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Transcript
Petrox 2003
Author: Ole Torsæter, NTNU
Assistant producers: Nils Arne Øksenvåg and Bjørn Arild Mythen
Petroleum Geology and Reservoirs of the
Wessex Basin, Southern England and
Northern France
ENTER
Introduction
Introduction
What is Petrox?
The Wessex Basin
- Evolution
- Stratigraphic
- Structur geology
- Source rocks
- Reservoir rocks
The Localities
- Overview map
This interdisciplinary course is based on lectures,
laboratory analysis, field work (southern UK,
northern France) and group project work. It
integrates studies in petroleum geology,
sedimentology, structural history, and reservoir
characterization. The course is assessed by a 6
day field based reservoir study of Cretaceous
chalk undertaken by groups of 4-6 students, which
will simulate work undertaken in exploration of
chalk fields.
- Day 1
- Day 2
- Day 3
- Day 4
- Day 5 and 6
Summary Questions
References
Lectures and analytical data. The lectures will
precede the course and provide a stratigraphical,
sedimentological and structural background to the
field study area, plus an introduction to local
petroleum geology and Chalk. 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 6 day field course builds upon
preceding lecture and practical laboratory work,
and provides a structural and sedimentological
context to oil habitat.
•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.
•Characterise important reservoir units (Sherwood
Sandstone, Bridport Sands, Cornbrash) using
sedimentology, poroperm data and evidence of
diagenetic modification
•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.
•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.
Characterisation of Chalk reservoirs. The concluding
part of the course will be undertaken by groups (4-6),
working as teams, based on the chalk cliffs along the
northern coast of France (Dieppe-Fecamp). It will
involve:
•Detailed sedimentological logging, to identify
sedimentary facies (hardgrounds, redeposited chalks,
pelagic chalks) and identify quality reservoir units.
•Geophysical logging, using micropermeters and
spectral gamma meters.
•Fracture characterization (spacing and orientation), and
estimation of water flow rates through fractures.
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. A summary will be presented orally on the
final day by each group.
The Wessex Basin
Introduction
Introduction to the Wessex Basin
The Wessex Basin
- Evolution
- Stratigraphic
- Structur geology
- Source rocks
- Reservoir rocks
The Localities
- Overview map
- Day 1
- Day 2
- Day 3
- Day 4
- Day 5 and 6
Summary Questions
References
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.
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
The Wessex Basin
Introduction
Evolution of the basin:
The Wessex Basin
- Evolution
- Stratigraphic
- Structur geology
- Source rocks
There were four distinct phases.
1.Permo-Triassic: A period of continental desert sedimentation,
initially simultaneously with igneous activity and rifting.
- Reservoir rocks
The Localities
- Overview map
- Day 1
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.
- Day 2
- Day 3
- Day 4
- Day 5 and 6
Summary Questions
References
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.
The Wessex Basin
Introduction
The Wessex Basin
- Evolution
- Stratigraphic
Stratigraphic summary of the basin
The Wessex Basin
The Jurassic Strata of West Dorset
(Lower and Middle Jurassic – Lithological Units)
- Structur geology
- Source rocks
- Reservoir rocks
The Localities
- Overview map
- Day 1
- Day 2
- Day 3
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.
- Day 4
- Day 5 and 6
Summary Questions
References
The basement rocks is of
Devonian and Carboniferous
geosynclinal deposits. The beds
were strongly folded and locally
metamorphosed.
(click on the green box to get more details and click a second time to remove it)
The Wessex Basin
Introduction
Structure geology of the basin
The Wessex Basin
- Evolution
- Stratigraphic
- Structur geology
- Source rocks
- Reservoir rocks
The Localities
- Overview map
- Day 1
- Day 2
- Day 3
- Day 4
- Day 5 and 6
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.
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.
