Download Petroleum Geology and Reservoirs of the Wessex Basin

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
ENTER
Petrox 2004
Introduction
What is Petrox?
The Wessex Basin
Evolution
Stratigraphic
Structure geology
Source rocks
Reservoir rocks
The Localities
Overview map
Locality 1
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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.
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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.
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Summary
Questions
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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.
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Petrox 2004
Introduction
Introduction to the Wessex Basin
The Wessex Basin
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
REFERENCES
ABOUT
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.
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Click on map to expand
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
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Petrox 2004
Introduction
Evolution of the basin:
The Wessex Basin
Evolution
Stratigraphic
Structure geology
There were four distinct phases.
Source rocks
Reservoir rocks
1.Permo-Triassic: A period of continental desert sedimentation,
initially simultaneously with igneous activity and rifting.
The Localities
Overview map
Locality 1
Locality 2
Locality 3
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.
Locality 4
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.
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4.The final stage of the basin evaluation was a culmination of the
reversed movement causing sharp flexuring down to the
north.
Summary
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Petrox 2004
Introduction
Stratigraphic summary of the basin
The Wessex Basin
Evolution
The Wessex Basin
The Jurassic Strata of West Dorset
(Lower and Middle Jurassic – Lithological Units)
Stratigraphic
Structure geology
Source 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.
Reservoir rocks
The Localities
Overview map
Locality 1
Triassic – mainly sandy in the lower
part and more clay towareds the top.
The deposits are mainly fluvial.
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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)
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Petrox 2004
Introduction
Structure geology of the basin
The Wessex Basin
Evolution
Stratigraphic
Structure geology
Source rocks
Reservoir rocks
The Localities
Overview map
Locality 1
Locality 2
Locality 3
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:
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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.
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Summary
Questions
REFERENCES
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.
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The
Petrox
Wessex
2004
Basin
Introduction
Source rocks in the Wessex Basin
The Wessex Basin
Evolution
Stratigraphic
Structure geology
Source rocks
The Jurassic contains three potential source
rock intervals:
Reservoir rocks
1.Kimmeridge Clay
The Localities
Overview map
Locality 1
2.Oxford Clay
Locality 2
3.Lower Lias
Locality 3
The Kimmeridge Clay is well exposed in and
around Kimmeridge Bay, where it approaches
500m in thickness, and mostly comprises oilprone 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.
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Locality 10
Summary
Questions
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Petrox 2004
Introduction
Reservoir rocks
The Wessex Basin
Evolution
Stratigraphic
Structure geology
Source rocks
Reservoir rocks
The Localities
Overview map
Locality 1
Locality 2
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.
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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.
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Summary
Questions
On the field trip we will
examine the Bridport
and the Sherwood
reservoirs.
90% of the recoverable reserves lie in
the Sherwood reservoir.
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Petrox 2004
Introduction
An overview map of the localities
The Wessex Basin
Evolution
Stratigraphic
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”.
Structure geology
(click on the green box to get a more detailed map of the Weymouth area)
Source rocks
Reservoir rocks
The Localities
Overview map
Locality 1
L1
L2
L 10
L9
L3
Locality 2
Locality 3
Locality 4
Locality 5
Locality 6
L5
Locality 7
L6
L7
L8
Locality 8
L4
Locality 9
Locality 10
Summary
Questions
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Petrox 2004
Introduction
Locality 1: Sidmouth- The Sherwood Sandstone Group
The Wessex Basin
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
The Sherwood Sandstone Group is divided into the
early Budleigh Salterton Pebble beds, which is a 20
- 30m thick formation of conglomerates and
sandstones, which is not seen at Sidmouth and the
Otter Sandstone Formation, which consists of fluvioaeolian 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.
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Petrox 2004
Introduction
Locality 1: Sidmouth- The Sherwood Sandstone Group
The Wessex Basin
Evolution
Stratigraphic
Structure geology
Source rocks
Sample nr.
