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Hydrocarbon Habitat |
British Gas Exploration and Production Limited, 100 Thames Valley Park Drive, Reading, Berkshire, RG6 1PT, UK
Conoco Inc., 600 North Dairy Ashford, Houston, TX 77079, USA
The Wessex basins were formed during Mesozoic extension and were subsequently modified by Cretaceous uplift and Alpine inversion events. The basin geometry and evolution of the area is strongly controlled by the long-lived fault systems which cross the area.
Two primary plays are recognized within the Wessex basins; the Triassic Sherwood Sandstone and the Jurassic Bridport Sandstone. These plays contribute the vast majority of the recoverable reserves in the area. The Triassic play consists of a Sherwood Sandstone Group reservoir, a Liassic mudstone source rock and a Mercia Mudstone Group regional seal. Hydrocarbons were generated from the Late Jurassic to Tertiary in the main kitchen area and migrated into a range of fault-related traps. The main risks on the play are reservoir quality together with the timing and the route of hydrocarbon migration into valid traps.
The Jurassic play consists of the Bridport Sandstone Formation which is also sourced from the Liassic mudstones and the top seal is provided by the Fuller’s Earth Formation. The dominant risk on this play is the extent and quality of reservoir facies. In the onshore domain trap definition, using two-dimensional seismic data, is an additional risk on both plays.
The basins of southern England have been explored for hydrocarbons for over 50 years. The initial phase of exploration focused on the onshore and tested anticlinal structures which had been mapped at surface. Onshore activity reached a peak in the successful drilling by British Gas of the Jurassic and Triassic reservoirs in the Wytch Farm Field in 1973 and 1977, respectively.
The first offshore well in the United Kingdom Continental Shelf (UKCS) was drilled off Lulworth Cove by BP in 1963. More recently, offshore exploration concentrated on fault-related traps within or on the edges of the Portland-Wight Basin. Although offshore activity has been reduced in the last 5 years, the 14th UKCS Licensing Round rekindled interest in the area as companies were awarded previously unlicensed acreage for exploration.
Successful future exploration in the Wessex basins will require a more complete understanding of reservoir development and three-dimensional basin evolution. Future drilling will focus on untested fault bounded prospects and stratigraphic traps with suitable reservoir quality.
