Leader Dave Richards Chief Geologist BP Oil Field Wytch Farm accompanied by Matt Dunning Petrologist.

Report by Sheila & Bob Alderman

 A few OUGS Wessex members were able to join in with this trip which had a limit on numbers. If possible I plan to organise a further visit later in the year for others who are interested.

We met at the viewpoint at Godlington between Corfe Castle and Studland. From here we could see the extent of the oil field, underground it stretches across an area of 6 x 12 km; out to sea from Poole Harbour as far as Bournemouth pier. What was significant was there was no evidence at all of any industry in the oil field area. The harbour and surrounding area is a Heritage site with several SSSI’s. This has meant that BP has been required to be environmentally very sensitive and to take precautions not to allow pollution or to have an environmentally negative impact in the area. The outcome is that this private land is a haven for wildlife. There are rare species such as sand lizards and smooth snakes, Dartford Warblers and the even rarer Setis Warbler as well as the famous Red Squirrels on the Poole Harbour islands of Brownsea and Furzey where on the latter there is a BP wellsite.

After setting the scene we travelled by BP minibus to the Gathering Station. All the extraction and production sites are very well screened by woodland with the sites excavated so they are lower than the surrounding land. The sites have a concrete base with a waterproof seal. Some sites have a water moat, which is tested to ensure there is no contamination by runoff. The buildings are painted in what can best be described as "blend in brown". At the Gathering Station the fluids extracted from the reserves, (gas, water, LPGs and oil) are separated out. The oil is fractionated so more gas can be removed. The lighter "domestic" gas is piped away to Sopley, Christchurch after the smell is added. The heavier Butane and Propane is piped to the LPG storage area and then piped to Furzebrook, Wareham where it is taken in tankers by rail to Avonmouth. This leaves the oil and water. Oil is lighter than water so the two can be easily separated. The oil pipeline goes around Wareham, Poole, south of the New Forest, Beaulieu, below Southampton Water to Hamble. In the event of an emergency, such as a drop in pressure, the whole operation can be shut down immediately at the touch of a switch. Should this occur then any oil, gas or LPG at the Gathering Station can be diverted to vents to be burned off. This would be the only time when a major "flare" would occur across the whole site.

Wytch Farm site initially extracted oil from the Jurassic Bridport sandstone reservoir at 3000 feet below sea level. Extraction then extended to 5000 feet into the older Permian Sherwood Sandstone, which is the principle reservoir in the North Sea. More recently oil is being extracted at 2500 feet from the highly fossiliferous limestone of the Frome Clay formation. There is an annual production of 490 million barrels (35 gallons per barrel/40 gallons US) of oil from the Wytch Farm, Wareham and Kimmeridge fields. The Sherwood Reservoir generates 90% of the production. The reserves at Wytch Farm are projected to run out in 20 years time. In all 110 wells have been drilled, 75 of these are producing oil and 25 are injecting water.

At the injector wells water is pumped into the wells to keep up the pressure and displace the oil as oil is extracted. The water separated in processing the oil with additional seawater is injected. The natural salinity of the water in these reservoirs is very high; three times the salinity of seawater. This highly saline water has the potential of being more environmentally harmful that the oil in the event of a spill as it will soak into the ground unlike the oil. The high salinity is due to the geology of the Sherwood sandstone positioned above the Mercia Mudstones. These formed in a desert lake with subsequent high salt levels. In some locations there is a 30m thick layer of halite above the Sherwood, but not at Wytch Farm.

The oil source rock is the lower Jurassic Blue Lias. This reached at a depth of 2.5 km in the Portland Wight Basin. The source rock in the North Sea is the Kimmeridge formation. In Dorset this has not reached sufficient depth in this area for oil to be formed. Note that although there is a well site at Kimmeridge the source rock is Blue Lias, not Kimmeridge Clay. Interestingly all along the south coast of Dorset to East Devon all the oil producing source rocks and reservoir rocks and are exposed, so can be studied above ground.

The Sherwood Sandstone has a granitic source from a landmass that extended from the present day Brittany across the English Channel ending at around Sidmouth in Devon. Usually sandstone gives a low radioactivity count but because of its origins, but the Sherwood is unusually high. This radioactivity can coat the metalwork during the extraction so precautions have to be taken. The Bridport Sand reserves have a high hydrogen sulphide content. The gas evolved is trapped in the cellars that surround the wells to prevent direct leakage into the environment. The high iron content of the Sherwood Reservoirs would swiftly have broken down any hydrogen sulphide that may have been passed into it.

The second location visited was Wellsite "D". Usually there is little activity here, but we saw them repositioning a drill head above the Sherwood well. This part of the reservoir is nearly exhausted but research has indicated that further extraction of oil can be achieved by injection of fresh water instead of saline. This is being attempted here. Also at this site there is one injector well and two Kimmeridge Formation wells. Here the extraction uses relatively energy efficient but slow extraction of "Nodding Donkeys". Electric Submersible Pumps are used in the deeper and more prolific Sherwood reserves. This is a faster but uses more energy. Wytch Farm is second only to Heathrow Airport as a "single site" consumer of electricity in the UK.

After lunch we drove to the top of the Purbeck hill to Creech Hill to see the geology of the area. The chalk is inclined almost to the vertical here and in places is overturned. This occurred as a result the re-activication of faults in the Alpine Orogeny when the African and European plates collided. We were able to have an overview of the succession of the Portland Formation, Purbeck Formation, Wealdon, Upper Greensand and Chalk exposed by erosion along the coast at Worbarrow Bay. At Wytch Farm below the Tertiary sediments are Chalk, Upper Greensand, Gault Clay and then the Wealden, underlain by Oxfordian strata and the Kimmeridge Clays and Limestones. There is no evidence of the Portland or Purbeck. See cross section.

The views were tremendous and the explanation of the processes and Geology were excellent. Many thanks to Dave and Matt for their time and efforts on our behalf and to Jan Baker for organising the visit.