Porosity φ
Permability (mD)
1.1
36.20%
100 - 140
1.2
10.60%
1-2
1.3
8.70%
0.00254
Sample 1.1: Fine sand, crossbedded
Reservoir rocks
Sample 1.2: Coarse sandstone, base channel
The Localities
Overview map
Locality 1
Locality 2
Sample 1.3: Silty sand, top channel
Sample1.1
Locality 3
Core samples info
Back to the Map
Locality 4
Locality 5
Locality 6
Locality 7
Locality 8
Locality 9
Locality 10
Summary
Questions
Sample1.3
Sample1.2
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Petrox 2004
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
Overview map
Locality 1
Locality 2
Locality 3
Locality 4
Locality 5
Locality 6
Locality 7
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 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.
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Summary
Questions
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.
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).
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Petrox 2004
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 2.1: Laminated dark shale
Sample 2.3
Overview map
Locality 1
Sample 2.2: Laminated dark shale
Sample 2.3: Laminated dark shale
Locality 2
Sample 2.2
Locality 3
Locality 4
Core samples info
Sample 2.1
Locality 5
Back to the Map
Locality 6
Locality 7
Locality 8
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
0.02
41.36
418
2.3
0.99
38.09
8.00
476
0.03
39.08
418
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Summary
Questions
Comments:
HI =Hydrogen Index (S2*100/(TOC)
PI =Production Index (weight ratio)
PP =Petroleum Potential (S1 + S2)
Tmax =Temperature at maximum of S2 peak
Sample 2.2
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Sample 2.1
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Petrox 2004
Introduction
Locality 3: The Bridport Sands reservoir at Burton Bradstock
The Wessex Basin
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
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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.
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Petrox 2004
Introduction
Locality 3: The Bridport Sands reservoir at Burton Bradstock
The Wessex Basin
Evolution
Sample 3.1: Soft sands (fine)
Stratigraphic
Structure geology
Sample 3.2: Soft sands (fine)
Source rocks
Sample 3.3: Quaternary layer
Reservoir rocks
Core samples info
Back to the Map
The Localities
Overview map
Locality 1
Locality 2
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
Perma
bility
(mD)
3.1
0.01
0
0.18
0
1.00
0.01
ndp
-
-
3.2
-
-
-
-
-
-
-
28.10%
15 - 25
3.3
-
-
-
-
-
-
-
14.80%
0.08
Locality 3
Locality 4
Locality 5
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 6
Locality 7
Locality 8
Locality 9
Locality 10
Sample 3.1
Summary
Questions
Sample 3.2
Sample 3.3
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Petrox 2004
Introduction
Locality 4: Isle of Portland
The Wessex Basin
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
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.
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Summary
Questions
In the Portland Cherty Series one might
observe a small transgression before the
large regression continues in the Portland
Freestone.
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Petrox 2004
Introduction
Locality 5: Oil seeps; Osmington Mills
The Wessex Basin
Evolution
Stratigraphic
Structure geology
Source rocks
Reservoir rocks
The Localities
Overview map
Locality 1
Sample 4.3
Sample 4.1: Hard layer seeping oil
Sample 4.2: Soft cross-bedded fine sand
Sample 4.1
Sample 4.3: Fine sand smelling of oil, top
Locality 2
Locality 3
Core samples info
Sample 4.2
Back to the Map
Locality 4
Locality 5
Locality 6
Locality 8
Sample nr.
S1
(mg/g
rock)
Locality 9
4.1
33.73
40.52
9.33
434
0.45
74.25
416
-
-
Locality 10
4.2
10.47
14.07
3.13
450
0.43
24.54
419
-
-
4.3
-
-
-
-
-
-
-
20.30%
-
Locality 7
Summary
Questions
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S2
(mg/g
rock)
TOC
(wt%)
HI
(mg/g
TOC)
PI
(S1/(S1+S2)
PP
(mg/g
rock)
Tmax
(°C)
Porosity φ
Permability
(mD)
Comments:
HI =Hydrogen Index (S2*100/(TOC)
Sample 4.1
PI =Production Index (weight ratio)
PP =Petroleum Potential (S1 + S2)
Tmax =Temperature at maximum of S2 peak
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Sample 4.1
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Petrox 2004
Introduction
Locality 6: Lulworth Cove and Mupe Bay
The Wessex Basin
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
REFERENCES
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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.
