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Petroleum Geology and Reservoirs of the Wessex Basin, Southern England

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1 Petroleum Geology and Reservoirs of the Wessex Basin, Southern England
Petrox 2004 Author: Ole Torsæter, NTNU Assistant producers: Nils Arne Øksenvåg, Bjørn Arild Mythen and Per Olav Paulsen Petroleum Geology and Reservoirs of the Wessex Basin, Southern England

2 What is Petrox? Introduction The Wessex Basin The Localities Summary
This interdisciplinary course is based on lectures and field work in southern UK and group project work. It integrates studies in petroleum geology, sedimentology, structural history, and reservoir characterization. The course is assessed by a 5 day field based reservoir study of sandstone and chalk undertaken by groups of 4-6 students, which will simulate work undertaken in exploration of hydrocarbon reservoirs. The groups will give oral presentations about their work and submit a written report. Lectures and analytical data. The lectures during the field work provide a stratigraphical, sedimentological and structural background to the field study area, plus an introduction to local petroleum geology. Geophysical and geochemical analysis of source and reservoir samples from the field area will be undertaken. Petroleum and reservoir geology of the Dorset oilfield, UK. This 5 day field course builds upon preceding lecture and practical laboratory work, and provides a structural and sedimentological context to oil habitat. 1. Examine Jurassic source rocks in the field (Blue Lias, Kimmeridge Fm) in the light of analytical data (TOC, hydrogen index, kerogen typing, maturation) and demonstrate how palaeoenvironment and burial history control source rock quality. 2. Characterize important reservoir units (Sherwood Sandstone, Bridport Sands, Cornbrash) using sedimentology, porosity and permeability data and evidence of diagenetic modification 3. Understand hydrocarbon migration history in the Wessex Basin in the context of the structural evolution of the region (Jurassic-Early extension, Late Cretaceous-Tertiary inversion), utilizing field, seismic and well-log data. 4. Use a visit to the Wytch Farm BP oilfield to show how geological understanding and evolving technology have contributed to exploration of and production from the field, and how conservation is a priority in this environmentally sensitive region. Characterization of Chalk reservoirs. The concluding part of the course will be undertaken by groups (4-6), working as teams, based on the chalk cliffs at Eastbourne and Brighton. It will involve: 1. Detailed sedimentological logging, to identify sedimentary facies (hardgrounds, redeposited chalks, pelagic chalks) and identify quality reservoir units. 2. Geophysical logging, using micropermeters and spectral gamma meters. 3. Fracture characterization (spacing and orientation), and estimation of water flow rates through fractures. 4. The data collected by each group will form the basis of a reasoned assessment of reservoir potential, suitable production methods and problems which are likely to be encountered. Evolution Stratigraphic Structure geology Source rocks Reservoir rocks The Localities Overview map Locality 1 Locality 2 Locality 3 Locality 4 Locality 5 Locality 6 Locality 7 Locality 8 Locality 9 Locality 10 Summary Questions

3 Introduction to the Wessex Basin
Evolution The Wessex Basin is one of a system of linked Mesozoic basins that cross southern England, the Channel and Northern France. The Wessex Basin covers more than 20,000 km2 of southern England, principally within the counties of Dorset and Hampshire, and extends offshore into the English Channel. It is a significant petroleum province, containing a fairly complete sequence of Permian to Cretaceous sediments, within which are source rocks for petroleum generation, reservoir rocks in which oil may accumulate, and seals (or caprocks) to keep oil accumulations in place. The complex tectonic history of the basin has allowed burial of source rocks into the zone of oil generation, and also led to the formation of structures (traps) for oil accumulation. The basin contains Wytch Farm, the largest known onshore oilfield in western Europe, and larger than most UK offshore fields. Two much smaller fields (Wareham and Kimmeridge) also produce oil in the basin, and a series of oil seepages are known in outcrops on the Dorset coast. Exploration of new prospects is ongoing in the basin. Stratigraphic Structure geology Source rocks Reservoir rocks Click on map to expand The Localities Overview map Locality 1 Locality 2 Locality 3 Locality 4 Locality 5 Locality 6 Locality 7 Locality 8 Locality 9 Locality 10 Figure: Surface geology of the Wessex Basin, Weald Basin and the English Channel. The position of WytchFarm Oil field  is marked by the red box Summary Questions

4 Evolution of the basin:
Introduction Evolution of the basin: The Wessex Basin Evolution Stratigraphic Structure geology There were four distinct phases. 1.Permo-Triassic: A period of continental desert sedimentation, initially simultaneously with igneous activity and rifting. 2.Shallow marine sedimentation in the Jurassic with exposure at the end of the period leading to continental deposition. The sediments were affected by syn-sedimentary extensional faulting trending east-west, and causing Southern Dorset to be down-thrown to the south. 3.Further faulting, eastwards tilting and erosion followed by stability and marine chalk sedimentation in Late Cr. Tectonic inversion occurred in Early Tertiary leading to down-thrown to the north and this controlled the deposition of Lower Tertiary fluvial and shallow marine sediments. 4.The final stage of the basin evaluation was a culmination of the reversed movement causing sharp flexuring down to the north. Source rocks Reservoir rocks The Localities Overview map Locality 1 Locality 2 Locality 3 Locality 4 Locality 5 Locality 6 Locality 7 Locality 8 Locality 9 Locality 10 Summary Questions