The Wessex Basin
Introduction
Source rocks in the Wessex Basin
The Wessex Basin
- Evolution
- Stratigraphic
- Structur geology
- Source rocks
The Jurassic contains three potential
source rock intervals:
- Reservoir rocks
1.Kimmeridge Clay
The Localities
2.Oxford Clay
- Overview map
- Day 1
3.Lower Lias
- Day 2
- Day 3
- Day 4
- Day 5 and 6
Summary Questions
References
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 organicrich source rock facies) are exposed in
the Lyme Regis area.
This section has also been drilled by a
borehole near Weymouth.
The Wessex Basin
Introduction
Reservoir rocks
The Wessex Basin
- Evolution
- Stratigraphic
- Structur geology
The three reservoirs in the Wytch Farm oil
field are:
- Source rocks
- Reservoir rocks
The Localities
- Overview map
The Frome is a complex reservoir
from the middle Jurassic,
consisting of isolated accumulation
of oyster shells surrounded by
mudstone.
- Day 1
- Day 2
- Day 3
- Day 4
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.
- Day 5 and 6
Summary Questions
References
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.
On the field trip we will
examine the Bridport
and the Sherwood
reservoirs.
The Localities
Introduction
An overview map of the localities
The Wessex Basin
- Evolution
- Stratigraphic
The localities we are visiting, day by day, during Petrox is shown by the map below. A more detail
explanation of each day becomes available when you click on the day icon.
- Structur geology
(click on the green box to get a more detailed map of the Weymouth area)
- Source rocks
- Reservoir rocks
The Localities
Day 1
Day 1
Day 1
Day 4
- Overview map
- Day 1
- Day 2
- Day 3
Day 3
- Day 4
- Day 5 and 6
Day 2
Day 2
Summary Questions
References
Day 2
Day 5-6
Day 1
Descriptions of localities
Introduction
Day 1: Source and Reservoir rocks
The Wessex Basin
- Evolution
Locality 1: Sidmouth- The Sherwood Sandstone Group
- Stratigraphic
- Structur geology
- Source rocks
- Reservoir rocks
The Localities
- Overview map
- Day 1
- Day 2
- Day 3
- Day 4
- Day 5 and 6
Summary Questions
References
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.
The Localities
Introduction
Sample
nr.
Porosity
φ
Permability
(mD)
- Evolution
1.1
36.20%
100 - 140
- Stratigraphic
1.2
10.60%
1-2
1.3
8.70%
0.00254
The Wessex Basin
- Structur geology
- Source rocks
Sample 1.1: Fine sand, crossbedded
- Reservoir rocks
Sample 1.2: Coarse sandstone, base channel
The Localities
Sample 1.3: Silty sand, top channel
- Overview map
- Day 1
- Day 2
Sample1.1
Core samples info
- Day 3
- Day 4
- Day 5 and 6
Summary Questions
References
Sample1.3
Sample1.2
Back to the Map
Introduction
Locality 2: Lyme Regis – west cliff, Early Jurassic Blue Lias (source)
The Wessex Basin
- Evolution
- Stratigraphic
- Structur geology
- Source rocks
- Reservoir rocks
The Localities
- Overview map
- Day 1
- Day 2
- Day 3
- Day 4
- Day 5 and 6
Summary Questions
References
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.
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 kilometres 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.
The Localities
Introduction
The Wessex Basin
- Evolution
- Stratigraphic
- Structur geology
- Source rocks
- Reservoir rocks
The Localities
Sample 2.1: Laminated dark shale
Sample 2.3
Sample 2.2: Laminated dark shale
- Overview map
Sample 2.3: Laminated dark shale
- Day 1
Sample 2.2
- Day 2
- Day 3
Core samples info
Sample 2.1
- Day 4
- Day 5 and 6
Summary Questions
References
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
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
Sample 2.1
Back to the Map
Introduction
The Wessex Basin
Locality 3: The Bridport Sands reservoir at Burton Bradstock
- Evolution
- Stratigraphic
- Structur geology
- Source rocks
- Reservoir rocks
The Localities
- Overview map
- Day 1
- Day 2
- Day 3
- Day 4
- Day 5 and 6
Summary Questions
References
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 channelled
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 colour 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.
