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 1959 that a borehole at Kimmeridge showed that oil was seeping out of the Cornbrash - a layer of rock over 500m below the surface. In 1960 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 1998) 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. The history of Purbeck Oil began in 1936 with the first wells drilled at Broad Bench near Kimmeridge by D'Arcy Exploration. These wells were unsuccessful.

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    (Bridport Reservoir - 900M deep and Sherwood Reservoir - 1600M deep)

British Gas discovered reserves of natural gas while drilling at Wytch Farm, North of Corfe Castle in 1974 and from this sprang the large undertaking run by B.P.  Amoco Ltd and sold on to Perenco in May 2011. Production started in March 1979 at 6,000 barrels a day and in the 1980's the larger and deeper reservoirs below Poole Bay came into production. British Gas had sold out to B.P.in 1983/4.

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 July 1999. Extended Reach Drilling (ERD) has been used from a Rig on the Goathorn Peninsular This process allows oil under Poole Bay without the need to install unsightly rigs in the bay or go to the expense of creating an artificial island as was once proposed, but dropped and ERD used instead. A new world record for ERD was set in September 1995 when the drill reached a step-out distance of 8,035m and a depth pf 1611m. This record was to be superseded as drilling continued and a step distance of 10,728m has since been achieved and drilling goes on.

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 well sites were 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 produced about 300-400 barrels of oil a day which was taken by small diameter pipeline to the main gathering station about 10 kilometers away and was processed with oil from the Wytch Farm well sites. Production has stopped, but may restart.

The Bridport sandstone from which the Wareham oil was 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.