5 Stratigraphic summary of the basin
Introduction Stratigraphic summary of the basin The Wessex Basin The Wessex Basin The Jurassic Strata of West Dorset (Lower and Middle Jurassic – Lithological Units) Evolution Stratigraphic Structure geology Source rocks Reservoir rocks Permian – fluvial and aeolian sands and red mudstones. Braided stream deposits, sheet-flood conglomerates, mudflow breccias and massive local derived breccias. The general provenance was from the west. Triassic – mainly sandy in the lower part and more clay towareds the top. The deposits are mainly fluvial. The Localities Overview map Locality 1 Locality 2 Locality 3 Locality 4 Locality 5 Locality 6 Locality 7 Locality 8 Locality 9 Locality 10 The basement rocks is of Devonian and Carboniferous geosynclinals deposits. The beds were strongly folded and locally metamorphosed. Summary Questions (click on the green box to get more details and click a second time to remove it)

6 Structure geology of the basin
Introduction Structure geology of the basin The Wessex Basin Evolution Stratigraphic Structure geology At the surface the Wessex Basin, including the extensions offshore into the English Channel, appears to comprice wide areas of very gentle dips and layer-cake geology separated by narrow belts of monoclinal flexuring. It can be with or without faulting and where the dips become steep to vertical. Small anticlinal folds may be found adjacent to the lines of disturbance. These structural lines are mainly east-west in southern England, but they form part of an interconnected system which crosses the Channel into northern France: They give the impression of lines of relative movement between the intervenig stable regions, and clearly represents a basement influence on the sedimentary layers, probably reactivating earlier features. Source rocks Reservoir rocks The Localities Overview map Locality 1 Locality 2 Locality 3 Locality 4 Locality 5 Locality 6 Locality 7 Locality 8 Locality 9 Locality 10 Summary Questions

7 Source rocks in the Wessex Basin
Introduction Source rocks in the Wessex Basin The Wessex Basin Evolution Stratigraphic Structure geology The Jurassic contains three potential source rock intervals: 1.Kimmeridge Clay 2.Oxford Clay 3.Lower Lias The Kimmeridge Clay is well exposed in and around Kimmeridge Bay, where it approaches 500m in thickness, and mostly comprises oil-prone source rock facies. The Oxford Clay is exposed in Furzy Cliff to the east of Weymouth, but is not easily accessible for study.  The Lower Lias, including the Blue Lias, Black Ven Marls and Belemnite Marls (all of which contain oil-prone organic-rich source rock facies) are exposed in the Lyme Regis area. This section has also been drilled by a borehole near Weymouth. Source rocks Reservoir rocks The Localities Overview map Locality 1 Locality 2 Locality 3 Locality 4 Locality 5 Locality 6 Locality 7 Locality 8 Locality 9 Locality 10 Summary Questions

8 Reservoir rocks Introduction The Wessex Basin
Evolution The three reservoirs in the Wytch Farm oil field are: The Frome is a complex reservoir from the middle Jurassic, consisting of isolated accumulation of oyster shells surrounded by mudstone. The Bridport is a 60m thick pile of fine grained sandstones that were deposited near the shore of a shallow sea in the Early Jurassic. The Sherwood reservoir consists of a number of sandstones stacked on top of one another to a thickness of about 120m, laid down by rivers in the Triassic period in an environment similar to the interior of Australia today. 90% of the recoverable reserves lie in the Sherwood reservoir. Stratigraphic Structure geology Source rocks Reservoir rocks The Localities Overview map Locality 1 Locality 2 Locality 3 Locality 4 Locality 5 Locality 6 Locality 7 On the field trip we will examine the Bridport and the Sherwood reservoirs. Locality 8 Locality 9 Locality 10 Summary Questions

9 An overview map of the localities
Introduction An overview map of the localities The Wessex Basin Evolution The localities we are visiting during Petrox is shown by the map below. A more detail explanation of each locality becomes available when you click on the ”locality icon”. Stratigraphic Structure geology (click on the green box to get a more detailed map of the Weymouth area) Source rocks Reservoir rocks The Localities L 2 L 1 L 3 L 10 Overview map L 9 Locality 1 Locality 2 Locality 3 Locality 4 Locality 5 L 8 Locality 6 L 7 L 5 Locality 7 L 6 Locality 8 Locality 9 L 4 Locality 10 Summary Questions

