| The Purbeck Mineral & Mining Museum |
| Oil - The three oilfields of Purbeck |
Kimmeridge oilfield
Small seepages of oil can be found in a variety of places in Purbeck, but their source remained a mystery for years. It was known that oil moves around or "migrates" within the rocks but it was not until the late 1950s that a borehole at Kimmeridge showed that oil was seeping out of the Cornbrash - a layer of rock over 500m below the surface. In 1959 the Kimmeridge Oil Well of British Petroleum started production and it can still be clearly seen on the top of the cliffs in the north end of the bay. The nodding donkey still pumps oils today at a rate of about 100 barrels a day - compare that to the the peak production of 110,000 barrels a day (reached in 1997) coming out of the Wytch Farm Oilfield!
It is probable that the Kimmeridge oil originates from even deeper, in the main oil-bearing strata that feed the Wytch Farm oilfield. These strata extend far out to sea and a number of oil companies have started to explore the sea-bed off the Dorset coast in recent years in an attempt to discover the extent of these reserves.
The oil-shale, or "Kimmeridge Coal" that has been won from the cliffs to the east of Kimmeridge since the early 17th century has nothing to do with the "free oil" being pumped by the nodding donkey. The "Coal" is actually a shaly bituminous stone that burns with a bright flame, gives off an offensive smell and leaves copious quantities of grey ash. It is no longer used commercially.
Wytch Farm oilfield
British
Gas discovered
reserves of natural gas while drilling at Wytch Farm,
BP
has exploited the oil and gas reserves in the Wytch and Poole Harbour areas to an amazing degree, which resulted in the building of a large
processing plant at Wytch and a new access road. Co-operation between the
company and the planners has resulted in minimum disturbance to the
countryside and yet to produce Europe's largest onshore oil field (It is known
as the Hidden Oilfield). One well is
the longest ever drilled at over 10km in June 1999.
The three main reservoirs are in the Frome Clay limestone (800m depth), Bridport sands (924m depth) and the Sherwood sandstone (1.5km depth) The light/sweet crude oil contains very little Hydrogen Sulphide, so it has a high value and some is exported.
Total estimated recoverable reserves are 480 million barrels, of which over 90% lie in the Sherwood reservoir making it the sixth largest in the UK.
Wytch
continues to produce vast quantities of oil and gas, the being piped to a
terminal at Hamble on Southampton Water and the gas is piped to Sopley where it
joins the British Gas National Grid. Some gas leaves in lorries. (This previously left
in long trains of pressurised
containers via part of the old railway line which formed the Swanage branch but
with a reduction in the gas being produced is now no longer economic.)
Wytch Farm oil field is the biggest user of Electricity in Southern England after Heathrow Airport.
For an excellent report see this Wytch Farm Field Trip 11.02.01 with Southampton Geology Field Study Group
Wareham oilfield
This underlies the town of Wareham. Two producing wellsites are located to the west of the town at Worget.
Wareham is a separate oilfield from Wytch Farm. It was discovered in 1964 but was not brought into production until 1991. It currently produces about 300-400 barrels of oil a day which is taken by small diameter pipeline to the main gathering station about 10 kilometers away and processed with oil from the Wytch Farm well sites.
The Bridport sandstone from which the Wareham oil is produced, contains reserves of about six million barrels but is not physically connected to the Wytch Farm oilfield's Bridport reservoir. In terms of onshore oilfields in the UK, Wareham is quite big but the scale of its larger neighbour has overshadowed its significance.