The Localities
Introduction
The Wessex Basin
- Evolution
- Stratigraphic
- Structur geology
- Source rocks
- Reservoir rocks
Sample 3.1: Soft sands (fine)
Sample 3.2: Soft sands (fine)
Core samples info
Back to the Map
Sample 3.3: Quaternary layer
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
Perma
bility
(mD)
3.1
0.01
0
0.18
0
1.00
0.01
ndp
-
-
- Day 3
3.2
-
-
-
-
-
-
-
28.10%
15 - 25
- Day 4
3.3
-
-
-
-
-
-
-
14.80%
0.08
- Overview map
- Day 1
- Day 2
- Day 5 and 6
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
References
Sample 3.1
Sample 3.2
Sample 3.3
Introduction
The Wessex Basin
- Evolution
- Stratigraphic
- Structur geology
- Source rocks
- Reservoir rocks
The Localities
- Overview map
- Day 1
- Day 2
- Day 3
- Day 4
- Day 5 and 6
Summary Questions
References
Locality 4: Isle of Portland
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 occuring in Jurassic. The
Kimmeridge clay is a deep water deposit,
and as one comes into the Portland
Sands and the Portland Stone the
enviroment 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.
Back to the Map
The Localities
Day 2: Oil seeps
Introduction
The Wessex Basin
Locality 1: Osmington Mills
- Evolution
- Stratigraphic
- Structur geology
- Source rocks
- Reservoir rocks
The Localities
- Overview map
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
- Day 1
- Day 2
- Day 3
Core samples info
Sample 4.2
Back to the Map
- Day 4
- Day 5 and 6
Summary Questions
References
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)
Sample 4.1
PI =Production Index (weight ratio)
PP =Petroleum Potential (S1 + S2)
Tmax =Temperature at maximum of S2 peak
Sample 4.1
Introduction
Locality 2: Lulworth Cove and Mupe Bay
The Wessex Basin
- Evolution
- Stratigraphic
- Structur geology
- Source rocks
- Reservoir rocks
The Localities
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.
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.
- Overview map
- Day 1
- Day 2
- Day 3
- Day 4
- Day 5 and 6
Summary Questions
References
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 enviroment
under deposition was a freshwater lagoon.
• Middle: Also here it is shale and
limestone but the enviroment was differnet
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 dolorite.
On top of the Wealden Formation is a big
layer of chalk. In the upper part of this
chalk it includes black flint.
The Localities
Introduction
The Wessex Basin
- Evolution
- Stratigraphic
- Structur geology
- Source rocks
- Reservoir rocks
The Localities
- Overview map
- Day 1
- Day 2
- Day 3
- Day 4
- Day 5 and 6
Mupe Bay
HI
PI
S1 are the same
S2
TOC
(mg/g
(S1/(S1+
HereSample
the formations
as in
Lulworth
nr.
(mg/g rock)
(mg/g rock)
(wt%)
TOC)
S2)
Cove except here the Portland Stone is not seen.
The layers are generally thicker here than in Luworth
Cove, 5.1
but the interesting
observation
is2.05
the oilsands
5.27
7.32
357
0.42
in the Wealden formation. The conglomerates at the
channel
oilsaturated
seeps are
5.2 floors are
15.36
25.62and oil 7.04
364
0.37
visible on outcrop. It is interesting to notice that the
clasts 5.3
are impregnated
with dead
oil while
the 436
sand 0.37
5.68
9.81
2.25
itself contain live oil. This means that the clasts
Comments:
originate
from Index
an older
reservoir which has been
HI =Hydrogen
(S2*100/(TOC)
PI
=Production
Index
(weight
eroded and the clasts is ratio)
redeposited in a new
PP =Petroleum Potential (S1 + S2)
reservoir.