10 Locality 1: Sidmouth- The Sherwood Sandstone Group
Introduction Locality 1: Sidmouth- The Sherwood Sandstone Group The Wessex Basin Evolution Stratigraphic The Sherwood Sandstone Group is divided into the early Budleigh Salterton Pebble beds, which is a  m thick formation of conglomerates and sandstones, which is not seen at Sidmouth and the Otter Sandstone Formation, which consists of fluvio-aeolian deposits and is about 120m thick. The sequence is broadly upward fining with increasing mud content. This unit is a reservoir rock in the subsurface about 100 km to the east within the Wytch Farm oil field. The lower part of the cliff is a current-bedded fine to medium grained sandstone with isolated lenses of red claystone and abundant clay pebble conglomerates. The enviroment during deposition was proximal braided alluvial plain and the depositions are channel and point bar. The thin claystones are abandoned river channels. At the top of the cliff the sandstone is cleaner and more uniform of aeolian origin. Structure geology Source rocks Reservoir rocks The Localities Overview map Locality 1 Locality 2 Locality 3 Locality 4 Locality 5 Locality 6 Locality 7 Locality 8 Locality 9 Locality 10 Summary Questions

11 Locality 1: Sidmouth- The Sherwood Sandstone Group
Introduction Locality 1: Sidmouth- The Sherwood Sandstone Group The Wessex Basin Sample nr. Porosity φ Permability (mD) 1.1 36.20% 1.2 10.60% 1 - 2 1.3 8.70% Evolution Stratigraphic Structure geology Source rocks Reservoir rocks Sample 1.1: Fine sand, crossbedded Sample 1.2: Coarse sandstone, base channel Sample 1.3: Silty sand, top channel The Localities Overview map Locality 1 Locality 2 Sample1.1 Core samples info Back to the Map Locality 3 Locality 4 Locality 5 Locality 6 Locality 7 Locality 8 Locality 9 Locality 10 Summary Questions Sample1.3 Sample1.2

12 Locality 2: Lyme Regis – west cliff, Early Jurassic Blue Lias (source)
Introduction Locality 2: Lyme Regis – west cliff, Early Jurassic Blue Lias (source) The Wessex Basin The general pattern is that on the hill tops there is weathered Cretaceous Upper Greensand (brownish green) with much chert and with some Chalk (of about 100million years old). These units lie unconformably on grey Liassic (Lower Jurassic) marine clays (of about 150 million years old) with ammonites, belemnites and, occasionally, ichthyosaurs and plesiosaurs. The lower part of the cliffs in Chippel Bay is steep and consists of alternating dark grey shales and light grey, argillaceous limestones (cementstones). This is the Blue Lias. The Shales-with-Beef form an inclined crumbly cliff above. Higher up the cliff slopes back and consists of shale. Here are the Shales-with- Beef (fibrous calcite - cone-in-cone), the most organic-rich part of the sequence. Beef is a common indicator of organic-rich shales in other sequences (such as the Purbeck). The bituminous shales form part of the source rocks for the oil of the huge Wytch Farm oilfield. Total organic carbon in the Liassic bituminous shale beds (these are usually thinly-laminated - paper shales) is usually about 6 weight per cent. In argillaceous limestone beds, such as those of the Blue Lias, it is lower, round about 0.5 weight per cent . The organic matter is in the state of kerogen, a microscopic brown waxy substance dispersed through the shale. The kerogen is of sapropel origin (Type II - liptinic), a mixed type consisting of marine algal plankton with some zooplankton (microscopic marine animals). It can, and has, produced both oil and gas if buried for a long period at a few kilometers down where the temperature is round about 100 degrees centigrade. Here, at Lyme Regis, it has not been buried deeply enough and it is not thermally mature. Deeper basins, however, occur to the south and south-east under the sea, and there it is mature. Evolution Stratigraphic Structure geology Source rocks Reservoir rocks The Localities Overview map Locality 1 Locality 2 Locality 3 Locality 4 Locality 5 Locality 6 Locality 7 Locality 8 Locality 9 Locality 10 Summary Questions

13 Locality 2: Lyme Regis – west cliff, Early Jurassic Blue Lias (source)
Introduction Locality 2: Lyme Regis – west cliff, Early Jurassic Blue Lias (source) The Wessex Basin Evolution Stratigraphic Structure geology Source rocks Reservoir rocks The Localities Sample 2.3 Sample 2.1: Laminated dark shale Sample 2.2: Laminated dark shale Sample 2.3: Laminated dark shale Overview map Locality 1 Locality 2 Sample 2.2 Locality 3 Locality 4 Core samples info Back to the Map Sample 2.1 Locality 5 Locality 6 Locality 7 Sample nr. S1 (mg/g rock) S2 (mg/g rock) TOC (wt%) HI (mg/g TOC) PI (S1/(S1+S2) PP (mg/g rock) Tmax (°C) 2.1 0.58 33.67 6.51 517 0.02 34.25 416 2.2 0.89 40.47 8.01 505 41.36 418 2.3 0.99 38.09 8.00 476 0.03 39.08 Comments: HI =Hydrogen Index (S2*100/(TOC) PI =Production Index (weight ratio) PP =Petroleum Potential (S1 + S2) Tmax =Temperature at maximum of S2 peak Locality 8 Locality 9 Locality 10 Summary Questions Sample 2.2 Sample 2.1

