Seismic Interpretation and Subsurface Mapping

1
Seismic Interpretation and Subsurface Mapping

• An A level teaching resource based on the
• Eakring oil field, East Midlands, UK
• Developed by Dorothy Satterfield (University of
  Derby)
• and Martin Whiteley (Barrisdale Ltd) on behalf
  of the
• Earth Science Teachers Association (ESTA)

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Seismic Interpretation and Subsurface Mapping

1. Introduction
2. Basic principles
3. Eakring exercise
4. Additional information

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1. Introduction

• Seismic interpretation and subsurface mapping are
  key skills that are used commonly in the oil
  industry
• This teaching resource introduces the basic
  principles of seismic interpretation and then, if
  time permits, they can be applied in a practical
  exercise
• The resource dovetails with the A level Geology
  specifications

4
2. Basic principles

• Seismic acquisition
• Seismic processing
• Understanding the data
• Seismic interpretation

5
Seismic acquisition offshore

• An air gun towed behind the survey ship transmits
  sound waves through the water column and into the
  subsurface
• Changes in rock type or fluid content reflect the
  sound waves towards the surface
• Receivers towed behind the vessel record how long
  it takes for the sound waves to return to the
  surface
• Sound waves reflected by different boundaries
  arrive at different times.
• The same principles apply to onshore acquisition

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Seismic acquisition onshore (1)

• Onshore seismic acquisition requires an energy
  input from a thumper truck. Geophones arrayed
  in a line behind the truck record the returning
  seismic signal.

Vibrator (source)
Geophones (receivers)
Sub-horizontal beds Unconformity Dipping beds
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• Seismic acquisition onshore (2)

• Seismic horizons represent changes in density and
  allow the subsurface geology to be interpreted.

Lithology change Angular unconformity Lithology
change
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Seismic processing

• Wiggle trace to CDP gather
• Normal move out correction
• Stacking
• What is a reflector?

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Wiggle trace to CDP gather
Wiggle traces
CDP gather
Graphs of intensity of sound as received by the
recorders
Graphs of intensity for one location collected
into groups and shown in a sequence.
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Normal move out correction
CMP
Sound receiversR3 R2 R1
Sound sources S1 S2 S3
Slowest
Fastest
Wave reflected
Sound wave in
Change in lithology from mud to sand so
sound is reflected back to surface
CDP
Data for one point from different signals to
different receivers 1. More time needed to
reach distant receivers so the data look like a
curve. 2. Correcting for normal move out
restores the curve to a near horizontal display.
Original CDP gather
corrected for normal move out
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Stacking
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What is a reflector?
There are many reflectors on a seismic section.
Major changes in properties usually produce
strong, continuous reflectors as shown by the
arrow.
A seismic reflector is a boundary between beds
with different properties. There may be a change
of lithology or fluid fill from Bed 1 to Bed 2.
These property changes cause some sound waves to
be reflected towards the surface.
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Understanding the data

• Common Depth Points (CDPs)
• Floating datum
• Two way time (TWT)
• Time versus depth

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Common Depth Points
Common midpoint above CDP
Sound sources S1 S2 S3
Sound receiversR3 R2 R1
CDPs are defined as the common reflecting point
at depth on a reflector or the halfway point when
a wave travels from a source to a reflector to a
receiver.
Sound wavereflected
Sound wave in
Change in lithology reflecting horizon
Common reflecting point or common depth point
(CDP)
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Floating datum
The floating datum line represents travel time
between the recording surface and the zero line
(generally sea level). This travel time depends
on rock type, how weathered the rock is, and
other factors.
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Two way time (TWT)
Two way time (TWT) indicates the time required
for the seismic wave to travel from a source to
some point below the surface and back up to a
receiver. In this example the TWT is 0.5 seconds.
TWT
surface
0
0.25 seconds
0.25 seconds
0.5
seconds
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Time versus depth

• Two way time (TWT) does not equate directly to
  depth
• Depth of a specific reflector can be determined
  using boreholes
• For example, 926 m depth 0.58 sec. TWT

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Seismic interpretation

• Check line scale and orientation.
• Work from the top of the section, where clarity
  is usually best, towards the bottom.
• Distinguish the major reflectors and geometries
  of seismic sequences.

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• Scale and orientation

• Use the scale bar to estimate the length of the
  line
• Use CDPs to check theorientation of the line on
  the accompanying map

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• Top down approach

• Start at the top of the section, where
  definition is usually best
• Work down the section toward the zone where the
  signal to noise ratio is reduced and the
  reflector definition is less clear

