Petroleum System Of Wessex Basin

Petroleum System Of Wessex Basin

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  PETROLEUM SYSTEM OF WESSEXBASIN MSc Petroleum Engineering London South Bank University Muhammad Kamal
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  Wessex Basin- Abrief Introduction • The Wessex Basin • Principally covering Dorset and Hampshire Counties • Permian to Cretaceous Sediments containing source, reservoir and seal or cap rocks forming ideal petroleum Province • Cenozoic Intra-plate contraction, structural inversion Muhammad Kamal
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  Evolution of Basin Therewere four distinct phases. 1.Permo-Triassic 2.Shallow marine sedimentation ,Jurassic 3.Further faulting, 4.The final stage of the basin evolution Muhammad Kamal
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  Stratigraphic summary Muhammad Kamal •Permian & Triassic, Non Marine • Triassic Sherwood Sandstone • Penarth Group, Liassic Marine • Jurassic Marine Sediments • shallow marine mud rocks, sandstones and limestone's • Lower Cretaceous, Non Marine • Albian Marine Clays, Sandstones • Chalk group, thinning of western part • Tertiary, Marine and Non Marine mix, Paleocene
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  • Wytch-Farm (1970), Kimmeridge(1959) and Wareham(1964)-oil producing fields in Basin • Oil Seepages along the Dorset Coast • On-going exploration • Cumulative output of Wessex basin petroleum province Muhammad Kamal Fig. Producing and Discovered Oil fields along the Basin
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  Muhammad Kamal
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  Muhammad Kamal Source rocksin the Wessex Basin The Jurassic contains three potential source rock intervals: 1.Kimmeridge Clay 2.Oxford Clay 3.Lower Lias Potential oil-source rocks are confined to three intervals in the Jurassic: the Lower Lias, where total organic carbon contents of up to 7.36% have been measured (Ebukanson and Kinghorn, 1985); the Oxford Clay (Callovian-Lower Oxfordian) with up to 12.36% TOC; and the Kimmeridge Clay (Kimmeridgian) with TOC up to 20.48%, but which also contains a 1m bed of oil shale with some 70% TOC. The organic matter is predominantly sapropelic oil-prone kerogen derived from marine plankton, although minor amounts of terrestrial material, much of it recycled, are also present.
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  Muhammad Kamal Reservoir rocks Thethree 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. 90% of the recoverable reserves lie in the Sherwood reservoir. This unit is a reservoir rock in the subsurface about 100 km to the east within the Wytch Farm oil field.
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  Muhammad Kamal Sample 3 Sample1 Sample 2 Sample No. 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 Perma bility (mD) 1 0.01 0 0.18 0 1.00 0.01 ndp - - 2 - - - - - - - 28.10% 15 - 25 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 The Bridport Sands Reservoir Lose material Weathering Erosion Burrows due to animals and shell fossils Weathering and different mineralogy.
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  Muhammad Kamal Seals Shales occurringthroughout the pre-Upper Cretaceous succession are sufficiently thick to act as effective seals to all of the known reservoirs.
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  Muhammad Kamal Traps • Allpresently known traps are structural. • Both fault sealed and anticlinal traps are to considered. • Stratigraphic traps are very limited. • Wytch Farm field are in a gentle elongated halfdome structure, fault-sealed to the south. • Other small accumulations nearby are fault-controlled. • The oil accumulation at Kimmeridge is in one of these anticlines and shows that at least this structure retains its integrity as a trap.
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  GENERATION, MIGRATION & ACCUMULATION MuhammadKamal
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  GENERATION • When organicrich source rock is buried deeper through time, temperature and pressure increases to a point at which hydrocarbons will generated. • Temperature dependent process. • Lias source rock. Muhammad Kamal
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  Migration Vertical Migration  Throughactive seepages within the fault planes. Lateral Migration:  Along the reservoir formations horizontally.  The oil at Wytch Farm is believed to have migrated from south, up the underlying fault and then laterally through the Triassic Sherwood Sandstone and the Bridport Sands reservoir formations. Muhammad Kamal
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  Accumulation Surface Indications  Mainlydue to seepage at the time of deposition.  Mainly due to anticline structures. Subsurface Indications  Many wells have been drilled in subsurface structures of Wessex- Channel Basin (Offshore & Onshore). Muhammad Kamal
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  Kimmeridge Oilfield • Comprisesa shallow accumulation within a faulted inversion anticline. • Production over the last 30 years approx 3 mmbbls. • Core data analysis indicates an average porosity of 1% and virtually zero permeability. Muhammad Kamal
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  Wareham & WytchFarm Oilfield • Wareham discovered by BP in 1964. • Discovery well produced 20 bopd from Middle Jurassic. • At Wytch Farm mature source rocks (Tertiary age & Synclinal) lie offshore to the south, within the Channel Sub- basin. • This structural relationship reveals the importance to hydrocarbon migration. Muhammad Kamal
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  UNCERTAINTY AND SUMMARY Muhammad Kamal
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  Uncertainty • However muchdata we have, there will always be uncertainty in our predictions; our job is to try to minimise that uncertainty and risk. If we could know the answers for sure, we would never drill a dry hole. • The Bridport Sands are strongly interbedded with alternating clay-rich sands and harder calcite cemented sands. This undoubtedly causes significant heterogeneity in terms of transmissivity and it is probable that horizontal hydraulic conductivity is much greater than vertical hydraulic conductivity • Due to faulting and tectonics movement, Sherwood sandstone reservoir is below the Lower Lias source rock. • Uncertainty in geothermal gradient makes the modeling of subsidence history of the basins difficult and complex . Muhammad Kamal
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  Summary • Among allthe reservoirs, Sherwood Sandstone is the best reservoir due to its excellent porosity and permeability • 90% of the recoverable reserves lie in the Sherwood Reservoir Muhammad Kamal
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  • Wessex Basinis the main oil-producing basin of onshore Britain • 84% of UK onshore oil production and 42% of UK onshore gas has been produced by Wessex Basin Muhammad Kamal Summary
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  Muhammad Kamal
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  References 1. http://www.brookes.ac.uk/geology/8361/1999/sarah/bsin2.htm 2. Gale,A., 2002, Sedimentary history of the Anglo-Paris Basin, Field guide for NTNU excursion 2-9th June. 3. http://www.glg.ed.ac.uk/research/rsrchstr/index2.html 4. http://www.mdctech.com/corporate/bpwytch.htm 5. http://nrg.ncl.ac.uk/home.html 6. http://www.webscapades.com/france/normandy/region-guide.htm 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, http://www.soton.ac.uk/~imw/dorlist.htm 9. Great Britain Street & Road Map, http://www.streetmap.co.uk/ 10. THE HYDROCARBON PROSPECTIVITY OF BRITAIN’S ONSHORE BASINS 11. http://www.southhampton.ac.uk Muhammad Kamal