1. technology and computing

OTC 17639
How to Plan for a Successful C.S.E.M. Survey
D.G. Peace, AOA Geomarine Operations LLC A.G.O.
Copyright 2005, Offshore Technology Conference
This paper was prepared for presentation at the 2005 Offshore Technology Conference held in
Houston, TX, U.S.A., 2–5 May 2005.
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Abstract
Electro-Magnetic or EM Techniques & Surveys have been
around for decades now and most explorationist’s will have
limited exposure to one or more of them. Such EM techniques
include land magneto-tellurics MT, audio MT, marine MT and
now Controlled Source EM - CSEM. The first three have been
successful commercial exploration tools for some time now
and are very good for answering regional geological questions
in areas where seismic fails or is ambiguous...areas such as
sub-salt, sub-volcanics sub-carbonates...ie poor seismic
regions. The EM methods help the explorationist by
measuring the passive electrical energy flow in the earth
which allows for identification of regional conductor and
resistor rocks.
However the new CSEM technique that is getting the recent
strong attention is rather different. Although it uses similar
receiver technology to the passive method it then couples it
with an active dipole EM source. Thus CSEM offers an
entirely new set of applications for EM by its ability to image
much thinner resistors down to 10's of meters rather than the
100's of meters of passive EM.
CSEM has been effectively proven over the last 3 years as an
exceptional new geophysical tool that can and already is
changing the way companies work in exploration, appraisal
and development.
Of CSEM the CEO of a major US Oil Corporation recently
stated something like :
“Research in non-seismic electro-magnetic hydrocarbon
detection has led to new technology for remotely detecting and
imaging hydrocarbons from the earth’s surface. This new
technology “CSEM” holds great promise for accurately
assessing potential hydrocarbon resources and reducing
exploration risk”
….and others have described the technology as the most
significant new E & P development since the transition from
analog to digital…cdp stacking, 3D….and more recently
DHI’s and AVO.
This talk describes how to plan and carry out a successful
CSEM survey.
“How to Plan for a Successful CSEM Survey”
Contents :
A. Introduction – EM can be a Life Changing Experience !
B. So You Really Want to do a CSEM Survey ?
C. Some Critical Things To Consider About Your Prospect
D. Survey Layout and Design
E. Processing and Interpretation of Results
F. Summary…and Good Luck
A, Introduction
To all of you geologists and geophysicists used to working
with seismic and now about to read this paper, please be aware
that getting involved with EM surveys may be a life changing
experience…you may never be the same again, so consider
yourself warned. I just don’t want you blaming me later on
for not telling you upfront.
You see I used to be a “normal” seismic guy happily doing my
usual seismic geophysics and geology and exploring until
1990… then someone asked me to do an EM survey…and life
has somehow never ever been quite the same since.
This is not a hard complicated paper…in fact just the opposite,
what we have tried to do in our 20 minutes is to condense
some 30 years of hard won EM experience into a few basic
guidelines to help you with your first CSEM survey. We hope
we are able to help even in a modest way to your future
success with CSEM.
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it was so exciting and in doing so unfortunately missed most
of the talk, a fact you have never missed in the last 30 years of
your career …that is until now.
For most of us seismic based folks, EM at first seems
alarmingly simple… then almost trivial and easy to do,
especially when someone else is talking about it…….but on
closer inspection, usually around the week before you
mobilise the crew, you suddenly start to understand why you
should have stayed awake in the school those 30 years
back….the devil as they say is in the details…and boy are
there a few details !
Figure 1 The PolarBjorn - A Superb CSEM Vessel
So, here you are sitting at the OTC, you are in the special
CSEM session although you might not really know why. You
have seen the recent EM adverts, read the press hype, maybe
been visited by a couple of EM companies selling their
wares……..you have listened to the previous 5 papers this
morning… and managed to convince yourself that you really
ought to get your management and technical team on board
and give this CSEM thing a whirl as it seems to have some
interesting points that could maybe help you in your day to
day exploration business. This perception of yours is correct,
figure 2 might help shows why as it shows a high resistivity
response over a potential prospect.
What I want to do here in the next 20 minutes is to try and
make your life easier…to start to guide you to a successful
first CSEM survey over one of your prospects, to show you
some of the important things you need to know, to guide you
as to what type of prospect you might consider… then outline
a few of the practicalities of how you should go about it all.
