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 80,000 barrels a day 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.

British Gas discovered reserves of natural gas while drilling at Wytch  Farm, Corfe and from this sprang the large undertaking now run by B.P.  Amoco Ltd.

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. One well is the longest ever drilled at 8 Km.

Wytch continues to produce vast quantities of oil and gas, the former being piped out of the district underground, while 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.)  

Please use the link to Wytch Farm Field Trip 11.02.01 with Southampton Geology Field Study Group for an excellent report