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Reflector character and geometry
Continuous reflector truncating short ones
Next continuous reflector
Reflectors onlapping continuous one
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3. Eakring exercise
This exercise has been developed to illustrate,
in practice, how subsurface information can be
integrated and used to predict where an oilfield
may occur. It builds on the principles outlined
in the PowerPoint presentation and can be
completed by individuals or small teams according
to the time available or their level of
enthusiasm. The seismic lines and base map can
be obtained from Dorothy Satterfield
(d.satterfield_at_derby.ac.uk) in a format that is
suitable for photocopying on A3 paper. You will
also receive a CD that contains a copy of the
entire PowerPoint presentation that you can
customise as you wish. This is free! The aim is
to interpret the seismic data and then produce a
map that shows the subsurface structure in the
region of the Eakring oil field. Oil fields
typically form in simple dome-like structures in
the subsurface. The structure must enclose porous
and permeable rocks that are capable of
containing oil and in this example there are a
number of potential reservoirs developed in the
Namurian and Westphalian (Carboniferous)
sandstones. Oil is prevented from leaking to the
surface by overlying mudstones and coals which
are impermeable.
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Background information
Oil exploration in the East Midlands has a long
history. Eakring and the neighbouring Dukes
Wood oil fields were discovered in the 1930s.
Most oil wells at Dukes Wood date from World War
II, though this nodding donkey or oil pump may
be a little younger. Production at Eakring and
Dukes Wood was important to the war effort in
Britain. Oil production at Dukes Wood stopped in
1966, but it continued in Eakring until 2003.
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Project data
Map showing the location of the 5 seismic
lines The seismic data were acquired in 1984
(hence the prefix 84 to each line
number) Notice also the Eakring Village well and
the location of oil fields in the area
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Understanding the data (1)
CDPs are typically marked at intervals along the
top of seismic lines and they are regularly
spaced to form a horizontal scale. Here, 80 CDPs
represent about 1 kilometre (km).
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Understanding the data (2)
Gaps in land seismic data are due to omissions
where data could not be acquired
For example, it is not always possible to
transmit the signal above pipes, in sensitive
areas and above buildings
Signals from farther away will provide
information for deeper horizons
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Understanding the data (3)
Two way time (TWT) is recorded on the vertical
axis of the seismic line in fractions of a
second. Sometimes it is more convenient to
express time as milliseconds. TWT is the time
required for the seismic wave to travel from the
source to some point below the surface and back
up to the receiver.
0.0 seconds or sea level
0.5 seconds or 500 milliseconds
1.0 seconds or 1000 milliseconds
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Correlating well and seismic data

• Use the Eakring Village well, which is located
  near
• the intersection of lines 69 and 70, to tie
  seismic reflectors to known geological horizons
  identified in the well
• Base Permian at 150 milliseconds
• Blackshale Coal at 240 milliseconds
• Near Top Dinantian at 500 milliseconds
• The potential reservoirs are Namurian and
  Westphalian (Upper Carboniferous) sandstones that
  occur below the Blackshale Coal and above the
  Near Top Dinantian (Lower Carboniferous) horizon

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Well tie to seismic
Eakring Village (projected)
Eakring Village (projected)
Base Permian 150 ms
Base Permian 150 ms
Potential reservoir interval
Blackshale Coal 240 ms
Blackshale Coal 240 ms
Near Top Dinantian 500 ms
Two Way Time (TWT) in Seconds
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Correlating reflectors
Starting at the top of the section, interpret the
Base Permian unconformity away from the well on
line 69 and correlate it with intersecting lines
70 and 71. Continue this process around the
loops formed by lines 72 and 73, ensuring that
your interpretation is consistent and
geologically reasonable. Repeat this process for
the Blackshale Coal and Near Top Dinantian
reflectors, accepting that in some areas the data
quality is quite poor and a best-guess
interpretation is necessary. It may be helpful
to annotate the lines to highlight where possible
faults disrupt the gentle dip of the Blackshale
Coal.
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Correlating the Base Permian unconformity
Eakring Village (projected)
Start by interpreting the Base Permian
unconformity away from the well on line 69.
Next fold line 70 at the intersection with line
69 and match them up. Find and interpret the
Base Permian unconformity.
Finally, unfold line 70 and finish the
interpretation.
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Plotting the Base Permian data
Determine the time values (in milliseconds) for
the Base Permian at an appropriate CDP interval
and plot those values on the map. For example, on
line 69 you could start by plotting values at CDP
500, 600, 700, 800 and so on.
160 ms
150 ms
150 ms
150 ms
Base Permian unconformity
150 ms
150 ms
160 ms
150 ms
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Mapping the Blackshale Coal

• Because the potential reservoir interval is
  poorly imaged (the reflectors are weak and
  discontinuous) the closest and most prominent
  reflector to map is the overlying Blackshale
  Coal.
• Determine the time value (in milliseconds) for
  the Blackshale Coal at an appropriate CDP
  interval and plot that value on the map. For
  example, on line 69 you could start by plotting
  values at CDP 500, 600, 700, 800 and so on. In
  some areas it may be necessary to infill with
  data at a finer scale.
• Contour these values to make a time map. Take
  particular care to recognise where faults may
  complicate the interpretation.
• Normally, a time map is converted into a depth
  map using velocity functions, but for the purpose
  of this exercise the time/depth pairings at the
  top of each seismic line give an adequate
  representation of the depth to a given horizon.