Figure 1 Example CSEM data across a potential drilling prospect
The previous papers were good, they explained it all and you
now think you understand pretty well how it works. However,
if you are honest you know the second you get out of the room
and try to explain it to someone else, you find you don’t really
understand it too well and maybe it’s actually more
complicated than it first seemed. Please don’t worry…this is
normal, and you are not alone as around 95% of the
exploration community would have a hard time explaining
what CSEM really means.
It is a truth that for most seismic based geophysicists…their
total exposure to EM stopped that damp, wet grey afternoon in
the first year at grad school when the professor was going
through “alternate” geophysical methods….you fell asleep as
Figure 3 Nothing Will be the Same after the Leap into
the EM world ?
But first….are you really sure why you want to do this ??
Once you have taken this first step into the EM
world…nothing will be the same after…don’t say I didn’t
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warn you. You are a paid up member of the exploration
community, you love your job and you are a consenting adult
with an inquiring mind…… and most importantly no one has
actually coerced you to do this….you only have yourself to
blame.
From this highly typical but promising start…let’s see what
we can do to help you on your way.
B, So…You Really Want to Do a CSEM Survey ??
Ok lets keep this simple…you are right, there are in reality
many very good reasons why this is a very good thing to do.
Several large oil companies have spent the last 2-3 years
carrying out pretty secret trial surveys of which you have just
seen the first few results. For these companies now it is all
about how far they can push the method and what level of
detail is possible (see figure 4).
Number
Of
CSEM
Surveys
information can predict the fluid fill of a target hydrocarbon
reservoir ahead of the drilling bit.
4. CSEM provides a unique non seismic look at your reservoir
in a direct measurement (resistivity) that you as a geologist are
very familiar with. The data is worked together with available
seismic data to provide relevant seismic and structural
constraints on the Electro-Magnetic data for modelling and
inversion purposes. CSEM is also great to integrate with other
well data such as downhole, well to well and well to surface
resistivity data.
5. CSEM can be used for appraisal, exploration, development
and in the future probably reservoir monitoring purposes. One
of the great things about EM as well, is that prior to doing a
survey you can actually model whether it will work or not.
Modelling target prospects ahead of CSEM field surveys is
compulsory standard procedure for EM surveys and will not
only tell you if you prospect is a valid CSEM target, but will
also guide you to the best survey layout and choice of
equipment.
2004
Widespread Commercial Surveys
More Complex Test Targets
Push the limits
New Companies taking an Interest
2003
Longer Term Trials
First Commercial Surveys
Wider Geological Areas
2002
Follow up CSEM Trials
New Areas
2001
First CSEM Trials
1975
1980
1985
1990
1995
2001
2002
2003
2004
Early R & D by Scripps & Southampton
Figure 4
2005
Time
Increasing Use of CSEM Technology
The Good Reasons to use CSEM include:
1. The method actually works!
MMT works by identifying regional resistivity variations due
to large scale structures at a basin scale level…whereas
CSEM works by being targeted by its geometry at specific
target reservoirs where it can establish the resistivity of the
target.
2. CSEM has been trialled extensively over the last 3-4 years
in many different geological basins and over many different
types of geological prospect in differing structural styles.
Research is now pushing the boundaries to see how far it can
be developed.
3. CSEM can truly identify the shape of thin resistive bodies
and establish their resistivity value, and with offset
Figure 5 A Suite of CSEM data showing the presence of a
shallow resistor
Unfortunately…. as in all the best things in life there are a few
catches along the way, some aspects you might have to be
wary of as you stick your toe tentatively into the EM water.
These Catches Include :
1. CSEM works best in 500mtrs of water or more…more is
good, beware shallow water first time around…..it can be
done but its not as simple to understand and interpret. Recent
experiments indicate that while reasonable data can be
obtained in shallow water, the results are not easy to process
and interpret. Hence the jury is partly out on this one at
present. These water depth considerations are caused by the
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presence of the energy air path (often called the air wave)
which comes in at around 1 to 1.5 times the water depth from
seabed. This energy can be likened in simple terms to a very
strong seismic multiple in as much as it is a very strong signal
many times larger than the CSEM signal we are seeking to
record from the target reservoir. Once the air wave arrives it
completely masks the target signal. See figures 6 and 8
This of course means that in deep water where the original
surveys were done the method can see deeply down to many
of today’s commercial target depths, however in shallow
water, the depth of investigation is presently severely limited.