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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.
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Petrox 2004
Introduction
Locality 6: Lulworth Cove and Mupe Bay
The Wessex Basin
Mupe Bay
Evolution
Sample
nr.
Stratigraphic
Structure geology
Source rocks
Reservoir rocks
The Localities
Overview map
Locality 1
Locality 2
Locality 3
Locality 4
Locality 5
Locality 6
S1
(mg/g rock)
S2
(mg/g rock)
TOC
(wt%)
HI
(mg/g
TOC)
PI
(S1/(S1+S
2)
PP
(mg/g
rock)
Tmax
(°C)
Porosity
φ
Permability
(mD)
0.42
12.59
420
26.40%
40 - 50
0.37
40.98
419
-
-
0.37
15.49
424
-
-
Here the formations are the same as in Lulworth
Cove except here the Portland Stone is not seen.
The5.1layers are5.27
generally thicker
here 2.05
than in Luworth
7.32
357
Cove, but the interesting observation is the oilsands
in the Wealden formation. The conglomerates at the
5.2
15.36
25.62
7.04
364
channel floors are oil-saturated and oil seeps are
visible on outcrop. It is interesting to notice that the
5.3
5.68
2.25
clasts
are impregnated
with9.81
dead oil while
the 436
sand
itself
contain live oil. This means that the clasts
Comments:
HI =Hydrogen
Indexan
(S2*100/(TOC)
originate
from
older reservoir which has been
PI =Production Index (weight ratio)
eroded
and Potential
the clasts
is redeposited in a new
PP =Petroleum
(S1 + S2)
Tmax =Temperature at maximum of S2 peak
reservoir.
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
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Petrox 2004
Introduction
Locality 7: The Kimmeridge clay and the Kimmeridge oilfield
The Wessex Basin
Evolution
Stratigraphic
Structure geology
Source rocks
Reservoir rocks
The Localities
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.
Overview map
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Sample 6.3
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Sample 6.2
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Locality 7
Sample 6.1
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Sa
mpl
e
nr.
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Summary
Questions
Core samples info
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PI
(S1/(
S1+S
2)
PP
(mg
/g
rock
)
T
ma
x
(°
C)
Por
osit
yφ
Perma
bility
(mD)
494
0.04
48.
58
42
0
10.
00
%
0.01
6.2
4
513
0.04
33.
27
42
1
-
-
2.2
5
436
0.37
15.
49
42
4
-
-
S1
(mg/
g
rock)
S2
(mg/
g
rock)
TO
C
(wt
%)
6.1
1.77
46.8
1
9.4
8
6.2
1.26
32.0
1
6.3
5.68
9.81
HI
(mg/
g
TOC
)
Comments:
HI =Hydrogen Index (S2*100/(TOC)
PI =Production Index (weight ratio)
PP =Petroleum Potential (S1 + S2)
Tmax =Temperature at maximum of S2 peak
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Petrox 2004
Introduction
Locality 7: The Kimmeridge clay and the Kimmeridge oilfield
The Wessex Basin
Evolution
Stratigraphic
Structure geology
Source rocks
Reservoir rocks
The Localities
Overview map
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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 1960.
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
(500-750 m deep)
Source: Lower Lias, Blue Lias
A faulting has caused a remigration from the
Bridport Sands to the Corntrash
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Summary
Questions
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.
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Petrox 2004
Introduction
Locality 8: Wytch Farm
The Wessex Basin
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
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Locality 10
Summary
Questions
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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.
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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.
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Petrox 2004
Introduction
Locality 8: Wytch Farm field characteristics
The Wessex Basin
Evolution
Reservoir rock:
Bridport: Jurassic sandstone 924m below surface at 40°C,
100 bar
600 tons of LPG
Stratigraphic
Structure geology
Export pipeline: 16-inch, 91km line to Hamble oil terminal
Source rocks
Reservoir rocks
Total number of wells: 29 injection and 74 producing
Well sites: 8 mainland sites, plus 2 on Furzey Island
The Localities
Overview map
Locality 1
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
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Frome: Clay rich, shelly limestone formed from ancient oyster
beds 750m below surface
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Reserves: 500 million barrels
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Wytch Farm
Peak production: 110,000 bpd in 1997
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Current production (1999): 90,000 barrels a day of oil
16 mmscfd of gas
Island
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Operations workforce: approx 150
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Area occupied by development sites: 105 acres (approx)
Summary
Questions
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
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Petrox 2004
Locality 9: Isle of Wight; Redcliff – Culver Cliff
Introduction
The Wessex Basin
Evolution
Stratigraphic
Structure geology
Source rocks
Reservoir rocks
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.