14 Locality 3: The Bridport Sands reservoir at Burton Bradstock
Introduction Locality 3: The Bridport Sands reservoir at Burton Bradstock The Wessex Basin Evolution Stratigraphic Structure geology At Burton Bradstock the Bridport Sands are clearly shown. These sands are from the Upper Liassic and form the higher reservoir at Wytch Farm oilfield. The sands are very fine to fine grained, well sorted and the clay content is low. The prominent ledges in the cliff are formed by calcite-cemented bands. The rock is generally massive but at some places current-bedded and channeled deposits can be seen. The Bridport Sands are overlain by inferior Oolite. This limestone is not accessible in the cliff but can usually be seen as rock falls on the beach. The limestone is usually bluish, grey in color and can contain large brown limonitic concretions known as ”snuff boxes” . Other things that might be observed is marl with limestone partings crowded with sponge and limonitic ooids. Source rocks Reservoir rocks The Localities Overview map Locality 1 Locality 2 Locality 3 Locality 4 Locality 5 Locality 6 Locality 7 Locality 8 Locality 9 Locality 10 Summary Questions

15 Locality 3: The Bridport Sands reservoir at Burton Bradstock
Introduction Locality 3: The Bridport Sands reservoir at Burton Bradstock The Wessex Basin Evolution Sample 3.1: Soft sands (fine) Sample 3.2: Soft sands (fine) Sample 3.3: Quaternary layer Stratigraphic Structure geology Core samples info Back to the Map Source rocks Reservoir rocks The Localities Sample nr. S1 (mg/g rock) S2 (mg/g rock) TOC (wt%) HI (mg/g TOC) PI (S1/(S1+S2) PP (mg/g rock) Tmax (°C) Porosity φ Permability (mD) 3.1 0.01 0.18 1.00 ndp - 3.2 28.10% 3.3 14.80% 0.08 Comments: HI =Hydrogen Index (S2*100/(TOC) PI =Production Index (weight ratio) PP =Petroleum Potential (S1 + S2) Tmax =Temperature at maximum of S2 peak Overview map Locality 1 Locality 2 Locality 3 Locality 4 Locality 5 Locality 6 Locality 7 Locality 8 Locality 9 Locality 10 Summary Questions Sample 3.1 Sample 3.2 Sample 3.3

16 Locality 4: Isle of Portland
Introduction Locality 4: Isle of Portland The Wessex Basin Simplified geological map of the Isle of Portland The Isle of Portland is mostly composed of Upper Jurassic marine strata with a small thickness of basal Cretaceous Purbeck Formation. The Upper Jurassic, Kimmeridge Clay occurs at certain places. On top of this lies the Portland Sand, which is largely marls with some sandy horizons. On the Portland Sand lies the Portland Stone consisting of the Portland Cherty Series overlain by the Portland Freestone, mostly oolitic limestone. Over this is the Purbeck Formation which is thin bedded limestone and shale, and this formation was deposited in the Jurassic and basal Cretaceous. This section from the Kimmeridge clay to the Purbeck Formation is the result of a regression occurring in Jurassic. The Kimmeridge clay is a deep water deposit, and as one comes into the Portland Sands and the Portland Stone the environment is getting shallower ending in the Purbeck Formation. In the Portland Cherty Series one might observe a small transgression before the large regression continues in the Portland Freestone. Evolution Stratigraphic Structure geology Source rocks Reservoir rocks The Localities Overview map Locality 1 Locality 2 Locality 3 Locality 4 Locality 5 Locality 6 Locality 7 Locality 8 Locality 9 Locality 10 Summary Questions Back to the Map

17 Locality 5: Oil seeps; Osmington Mills
Introduction Locality 5: Oil seeps; Osmington Mills The Wessex Basin Evolution Stratigraphic Structure geology Source rocks Sample 4.3 Reservoir rocks Sample 4.1: Hard layer seeping oil Sample 4.2: Soft cross-bedded fine sand Sample 4.3: Fine sand smelling of oil, top The Localities Sample 4.1 Overview map Locality 1 Locality 2 Locality 3 Core samples info Back to the Map Sample 4.2 Locality 4 Locality 5 Locality 6 Locality 7 Sample nr. S1 (mg/g rock) S2 (mg/g rock) TOC (wt%) HI (mg/g TOC) PI (S1/(S1+S2) PP (mg/g rock) Tmax (°C) Porosity φ Permability (mD) 4.1 33.73 40.52 9.33 434 0.45 74.25 416 - 4.2 10.47 14.07 3.13 450 0.43 24.54 419 4.3 20.30% Comments: HI =Hydrogen Index (S2*100/(TOC) PI =Production Index (weight ratio) PP =Petroleum Potential (S1 + S2) Tmax =Temperature at maximum of S2 peak Locality 8 Locality 9 Locality 10 Summary Questions Sample 4.1 Sample 4.1

