1. PETROLEUM SYSTEM
OF WESSEX BASIN
MSc Petroleum Engineering
London South Bank
University
Muhammad Kamal

2. 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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3. 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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4. 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

5. • 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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7. Muhammad Kamal
Source rocks in 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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Reservoir rocks
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.
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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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
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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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.

12. GENERATION, MIGRATION
& ACCUMULATION
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13. 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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14. 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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15. 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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16. 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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17. 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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18. UNCERTAINTY AND
SUMMARY
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19. 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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20. 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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21. • 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
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Summary

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23. 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
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