2. There are target-water depth-depth to target considerations
that may make your target more difficult to evaluate. At
present. 1-1.5 times water depth is a good target depth below
seabed…deeper is possible but needs more care and attention.
Energy Propagation Modes
4. Simple big target structures are great, more complex ones
are tougher, closely stacked targets are hard to separate while
wider spaced stacked targets are easier. EM data interpretation
gets more interesting in complex geology…large bumps are
straightforward, but complex and stacked or inter-twining
channels are more difficult. Targets that are close over or
under salt or basement type rocks are more challenging and
will surely sort the men from the boys.
5. Target aerial size is important. A rule of thumb suggests
that targets bigger than their depth of burial (from WB) are
easily imaged, targets around the same size as the depth of
burial should be ok but need to be modelled to be sure,
however targets smaller than their depth of burial may not be
possible to see and detailed modelling will be required to
establish what is possible.
Seawater
transmitter
Think diffusion not wave fronts
-10
Direct
Wave
-11
Air Path
Sediment
1 Ohm-m
Target
Wave
-12
-13
-14
Towed
Dipole
Source
Direct Path
Log10(E-Field)
V/Am2
-15
Air Wave
-16
200
0
Recent sediments 1-3 ohm-m
Target Path
200
400
800
12000
Source-Receiver Separation (m)
oil – gas sand 10 – 50 - 80 ohm-m
Figure 6 Showing CSEM energy propagation pathways
3. CSEM identifies changes in resistivity. Thus if there is no
resistivity contrast, or only a very small resistivity contrast
between the target and the surrounding matrix rock then there
is nothing to observe, so usually don’t do an EM Survey in
such areas. In this case of little or no resistivity contrast then it
is unlikely you are looking at a commercial size target, most
of which have good resistivity contrasts!!
CSEM
CSEM Survey Ship
Towed Dipole Source
50 meters from Seabed
Seawate
transmitte
-10
Sediments
1 Ohm-m
-11
Oil Sat
50 Ohm-m
Sediments
1 Ohm-m
Log10(EField)
Direct
Wave
Target
Wave
-12
-13
-14
200
-15
Air Wave
-16
200
0
Sea Water
2 Km
400
800
12000
Source-Receiver Separation (m)
Figure 8 showing plots of the different “waves” with and
without a target
Sea Bottom
Sands/shales
0.5-2 ohm-meters
Sands/shales
2 Km
Oil Sand ~40+ ohm-meters
Description - Resistor Mapping
Figure 7
Showing good Target Parameters
6. Unfortunately, these are early days in the use of CSEM and
there are actually are not many people who really really
understand how the whole CSEM thing works, and of those
who think they do, I am afraid many are wrong or misguided
or even swayed by heavy commercial pressures (heaven
forbid). …thus there is a real EM brains shortage at present.
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C, Some Critical Things To Consider About Your Prospect
More than anything else we want your first CSEM survey to
be a success.
We want that because it will reinforce the CSEM method and
its future usage while at the same time increase your standing
in your company and at the same time will probably give you
access to more funds to do more CSEM surveys…..a real winwin-win situation.
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3. Most important of all you will have a discovery well or two
that will confirm that the hydrocarbon reservoir section has a
higher resistivity value than the surrounding matrix
rocks…this is very important as it is both the value that CSEM
can measure…i.e. the difference in resistivity between the
matrix and the reservoir…..AND it will help confirm that a
high resistivity thin feature actually is an economic
hydrocarbon bearing reservoir…… RATHER than some other
thin high resistivity feature like a sill, or tight sand, or salt etc.
See figures 9 and 10.
There are several target considerations for your first survey
such as:
1. Water depth over target and nature of water bottom…flat or
channelised, steeply sloping, does the target sit above the
water bottom as you go off a shelf edge ?...or combinations of
these
2. Target depth from water bottom and the aerial size of the
target compared to its depth of burial from water bottom.
Targets smaller in aerial extent than their depth of burial are
tougher to find than targets aerially bigger than their depth of
burial.
3. Geologic type of target…big bump/fault closure/channels/
salt play?
4. Wide and fat or slim and thin ?, single reservoir or stacked ?
5. Is there a resistivity contrast between the target and host
rock? If not then please don’t do an EM survey.
6. Are there any other resistors near your target that could be
mistaken for your target, such as authigenic carbonates, gas
hydrates, tight sands, salt or volcanics
7. What questions do you want the CSEM to answer…simple
YES-NO or more complex YES-NO, details of reservoir edge
and thicknesses etc?