Tmax =Temperature at maximum of S2 peak
Core samples info
PP
(mg/g
rock)
Tmax
(°C)
Porosity
φ
Permability
(mD)
12.59
420
26.40%
40 - 50
40.98
419
-
-
15.49
424
-
-
Back to the Map
Summary Questions
References
Sample 5.3
Sample 5.3
Sample 5.2
Sample 5.2
Sample 5.1
The Localities
Introduction
The Wessex Basin
- Evolution
- Stratigraphic
- Structur geology
- Source rocks
- Reservoir rocks
The Localities
Locality 3: The Kimmeridge clay and the Kimmeridge oilfield
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 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.
Sample 6.3
- Overview map
- Day 1
- Day 2
- Day 3
Sample 6.2
- Day 4
- Day 5 and 6
Summary Questions
Sample 6.1
Sa
mpl
e
nr.
References
Core samples info
Back to the Map
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
The Localities
Introduction
The Wessex Basin
- Evolution
- Stratigraphic
- Structur geology
- Source rocks
- Reservoir rocks
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 licence
granted solely to BP many years before
Wytch Farm was discovered and production
began in 1960.
The Localities
- Overview map
- Day 1
- Day 2
- Day 3
- Day 4
- Day 5 and 6
Summary Questions
References
Oil from midddle 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
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.
Back to the Map
The Localities
Introduction
Day 3: Wytch Farm
The Wessex Basin
- Evolution
- Stratigraphic
- Structur geology
- Source rocks
- Reservoir rocks
The Localities
- Overview map
- Day 1
- Day 2
- Day 3
- Day 4
- Day 5 and 6
Summary Questions
References
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.
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.
The Localities
Introduction
The Wessex Basin
Wytch Farm field characteristics
Reservoir rock:
- Stratigraphic
Bridport: Jurassic sandstone 924m below surface at 40°C,
100 bar
600 tonnes of LPG
- Structur geology
Export pipeline: 16-inch, 91km line to Hamble oil terminal
- Source rocks
Total number of wells: 29 injection and 74 producing
- Evolution
- Reservoir rocks
The Localities
- Overview map
- Day 1
- Day 2
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
- Day 3
Frome: Clay rich, shelly limestone formed from ancient oyster
beds 750m below surface
- Day 4
Reserves: 500 million barrels
- Day 5 and 6
Peak production: 110,000 bpd in 1997
Wytch Farm
Summary Questions
Current production (1999): 90,000 barrels a day of oil
16 mmscfd of gas
Island
References
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 kilometres
Participants: BP, ARCO, Premier, Kerr McGee, ONEPM, Tal
Back to the Map
The Localities
Introduction
The Wessex Basin
- Evolution
Day 4: Isle of Wight
Locality 1: Redcliff – Culver Cliff
- Stratigraphic
- Structur geology
- Source rocks
- Reservoir rocks
The Localities
- Overview map
- Day 1
- Day 2
- Day 3
- Day 4
- Day 5 and 6
Summary Questions
References
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
Introduction
The Wessex Basin
The Lower Greensand Group is divided into four
formations:
- Evolution
- Stratigraphic
- Structur geology
1.The Carstone
2.The Sandrock
- Source rocks
3.The Ferruginous Sands
- Reservoir rocks
4.The Atherfield Clay
The Localities
- Overview map
- Day 1
- Day 2
- Day 3
- Day 4
- Day 5 and 6
Summary Questions
References
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 rom a
transgression. This part has a thickness of about 90 m.
The Ferruginous Sands consists of sandstone and
clayey sands of different colour, mostly yellow, brown
and red-brown. Glauconite, gravel and pebbles of
phosphate can be observed and the Sands is 80 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.
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 og blue-grey marls and pale grey
limestone.
The Sandrock is a eustarine deposition. It is fine sands
yellow, white and brown in colour, and at the base
Grey Chalk is a grey-white limestone.
there is a grey-blue sandy shale. The Sandrock is 27 m
thick.
Chalk Marl consists of mottled blue-grey limestone and
marl.
The Carstone is a coarse, poorly sorted sandstone. It is
red-brown in colour and the tickness is 20 m.
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 350-400 m thick.