18 Locality 6: Lulworth Cove and Mupe Bay
Introduction Locality 6: Lulworth Cove and Mupe Bay The Wessex Basin Evolution Lulworth Cove In this cove one can observe strata from Cretaceous and also a small portion of the upper Jurassic Portland Stone. The strata from Cretaceous is Purbeck Formation, Wealden and Chalk. The Purbeck Formation is a thin bedded limestone and shale and the formation can be divided into three parts: Upper: Here the shale is pyritic, glauconitic and also include siderite. The limestone is called Purbeck marble and is a gastropod limestone. The environment under deposition was a freshwater lagoon. Middle: Also here it is shale and limestone but the environment was different than for the upper part. Here the lagoon was in contact with the sea, and this is observed in the change in fossils found here. Lower: The lagoon did not have contact with the sea but it was hyper saline. The deposits are also here limestone and shales, but in addition there is some marl and marlstone. The limestone might also be dolerite. Wealden is a fluvial deposition with marls, shale and sandstone. There are channels with conglomerate at the channel floor and point bars with crossbedding. These channels are incased in shale. On top of the Wealden Formation is a big layer of chalk. In the upper part of this chalk it includes black flint. Stratigraphic Structure geology Source rocks Reservoir rocks The Localities Overview map Locality 1 Locality 2 Locality 3 Locality 4 Locality 5 Locality 6 Locality 7 Locality 8 Locality 9 Locality 10 Summary Questions

19 Locality 6: Lulworth Cove and Mupe Bay
Introduction Locality 6: Lulworth Cove and Mupe Bay The Wessex Basin Mupe Bay Here the formations are the same as in Lulworth Cove except here the Portland Stone is not seen. The layers are generally thicker here than in Luworth Cove, but the interesting observation is the oilsands in the Wealden formation. The conglomerates at the channel floors are oil-saturated and oil seeps are visible on outcrop. It is interesting to notice that the clasts are impregnated with dead oil while the sand itself contain live oil. This means that the clasts originate from an older reservoir which has been eroded and the clasts is redeposited in a new reservoir. Sample nr. S1 (mg/g rock) S2 (mg/g rock) TOC (wt%) HI (mg/g TOC) PI (S1/(S1+S2) PP (mg/g rock) Tmax (°C) Porosity φ Permability (mD) 5.1 5.27 7.32 2.05 357 0.42 12.59 420 26.40% 5.2 15.36 25.62 7.04 364 0.37 40.98 419 - 5.3 5.68 9.81 2.25 436 15.49 424 Comments: HI =Hydrogen Index (S2*100/(TOC) PI =Production Index (weight ratio) PP =Petroleum Potential (S1 + S2) Tmax =Temperature at maximum of S2 peak Evolution Stratigraphic Structure geology Source rocks Reservoir rocks The Localities Overview map Locality 1 Locality 2 Locality 3 Locality 4 Locality 5 Locality 6 Core samples info Back to the Map Locality 7 Locality 8 Locality 9 Locality 10 Summary Questions Sample 5.3 Sample 5.3 Sample 5.2 Sample 5.2 Sample 5.1

20 Locality 7: The Kimmeridge clay and the Kimmeridge oilfield
Introduction Locality 7: The Kimmeridge clay and the Kimmeridge oilfield The Wessex Basin Kimmeridge is an excellent and exciting coastal cliff locality for the geologist. Here is the type section for the Kimmeridge Clay Formation, of Upper Jurassic age, the source rock for oil in the North Sea. It reveals much about Jurassic sea environments, about ichthyosaurs, plesiosaurs and other vertebrates. Important vertebrate discoveries have been made, particularly in the bituminous shales which favour the preservation of bones. Features of interest include the abundant ammonites, mostly but not always crushed, much-discussed cycles of sedimentation, diagenetic dolomite beds and the "Kimmeridge Coal" or "Blackstone" is an oil-shale much mined in the past and used for fuel, but also carved since Roman times like a type of jet. The oil shale occasionally burns in cliff-fires. Further interest is provided by an oil well on the top of the cliffs and which is pumping oil from Middle Jurassic strata beneath. Evolution Stratigraphic Structure geology Source rocks Reservoir rocks The Localities Overview map Locality 1 Sample 6.3 Locality 2 Locality 3 Locality 4 Locality 5 Sample 6.2 Locality 6 Locality 7 Sample 6.1 Sample nr. S1 (mg/g rock) S2 (mg/g rock) TOC (wt%) HI (mg/g TOC) PI (S1/(S1+S2) PP (mg/g rock) Tmax (°C) Porosity φ Permability (mD) 6.1 1.77 46.81 9.48 494 0.04 48.58 420 10.00% 0.01 6.2 1.26 32.01 6.24 513 33.27 421 - 6.3 5.68 9.81 2.25 436 0.37 15.49 424 Comments: HI =Hydrogen Index (S2*100/(TOC) PI =Production Index (weight ratio) PP =Petroleum Potential (S1 + S2) Tmax =Temperature at maximum of S2 peak Locality 8 Locality 9 Locality 10 Summary Questions Core samples info Back to the Map