8. All the usual is it a single or multiple prospect area?, The
impact of CSEM increases when it is used as a risk-reduction
tool, and used to evaluate and rank a number of possible leads
in a block. Are there other companies who can share the
survey costs type of questions?
So with all this in mind we would suggest that your first
CSEM survey takes place over an appraisal prospect situation.
This was where the first ever CSEM surveys were carried out
and for some very good reasons:
1. In an appraisal situation you usually have very good
seismic, 3D, often good AVO on which you drilled your first
discovery well.
2. You will have a pretty good idea of the structural shape and
the reservoir shape which may or may not be the same.
Offset Discovery Well
Structure defined on 3D
seismic with similar AVO
amplitude effects
1st Discovery Well
Figure 9 An appraisal plan based on seismic & AVO doesn’t
know the reservoir resistivity and where it changes
4. Planning for a high cost high visibility development project,
you will have very good financial reasons to avoid drilling dry
or fizz gas wells, and the more £$£$ you can save your boss
the happier he or they will be.
5. As you have just had a discovery, your boss will most likely
not moan too much at the thought of forking out say $1.0 to
2.0 million $$$ to ensure you don’t drill dry wells in the
appraisal….a fact that somehow escaped his notice before
drilling the discovery. Spending say $1.5m on the survey to
save the cost of one or more $40-50m wells will get most
peoples favourable response.
(Small aside - Its amazing how many companies wont spend a
small amount of $$$ pre-spud to ensure an exploration well is
best located but once the initial discovery is made money is
less of a problem. Excuse me but a well is a well is a well.)
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Prospect Modelling - Try Before You Buy - MODEL
MODEL MODEL
So you have your appraisal project in-hand, you have checked
the water depth target depth relationship works, the boss say’s
OK why don’t you look at it more closely….what do you do
next ?
More accurate picture of
reservoir characteristics
pre-appraisal drilling.
CSEM will outline the high
resistivity reservoir
Figure 10 Whereas including the resistivity data highlights where
the reservoir is changing and shows where not to drill
NB - Technical note to very successful companies…IF you are
lucky enough to be drilling 100% successful wells from your
current methods… i.e. your seismic and AVO driven wells are
100% ok…then you don’t need CSEM. BUT please don’t
throw away this paper as your run of luck will for sure come
to an end sooner or later.
After Appraisal CSEM
Of course once you have established in a geological area that a
CSEM anomaly equates to a hydrocarbon bearing reservoir
and no other geological feature, then the method can be used
for near field and further away exploration purposes. A
positive CSEM anomaly in open acreage adjacent to a CSEM
discovery is going to have much more attractive risking than
one without. Having the CSEM anomaly also allows more
accurate economics to be considered and a better value placed
on any subsequent bid for the Acreage.
CSEM before 3D ? is a question being asked more and more.
Once you have proven that CSEM works in your area and you
have a 2D prospect with a related positive CSEM
anomaly…then why not drill and prove the prospect before
going to the additional expense of a 3D seismic survey?
Take this argument a bit further, then you will arrive at the
scenario where you have maybe 2 or 3 CSEM successes, some
on 3D some on 2D and you might ask well why don’t I cover
my block with CSEM and then if there are any other CSEM
anomalies present…just drill those first…even if you have
only limited regional 2D seismic or maybe only a line or two.
At this point you may think I have gone too far…but I can
assure you others in Oil companies are thinking this way
already…its all a matter of proving the method to suit the level
of confidence you require for any given geological area.
Well one of the very nice aspects of using EM surveys is that
you can model your target pretty accurately before committing
to acquire any MMT or CSEM survey data. From simple
geological cartoons of your prospect it is possible to model the
CSEM response and thus tell if it is both a suitable target for
CSEM and if so what survey parameters and equipment will
be required to conduct a successful survey..
So the next step is to meet with a CSEM contractor, explain
the project and problems you are trying to solve. They should
firmly point you in the direction of modelling the prospect in
detail. They will probably charge a little to do this properly.
We cannot over emphasise the importance of good modelling
pre-survey.
There are several simple 1D modelling packages around that
can be had for little or nothing; in fact there is a simple free
1D model generating programme on Prof Steve Constables
website (Scripps Institution of Oceanography www.
mahi.ucsd.edu/steve).