The Localities
Introduction
Locality 2: Whitecliff Bay
The Wessex Basin
- Evolution
- Stratigraphic
- Structur geology
- Source rocks
- Reservoir rocks
The Localities
- Overview map
- Day 1
- Day 2
- Day 3
- Day 4
- Day 5 and 6
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 kilometre 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.
Summary Questions
References
Click on map to expand and get a illustration of the geological cliff section
Back to the Map
The Localities
Introduction
Day 5 and 6: Dieppe-Senneville
The Wessex Basin
- Evolution
- Stratigraphic
- Structur geology
- Source rocks
- Reservoir rocks
The Localities
- Overview map
- Day 1
In France it will be focused on the Chalk from Upper
Cretaceous. This has been seen in Culver Cliff, but in the
area around Dieppe the Chalk is easily accessible and it
is continuous in its deposition. It is interesting in the
aspect of assessment of carbonated reservoirs. The
chalk can be divided into six main depositional facies:
-Pelagic Chalk
-Clay-rich Pelagic Chalk
-Nodular Pelagic Chalk
-Reworked Chalk
-Nodular Reworked Chalk
-Condensed Chalk
- Day 2
- Day 3
- Day 4
- Day 5 and 6
Summary Questions
References
The Pelagic Chalk Facies consists of white chalk and it is
homogenous. It is flintless or it can include beds of black
nodular flint. Since it lacks sedimentary structures and
consists mostly of pelagic skeletal debris it suggests that
it is a result of a slow accumulation of pelagic rain.
Reworked Chalk Facies consists of white to grey, coarse
grained chalks with a rough surface texture. It includes
large bioclasts which can be clearly seen, and one might
also observe chalk clasts, flaser wisps and beds of black
nodular flint. It is reworked in the sense of the finer debris
has been removed or winnowed.
The Clay-rich Pelagic Chalk Facies is thin marl interbeds
in the White Chalk Formation and marly, bioturbated
grey chalks and marls in the Lower Chalk Formation.
These interbeds occur throughout the Chalk and indicate
a sporadic change in the clay input.
Nodular Reworked Chalk Facies comprises of white,
bioclastic nodular calcisphere wackestones made up of
discrete carbonate concretions separated by soft burrow
fill. The finer debris have been winnowed and thus the
facies is enriched in coarser grain particles.
Nodular Pelagic Chalk Facies comprises of white,
nodular chalks. They are made up of carbonate
concretions with diffuse bondaries spareted by soft
burrow-fill. The concretions and the fill weather differently
and this makes the concretions to stick out in outcrops.
The facies is a result of pelagic rain where the
concretions are a result of diagenetic cementation.
The Condensed Chalk Facies comprises of thick (2-3m)
sequences composed of two or three stacked
hardgrounds or hardgrounds combined with nodular
chalks. It includes glauconite (green) and phosphate
(brown). The weathering of the glauconite and the iron
sulphides gives many hardgrounds a yellow appearance.
Back to the Map
Introduction
Summary Questions
The Wessex Basin
- Evolution
- Stratigraphic
- Structur 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?
4.
What is the reservoir rock of Kimmeridge Oilfield?
- Overview map
5.
From what time periods are the the rocks we see during the excursion?
- Day 1
6.
What are the three potensial source rocks in the Wessex Basin?
7.
How many facies do you find in the Chalk and what are their names?
- Day 4
8.
Walking from Redcliff to Culver Cliff what formations do you see?
- Day 5 and 6
9.
Wytch Farm and Kimmeridge have the same source rock but why do they not have the same
reservoir?
- Reservoir rocks
The Localities
- Day 2
- Day 3
Summary Questions
References
10. In the Bridport Sands and the Chalk there are several things that might cause problems for
permability in reservoirs, what are they?
References
Introduction
References
The Wessex Basin
- Evolution
- Stratigraphic
- Structur geology
1. http://www.brookes.ac.uk/geology/8361/1999/sarah/bsin2.htm
- Source rocks
- Reservoir rocks
The Localities
- Overview map
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
- Day 1
- Day 2
4. http://www.mdctech.com/corporate/bpwytch.htm.
