Muhammad Kamal

PETROLEUM SYSTEM OF WESSEX BASIN

MSc Petroleum EngineeringLondon South BankUniversity

Wessex Basin- A brief 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

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Evolution of Basin

There were four distinct phases.

1.Permo-Triassic

2.Shallow marine sedimentation ,Jurassic

3.Further faulting,

4.The final stage of the basin evolution

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Stratigraphic summary

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• 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

• 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

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Fig. Producing and Discovered Oil fields along the Basin

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Source, Reservoir, Trap & Seal

Wessex Basin

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Source rocks in the Wessex BasinThe 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 theJurassic: the Lower Lias, where total organic carbon contents of up to7.36% have been measured (Ebukanson and Kinghorn, 1985); the OxfordClay (Callovian-Lower Oxfordian) with up to 12.36% TOC; and theKimmeridge Clay (Kimmeridgian) with TOC up to 20.48%, but whichalso contains a 1m bed of oil shale with some 70% TOC. The organicmatter is predominantly sapropelic oil-prone kerogen derived frommarine plankton, although minor amounts of terrestrial material, muchof it recycled, are also present.

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Reservoir rocksThe 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.

90% of the recoverable reserves lie inthe Sherwood reservoir.

This unit is a reservoir rockin the subsurface about 100 km to the east within the Wytch Farm oil field.

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Sample 3

Sample 1

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

Permability (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 materialWeatheringErosion

Burrows due to animals and shell fossilsWeathering and different mineralogy.

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Seals

Shales occurring throughout the pre-Upper Cretaceous succession are sufficiently thick to act as effective seals to all of the known reservoirs.

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Traps

• All presently 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

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GENERATION

• When organic rich 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.

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Migration

Vertical Migration Through active 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.

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Accumulation

Surface Indications Mainly due 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).

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Kimmeridge Oilfield

• Comprises a 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.

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Wareham & Wytch Farm 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.

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UNCERTAINTY AND SUMMARY

Uncertainty• However much data 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 .

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Summary

• Among all the reservoirs, Sherwood Sandstone is the best reservoir due to its excellent porosity and permeability

• 90% of the recoverable reserves lie in the Sherwood Reservoir

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• Wessex Basin is 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

Summary

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References1. 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