21 Locality 7: The Kimmeridge clay and the Kimmeridge oilfield
Introduction Locality 7: The Kimmeridge clay and the Kimmeridge oilfield The Wessex Basin The Kimmeridge oilfield: Small quantities of oil (about 65 barrels a day) are still being produced from a well site on the cliffs at Kimmeridge a few miles west of Swanage. This accumulation is located in a license granted solely to BP many years before Wytch Farm was discovered and production began in Oil from middle Jurassic strata 500m down in the ground. To date, only the small Kimmeridge oilfield, which is situated in the core of a periclinal fold created in response to structural inversion, suggests that any hydrocarbon remigration into the younger structural inversion structures has taken place. Reservoir: Cornbrash, fossiliferous limestone ( m deep) Source: Lower Lias, Blue Lias A faulting has caused a remigration from the Bridport Sands to the Corntrash The oil is taken by road tanker to the rail terminal at Furzebrook, near Wareham, where it is stored in tanks and taken by pipeline in batches to the main gathering station. Evolution Stratigraphic Structure geology Source rocks Reservoir rocks The Localities Overview map Locality 1 Locality 2 Locality 3 Locality 4 Locality 5 Locality 6 Locality 7 Locality 8 Locality 9 Locality 10 Summary Questions Back to the Map

22 Locality 8: Wytch Farm Introduction The Wessex Basin The Localities
Wytch Farm site initially extracted oil from the Jurassic Bridport sandstone reservoir at 900 m below sea level. Extraction then extended to 1500 m into the older Permian Sherwood Sandstone, which is the principle reservoir in the North Sea. More recently oil is being extracted at 750 m from the highly fossiliferous limestone of the Frome Clay formation. There is an annual production of 490 million barrels 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. Evolution Stratigraphic Structure geology Source rocks Reservoir rocks The Localities Overview map Locality 1 Locality 2 Locality 3 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 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. Locality 4 Locality 5 Locality 6 Locality 7 Locality 8 Locality 9 Locality 10 Summary Questions

23 Locality 8: Wytch Farm field characteristics
Introduction Locality 8: Wytch Farm field characteristics Reservoir rock: Bridport: Jurassic sandstone 924m below surface at 40°C, 100 bar 600 tons of LPG Export pipeline: 16-inch, 91km line to Hamble oil terminal Total number of wells: 29 injection and 74 producing Well sites: 8 mainland sites, plus 2 on Furzey Island Seawater treatment capacity: 85,000 bpd from Cleavel Point pumping station on the mainland plus a maximum of 80,000 bpd from L site, on Furzey Sherwood: Triassic sandstone 1585m below surface at 65°C, 165 bar Frome: Clay rich, shelly limestone formed from ancient oyster beds 750m below surface Reserves: 500 million barrels Peak production: 110,000 bpd in 1997 Current production (1999): 90,000 barrels a day of oil 16 mmscfd of gas Island Operations workforce: approx 150 Area occupied by development sites: 105 acres (approx) Area under land management by BP: over 310 acres including 64 acre conservation area Trees: 100,000 trees and shrubs planted around oilfield and 32,000 on pipeline route Distance from Aberdeen: 880 kilometers Participants: BP, ARCO, Premier, Kerr McGee, ONEPM, Tal The Wessex Basin Evolution Stratigraphic Structure geology Source rocks Reservoir rocks The Localities Overview map Locality 1 Locality 2 Locality 3 Locality 4 Wytch Farm Locality 5 Locality 6 Locality 7 Locality 8 Locality 9 Locality 10 Summary Questions Back to the Map

24 Locality 9: Isle of Wight; Redcliff – Culver Cliff
Introduction Locality 9: Isle of Wight; Redcliff – Culver Cliff The Wessex Basin Evolution Stratigraphic Structure geology On top of the Wealden Group seen in Lulworth Cove and Mupe Bay lie the Lower Greensand Group, the Gault, the Upper Greensand Group and the Chalk. The figure below shows how these formations is seen from Redcliff towards Culver Cliff. Source rocks Reservoir rocks The Localities Overview map Locality 1 Locality 2 Locality 3 Locality 4 Locality 5 Locality 6 Locality 7 Locality 8 Locality 9 Locality 10 Summary Questions