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Plotting data for the Blackshale Coal
In some cases it may be easier to choose
convenient time values for contouring (say, 250
ms, 300 ms, 350 ms, etc.) and plot these against
the appropriate CDPs.
250
250
250
250
280
210
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Contouring the data
Use the time values to produce contours.
Label them in milliseconds to create a subsurface
time structure map.
300
250
250
250
300
210
250
300
300
250
250
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Interpreting the map

• 1. What does the map show?
• 2. Using the time/depth pairings, what is the
  approximate depth in metres to the top of the
  potential reservoir interval at the crest of the
  mapped structure?
• To answer this, plot the time/depth pairings on
  a graph, insert a line of best fit and use it to
  derive the approximate depth of the reservoir
  interval.
• 3. Where would you locate additional seismic data
  to confirm the size and shape of the potential
  structural trap that you have mapped?

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4. Additional information

• Specimen answers
• Extension activities
• Web-based resources
• Further reading
• Contact us
• Acknowledgements

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Specimen answers
The Blackshale Coal dips gently towards the NE
and reaches a high point in the vicinity of the
intersection of lines 69 and 70. Faults can be
extrapolated in a variety of ways in the SW part
of the map to create a potential trap.
1.
The crest of the potential structure as defined
by the Blackshale Coal is at 210 milliseconds (at
CDP 540 on line 69), but the potential reservoir
unit is at about 300 milliseconds. Inspection of
the time/depth pairings in the area shows that
300 milliseconds corresponds to about 350 metres
below surface.
2.
The potential trap would need to be better
defined by extending the seismic lines in a
southerly direction. The extent of the Eakring
Field is shown on the seismic line location map
(Slide 24) and it is evidently an elongate N-S
structure, of which only the northernmost
culmination is defined in this exercise.
3.
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Extension activities

• Individuals or groups with sufficient time and
  interest may want to tackle one of the following
  activities
• Research the economic and social impact of the
  wartime extraction of oil from the East Midlands
• Analyse the similarities and differences between
  onshore and offshore oil exploration in the UK
• Assess the remaining potential of onshore oil and
  gas in the UK
• Account for the differences between the small oil
  fields in the East Midlands and the much larger
  accumulation at Wytch Farm in Dorset

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Web-based resources (1)

• This website, developed by the University of
  Tromsø in Norway, contains a number of modules
  that summarise key geological topics through
  simple animated cartoons. In particular, the Oil
  and Gas module provides useful background
  information for teachers and students who may not
  be conversant with hydrocarbon geology.
• http//www.ig.uit.no/webgeology/

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Web-based resources (2)

• Oil and Gas UK provides educational information
  on its website including history of the North Sea
  and exploration and production techniques.
• http//www.oilandgas.org.uk/education/index.cfm
• More specifically, Oil and Gas UK, with the
  support of the Natural History Museum, has
  produced online and paper versions of Britain's
  Offshore Oil and Gas , which is an excellent
  introduction to the history, science and
  technology of the UK oil industry.
• http//www.oilandgas.org.uk/education/storyofoil/
  index.cfm

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Web-based resources (3)

• The UK Onshore Geophysical Library manages the
  archive and official release of seismic data
  recorded over landward areas of the UK. One of
  the Library's main objectives is to provide
  active support for academia, and there is limited
  support for provision of data to educational
  institutions.
• http//www.ukogl.org.uk/

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Web-based resources (4)

• This report, produced by the British Geological
  Survey (BGS) on behalf of the Department of Trade
  and Industry (DTI), provides a general synopsis
  of the petroleum systems of the UKs onshore
  basins
• It is a large (6MB) file that may take some time
  to open and download
• http//www.og.dti.gov.uk/UKpromote/geoscientific
  /
• Onshore_petroleum_potential_2006.pdf

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Web-based resources (5)

• This website provides information about the Dukes
  Wood Oil Museum and Nature Reserve, near Eakring.
  It is an interesting place to visit because it
  combines both natural and industrial history.
  School parties are welcome and the reserve is
  always open, but access to the oil museum needs
  to be pre-arranged
• http//www.dukeswoodoilmuseum.co.uk/

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Further reading

• The Sedimentary Record of Sea-Level Change,
  edited by Angela L. Coe, 2003. Co-published by
  The Open University and Cambridge University
  Press, 288 pages. 
• A regional assessment of the intra-Carboniferous
  play of Northern England, by Fraser, A. J. et al.
  in Classic Petroleum Provinces, edited by Jim
  Brooks, 1990. Geological Society Special
  Publication No. 50, pp.417-440.

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Contact us

• Dr Dorothy Satterfield
• Geography, Earth, and Environmental Sciences
• University of Derby (FEHS)
• Kedleston Road
• Derby DE22 1GB
• Email d.satterfield_at_derby.ac.uk
• Dr Martin J Whiteley
• Barrisdale Limited
• 16 Amberley Gardens
• Bedford MK40 3BT
• Email mjwhiteley_at_yahoo.co.uk
•  

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Acknowledgements
Data, images and advice were provided by the
following individuals and organisations Mark
Alldred from the UK Onshore Geophysical Library
(UKGOL) Oil and Gas UK for permission to
reproduce data contained in Slides 5 and
9-11 Tony Hodge and Mick Price from Roc Oil