Courtesy of ukooa
Oil and gas from west Shetland
Exploration of the basins to the west of Britain has been slower and more difficult than exploration in the North Sea.
The Faeroe-Shetland Basin began to develop in the Permian Period and continued to subside throughout the Mesozoic and Tertiary Eras. Oil and gas migrated from the Kimmeridge Clay source rock into a ridge of much older Devonian and Carboniferous rocks that separates the two basins. The Clair field is located on this ridge and contains a vast amount of oil. The oil is trapped in fractured Devonian and Carboniferous reservoir rocks. Much younger muddy sediments of the Upper Cretaceous cover the ridge and prevent the oil from escaping to the surface. The fractures and faults in the older rock allow the oil to rise easily to the highest part of the ridge. The fractures are only occasionally cut by a well so whether a well will produce a sufficient amount of oil to be profitable is very difficult to predict. The oil has been biodegraded by bacteria that were carried into the oilfield by water when the field was almost uncovered during the Cretaceous Period. As a result of the biodegradation the oil in the Clair field is a lot more viscous than normal North Sea oils and does not flow rapidly.
Exploration drilling continued in the Faeroe-Shetland Basin without significant success until BP discovered the Foinaven field in 1992 (F49), followed a year later by the Schiehallion field. These fields are 75 kms south-east of the Clair field and were discovered in much younger sandstones, that are similar in age to the Forties sandstone in the North Sea. The rivers that swept sands eastward into the North Sea, about 55 million years ago, were matched by smaller rivers carrying sands westwards onto the continental shelf. Earthquakes, along faults, repeatedly caused these sands to flow down into the deep waters along the Faeroe-Shetland Basin. Once in deep water, the sands were covered with mud to provide a seal and form a stratigraphic trap.
The present day water depth in the area is over 1,400 ft. Floating Production, Storage and Offloading (FPSO) vessels have been used in the development of the Foinaven and Schiehallion fields, while the giant Clair field is being developed with a fixed platform and a pipeline to an onshore terminal.
Foinaven was the first field in the world to have a fixed seismic array on the seafloor across the field. This will allow BP to monitor any changes to the seismic wave character throughout the life of the field and actually "see" the oil draining from the rock seismically. This will ensure all the sands are drained efficiently.