25 Locality 9: Isle of Wight; Redcliff – Culver Cliff
Introduction Locality 9: Isle of Wight; Redcliff – Culver Cliff The Wessex Basin The Lower Greensand Group is divided into four formations: 1.The Carstone 2.The Sandrock 3.The Ferruginous Sands 4.The Atherfield Clay The Atherfield Clay is right above the Wealden Group and consists of grey-brown clay and silt. At the base is a coarse sand containing glauconite, corals and Jurassic fossils. This coarse sand is a result from a transgression. This part has a thickness of about 90 m. The Ferruginous Sands consists of sandstone and clayey sands of different color, mostly yellow, brown and red-brown. Glauconite, gravel and pebbles of phosphate can be observed and the Sands is 80 m thick. The Sandrock is a eustarine deposition. It is fine sands yellow, white and brown in color, and at the base there is a grey-blue sandy shale. The Sandrock is 27 m thick. The Carstone is a coarse, poorly sorted sandstone. It is red-brown in color and the thickness is 20 m. On top of the Lower Greensand Group is The Gault Clay which is a blue-black mudstone about 30 m thick. It contains pyrite and phosohate concretions. Above this is The Upper Greensand Group which consists of fine, pale grey-green sandstone and siltstone. They are crossbedded and bioturbated, and contain carbonate semented concretions. The thickness is 30 m. Over this is The Chalk which can be divided into two parts: Lower Chalk Plenus Marls Grey Chalk Chalk Marl Glauconitic Marl Upper Chalk As one can see the Lower Chalk is divided into four parts. The thickness for the entire Lower Chalk is about 60 m. Plenus Marls consists of blue-grey marls and pale grey limestone. Grey Chalk is a grey-white limestone. Chalk Marl consists of mottled blue-grey limestone and marl. Glauconitic Marl is a blue-grey sandy marl. The Upper Chalk consists of the White Chalk. Here the amount of clay is smaller than for The Lower Chalk and it contain hard nodules of limestone and bands of black flint. Its is m thick. Evolution Stratigraphic Structure geology Source rocks Reservoir rocks The Localities Overview map Locality 1 Locality 2 Locality 3 Locality 4 Locality 5 Locality 6 Locality 7 Locality 8 Locality 9 Locality 10 Summary Questions

26 Locality 10: Isle of Wight; Whitecliff Bay
Introduction Locality 10: Isle of Wight; Whitecliff Bay The Wessex Basin Evolution Stratigraphic Whitecliff Bay is protected by a headland of Chalk cliffs in the south, but it is notable as probably the best exposure in southern England for Tertiary, Palaeogene, strata which are seen to the northeast of this. The cliffs of soft sands and clays provide one of the most important sections of Europe. Locally, they form a key, reference section for the younger strata of the Hampshire Basin. Most of the beds are vertical or steeply dipping and thus a large stratigraphical sequence is seen in a short geographical distance. In little more than a kilometer of coastline, about 500m of late Palaeocene to late Eocene clays and sands are well-exposed. Very fossiliferous sediments are present, particularly in the marine London Clay, the marine Bracklesham Group and the lacustrine and lagoonal Solent Group, which includes the Headon Hill Formation and the Bembridge Limestone and Bembridge Marls. Structure geology Source rocks Reservoir rocks The Localities Overview map Locality 1 Locality 2 Locality 3 Locality 4 Locality 5 Locality 6 Locality 7 Locality 8 Locality 9 Locality 10 Summary Questions Back to the Map Click on map to expand and get a illustration of the geological cliff section

27 Summary Questions Introduction The Wessex Basin
Evolution Stratigraphic Structure geology What is the source rock of Wytch Farm Oilfield? What is the source rock of Kimmeridge Oilfield? What is the reservoir rocks of Wytch Farm Oilfield? What is the reservoir rock of Kimmeridge Oilfield? From what time periods are the rocks we see during the excursion? What are the three potential source rocks in the Wessex Basin? How many facies do you find in the Chalk and what are their names? Walking from Redcliff to Culver Cliff what formations do you see? Wytch Farm and Kimmeridge have the same source rock but why do they not have the same reservoir? In the Bridport Sands and the Chalk there are several things that might cause problems for permability in reservoirs, what are they? Source rocks Reservoir rocks The Localities Overview map Locality 1 Locality 2 Locality 3 Locality 4 Locality 5 Locality 6 Locality 7 Locality 8 Locality 9 Locality 10 Summary Questions

28 References Introduction The Wessex Basin
1. 2. Gale, A., 2002, Sedimentary history of the Anglo-Paris Basin, Field guide for NTNU excursion 2-9th June. 3. 4. 5. 6. 7. Selley, R.C., Stoneley, R., A field guide to the petroleum geology of the Wessex Basin. 8. West, I., 2003, Geology of the central south coast of England, 9. Great Britain Street & Road Map, Evolution Stratigraphic Structure geology Source rocks Reservoir rocks The Localities Overview map Locality 1 Locality 2 Locality 3 Locality 4 Locality 5 Locality 6 Locality 7 Locality 8 Locality 9 Locality 10 Summary Questions