- Day 3
- Day 4
5. http://nrg.ncl.ac.uk/home.html
- Day 5 and 6
Summary Questions
References
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.
Full scale map of the Wessex basin
Introduction
INTRODUCTION
MODELLING
The
Wessex Basin
- Evolution
EXAMPLES
- Stratigraphic
SUMMARY
- Structur geology
- Source rocks
- Reservoir rocks
The Localities
- Overview map
- Day 1
- Day 2
- Day 3
- Day 4
- Day 5 and 6
Summary Questions
References
Figure: Surface geology of the Wessex Basin, Weald Basin and the English
BackChannel. The position of WytchFarm Oil field is marked by the red box
Introduction
The Wessex Basin
- Evolution
(One click on the circles shows the explanation and second click remove it)
- Stratigraphic
- Structur
geology
The
Bagshot
Sands
consistupmainly
of yellow
The
Reading
The
Formation
overlyingcontain
London
Clay
Formation
is made
of
This unit includes the Barton Clay (bottom), the Chama
sands
which
are
finely
grained.
But
it
also
includes
Source
rocks
caliche nodules
silts,and
sand
rare
andthin
clays, with glauconite at some levels,
Sand
(middle) and Becton Sand (top). All but the
thin
seams
of
pipe
clay
in
sequenses
of
yellow,
channel
sandstones.
and
comprises
These
four
depositional
cycles
which
Becton
Sand are poorly exposed on account of
- Reservoir rocks
white and grey sands.
represent overbank
may represent
deposits
sequences.
and
Beds of flint pebbles occur
landslip and encroaching sea-defeces.
channel fillsat
and
theare
bases
probably
of certain cycles. Septarian concretions
The
Localities
Sparnacian and
in age.
fossil wood are conspicuous.
- Overview map
- Day 1
- Day 2
1
2
3
4
5
6
- Day 3
- Day 4
- Day 5 and 6
Summary Questions
References
The Bracklesham Group comprises open marine glauconitic shellyThe
sands,
baseand
of the
estuarine
Headonsands
Hill Formation
and silts. is marked by a transition
The succession exposed in Whitecliff Bay is one of the most
complete
in northern
Europe,
from
the underlying
yellow
sandsand
to grey, greenish and black
has been the subject of detailed stratigraphical investigation,
fromclays
sequence
stratigraphy,
lignitic
containing
freshwater molluscs. Sandy levels are
magnetostratigraphy and nannofossil zonation.
also present. At least two brackish intercalations are known (the
Brockenhurst Bed” and the ”Venus Bed”), characterised by
The Wittering Formation comprises two marine glauconitic silty sands units and two laminated
containing diverse molluscs.
estuarine silty units. The base is well marked by a flint pebble bed which is probably a
transgressive beachdeposit. The higher marine sands contains
Bembridge
Nummulites planulatus at the base. A conspicuous rootedThe
lignite
probably Limestone
represent is a freshwater deposit containing
palaeosol
horizons,
and abundant freshwater molluscs.
estuarine channel abandonment.
The base
of the
overlying
Formation includes various
The Earnley Formation consists of open marine bioturbated glauconitic
sands
which
containBouldnor
a
brackish horizons, including oysters. The higher part of the
richmollusc fauna, and abundant large Nummulites laevigatus.
Bouldnor
Formation
is made
up of
The overlying Marsh Farm Formation is an estuarine deposit,
including
laminated
silts with
a freshwater and slightly
reported
brackish green and grey clays containing a few sands and
brackish water fauna, and a single, thin fully marine intercalation
limestones.
containing
The base
Nummulites
of the Oligocene is taken within the lower
laevigatus.
part of this formation on the basis of indirect evidence from the
Paris Basin and Belgium.
The Selsey Formation comprises marine glauconitic silty sands, but is presently very poorly exposed.
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