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
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Petrox 2004
Locality 9: Isle of Wight; Redcliff – Culver Cliff
Introduction
The Wessex Basin
Evolution
Stratigraphic
The Lower Greensand Group is divided into four
formations:
1.The Carstone
Structure geology
Source rocks
Reservoir rocks
The Localities
Overview map
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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.
Locality 4
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.
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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.
Locality 10
Summary
Questions
REFERENCES
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
The Carstone is a coarse, poorly sorted sandstone. It is amount of clay is smaller than for The Lower Chalk and it
red-brown in color and the thickness is 20 m.
contain hard nodules of limestone and bands of black flint.
Its is 350-400 m thick.
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.
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Petrox 2004
Introduction
Locality 10: Isle of Wight; Whitecliff Bay
The Wessex Basin
Evolution
Stratigraphic
Structure geology
Source rocks
Reservoir rocks
The Localities
Overview map
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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 wellexposed. 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.
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Summary
Questions
Click on map to expand and get a illustration of the geological cliff section
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Petrox 2004
Introduction
Summary Questions
The Wessex Basin
Evolution
Stratigraphic
Structure geology
1.
What is the source rock of Wytch Farm Oilfield?
Source rocks
2.
What is the source rock of Kimmeridge Oilfield?
3.
What is the reservoir rocks of Wytch Farm Oilfield?
The Localities
4.
What is the reservoir rock of Kimmeridge Oilfield?
Overview map
Locality 1
5.
From what time periods are the rocks we see during the excursion?
Locality 2
6.
What are the three potential source rocks in the Wessex Basin?
Locality 3
7.
How many facies do you find in the Chalk and what are their names?
8.
Walking from Redcliff to Culver Cliff what formations do you see?
9.
Wytch Farm and Kimmeridge have the same source rock but why do they not have the same
reservoir?
Reservoir rocks
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10. In the Bridport Sands and the Chalk there are several things that might cause problems for
permability in reservoirs, what are they?
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Locality 9
Locality 10
Summary
Questions
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Petrox 2004
Introduction
References
The Wessex Basin
Evolution
1. http://www.brookes.ac.uk/geology/8361/1999/sarah/bsin2.htm
Stratigraphic
Structure geology
Source rocks
Reservoir rocks
The Localities
Overview map
Locality 1
Locality 2
Locality 3
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
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6. http://www.webscapades.com/france/normandy/region-guide.htm.
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7. Selley, R.C., Stoneley, R., A field guide to the petroleum geology of
the Wessex Basin.
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8. West, I., 2003, Geology of the central south coast of England,
http://www.soton.ac.uk/~imw/dorlist.htm.
Summary
Questions
9. Great Britain Street & Road Map,
http://www.streetmap.co.uk/
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Petrox 2004
Introduction
About this module
The Wessex Basin
Evolution
Title: Petrox 2004
Stratigraphic
Structure geology
Author: Prof. Ole Torsæter
Source rocks
Reservoir rocks
The Localities
Overview map
Locality 1
Assistant producers: Nils Arne Øksenvåg(student)
Bjørn Arild Mythen (student)
Per Olav Paulsen (student)
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Size: 4,8 mb
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Publication date: 5. aug 2004
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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.
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Locality 10
Software required: PowerPoint XP/XP Viewer
Summary
Questions
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Estimated time to complete: 45 minutes
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Petrox 2004
Introduction
Help
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The Wessex Basin
Evolution
Stratigraphic
Structure geology
Source rocks
Reservoir rocks
The Localities
Overview map
Locality 1
Locality 2
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