29 About this module Introduction The Wessex Basin Title: Petrox 2004
Evolution Title: Petrox 2004 Author: Prof. Ole Torsæter Assistant producers: Nils Arne Øksenvåg(student) Bjørn Arild Mythen (student) Per Olav Paulsen (student) Size: 4,8 mb Publication date: 5. aug 2004 Abstract: This module are made for the fild trip ”Petrox 2004”. The module will give you the basic knowledge of what you are going to see during the days in the field. Software required: PowerPoint XP/XP Viewer Estimated time to complete: 45 minutes Stratigraphic Structure geology Source rocks Reservoir rocks The Localities Overview map Locality 1 Locality 2 Locality 3 Locality 4 Locality 5 Locality 6 Locality 7 Locality 8 Locality 9 Locality 10 Summary Questions

30 Help Navigation tools in the module
Introduction Help Navigation tools in the module The Wessex Basin Evolution Stratigraphic On every page, you will find the title at the top, and a menu with the main chapters in bold to the left. These are hyperlinks which enable you choose the chapters in whichever order you wish to view them. Keep in mind that the module is set up in the order the author believes is most appropriate for study. These chapters are also represented with an illustration on the introduction slide linked to the appropriate chapter. The chapter you are currently viewing in is shown with this marker: , while the subchapter (when applicable) is highlighted in orange. Within the main frame (the white area), you’ll find text and illustrations as well as animations and videos etc. Many pictures have enlargement buttons near them. At bottom of the slide you’ll find a few standardised buttons which occur on every page (some may not be present in the module): Structure geology Source rocks shows the list of references. Reservoir rocks shows information about the module (e.g. author and assistant producer). The Localities Overview map Locality 1 shows a list of frequently asked questions if there are any. Locality 2 Locality 3 takes you to previously viewed slide. Locality 4 is linked to the start of chapter and previously slide, respectively. Locality 5 Locality 6 Previous picture in an animation or sequence of pictures. is linked to the next chapter and slide, respectively. Locality 7 Next picture in an animation or sequence of pictures. Locality 8 you may turn off the sound, or turn it on (when available). Locality 9 Locality 10 you have figured it out! Summary Questions will end your session with the current module. If you have any problems, please let us know by sending an to Please include the title of module and description of the problem. We will respond as quickly as possible.

31 INTRODUCTION MODELLING EXAMPLES SUMMARY Figure: Surface geology of the Wessex Basin, Weald Basin and the English Channel. The position of WytchFarm Oil field  is marked by the red box Back

32 (One click on the circles shows the explanation and second click remove it)
The Reading Formation contain caliche nodules and rare thin channel sandstones. These represent overbank deposits and channel fills and are probably Sparnacian in age. The overlying London Clay Formation is made up of silts, sand and clays, with glauconite at some levels, and comprises four depositional cycles which may represent sequences. Beds of flint pebbles occur at the bases of certain cycles. Septarian concretions and fossil wood are conspicuous. The Bagshot Sands consist mainly of yellow sands which are finely grained. But it also includes thin seams of pipe clay in sequenses of yellow, white and grey sands. This unit includes the Barton Clay (bottom), the Chama Sand (middle) and Becton Sand (top). All but the Becton Sand are poorly exposed on account of landslip and encroaching sea-defeces. 1 2 3 4 5 6 The Bracklesham Group comprises open marine glauconitic shelly sands, and estuarine sands and silts. The succession exposed in Whitecliff Bay is one of the most complete in northern Europe, and has been the subject of detailed stratigraphical investigation, from sequence stratigraphy, magnetostratigraphy and nannofossil zonation. The Wittering Formation comprises two marine glauconitic silty sands units and two laminated estuarine silty units. The base is well marked by a flint pebble bed which is probably a transgressive beachdeposit. The higher marine sands contains Nummulites planulatus at the base. A conspicuous rooted lignite probably represent estuarine channel abandonment. The Earnley Formation consists of open marine bioturbated glauconitic sands which contain a richmollusc fauna, and abundant large Nummulites laevigatus. The overlying Marsh Farm Formation is an estuarine deposit, including laminated silts with a reported brackish water fauna, and a single, thin fully marine intercalation containing Nummulites laevigatus. The Selsey Formation comprises marine glauconitic silty sands, but is presently very poorly exposed. The base of the Headon Hill Formation is marked by a transition from the underlying yellow sands to grey, greenish and black lignitic clays containing freshwater molluscs. Sandy levels are also present. At least two brackish intercalations are known (the Brockenhurst Bed” and the ”Venus Bed”), characterised by containing diverse molluscs. The Bembridge Limestone is a freshwater deposit containing palaeosol horizons, and abundant freshwater molluscs. The base of the overlying Bouldnor Formation includes various brackish horizons, including oysters. The higher part of the Bouldnor Formation is made up of freshwater and slightly brackish green and grey clays containing a few sands and limestones. The base of the Oligocene is taken within the lower part of this formation on the basis of indirect evidence from the Paris Basin and Belgium. Back


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