of the offshore North Channel has been evaluated, integrating data from a number of onshore wells with seismic, gravity,magnetic and onshore outcrop studies. Interpretation of these data suggest that the North Channel can be divided into two sub-basins, separated by the offshore extension of the Southern Uplands Fault Zone (SUFZ). The narrow, southern, Portpatrick sub-basin has a NNW-SSE trend and consists of a series of simple tilted fault-blocks which dip east, towards the basin-bounding Portpatrick Fault. The northern sub-basin widens considerably into the Larne-Firth of Clyde basin complex, which has a predominantly Caledonide (NE-SW) orientation and is an extension of the Scottish Midland Valley. This sub-basin is characterised by westerly dipping fault blocks, the reversal in basin polarity apparently accommodated along the SUFZ. The primary reservoir interval is expected to occur in the Lower Triassic Sherwood Sandstone Group, although secondary targets may exist in the Permian and Carboniferous. Shallow coring and seismic facies analysis suggests that the Mercia Mudstone Group subcrops Quaternary-Recent deposits at sea bed and overlies the Sherwood Sandstone. Halites and shales within the Mercia Mudstone Group should provide an effective regional top-seal, except in areas where the Sherwood Sandstone is close to the surface. Large tilted-fault block and rollover anticline traps have been identified on seismic data, although the Carboniferous source rock potential and burial history are poorly understood and exploration risk is high. Regional outcrop patterns and the compaction state of the Mesozoic strata, together with vitrinite measurements suggest the basin was uplifted in the early Tertiary. In common with the East Irish Sea, maturity was probably reached prior to the end of the Cretaceous. This implies that source rocks within the basin will not currently be generating hydrocarbons, having effectively been 'switched' off by Tertiary uplift. Charge mechanics are uncertain but must involve early pre-uplift migration or remigration.