Most companies have got and use 1D and 2.5D modelling
packages. 1D is fine for quick and simple robust simple
geology models and will give you a quick feel for whether
your prospect is potentially do-able. The next level of
complexity is to use a 2.5D modelling programme, which is a
halfway house to full 3D capability and in fairly common use
nowadays. A full complex 3D modelling programme should
give the highest level of detail and flexibility but they require
more time, care and input data and many presently are rather
slow and sometimes not as accurate computationally as we
would like. However 2.5D or 3D modelling should be
mandatory for successful modelling of anything approaching a
complex geological target..i.e. for any real life exploration
target most of us look at nowadays !!
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D, Survey Layout and Design
One of the outputs from the modelling process will be that you
get some necessary information to help you plan the survey
design (See figure 11).
This information includes things such as:
1. The range of source to receiver offsets where the signal
change due to the presence of the resistor can be seen.
2. The offset which has the maximum difference in recorded
signal between the matrix host rock half-space and the ontarget response.
Figure 11 showing results from a modelling test
Good CSEM companies will work with you on the modelling
and be transparent about the implications of your models. See
fig 11.
Good 3D modelling packages will not only give you a YES
(it’s a target) or NO (It’s too difficult at present), but they will
also allow you to experiment with new or novel acquisition
techniques that can make the difference between the target
being seen or not.
As an example we recently were asked to look at a deep
stratigraphic lead located partially under a deep salt dome. The
salt was very accurately imaged from good 3D seismic; the
water depth was 1500 mtrs and the target depth approaching
3000 mtrs from water bottom. By our normal CSEM target
criteria we could see this was at best a difficult project maybe
an impossible target and certainly requiring detailed
modelling. From this unpromising start we were able to :
1. first see that the target alone (without the salt) could be
imaged by CSEM using simple 1D models
2. See that even with the salt included the target could be seen,
but unfortunately apparently requiring higher specification
recording system than was available at the time.
3. Later, by creatively experimenting with survey layouts, we
were able to make the salt our friend rather than the enemy at
which point we also found we could image the target with
lower specification equipment than was originally thought to
be required…ie today’s level of equipment. Full 3D model
So with detailed modelling we were able to take an un-doable
prospect and show how it could be done with today’s
equipment and survey techniques. This level of analysis
wouldn’t be possible with 1D or 2.5D models.
So Model Model Model closely with your EM contractor,
pick their brains and pay close attention to the detail and don’t
skimp on it.
3. The level of recording accuracy & sensitivity required to
“see” your target is limited by the overall signal to noise levels
of your system. These are usually measured in electrical field
strength normalised to the source electrical dipole moment. EField in V/Am2 typically are in the range of -13 to – 16
V/Am2. Early CSEM surveys were capable of measuring
signals as small as -13 to -14 V/Am2, but the latest cutting
edge surveys nowadays are a couple of orders of magnitude
better, and can measure signals as low as 10 -16 on a good
day.
Theoretical minimums are around 10 -17 V/Am2 so we are
now pretty close to the limits of recording; but maybe some
blue sky R & D thinking will move these goalposts soon?
4. Preferred positions for source and receiver line layouts.
Which also depends on the type of survey Question you want
to answer…ie simple Yes-No or to gain more detailed
reservoir information.
NB, as the method measures the difference in the signal
response between the on and of target section, it is by default
necessary to measure both the on and off target responses,
which becomes more interesting to achieve when separate
prospects are close or intertwined n some way (such as
channel complexes)
Survey Layout
To date there have been four main types of CSEM survey
layout :
a. Simple or detailed grids
b. Star pattern centred on the target centre (if you know
where that is ? and if you think you do is it really
correct ?
c. Simple two line cross again centred
d. Single line…dip or strike ?
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Early surveys tended towards the grid format, later surveys
used the Star pattern where the line directions can help define
the reservoir edges, and later as clients wanted to save money
….cross line and then single line surveys have also become
more popular although there are a few things to consider when
doing a single line survey.
and the CSEM method up for failure….which is not our
recommended approach.
As an example, let’s take six identical prospect blobs that we
have interpreted pretty rigorously from our seismic. They all
look pretty similar and the shape is more or less the same. The
AVO response looks similar on each and we are having a hard
time homing in on one or the other as the best target. We
know that our well commitment say’s that we should drill 3
wells, but we also know that our company will for sure pull
the plug after the first dry well and head for the hills….what
are we to do?. Figure 13.
Hey lets run a single CSEM line over each of the blobs and
that will tell us…great idea ? unfortunately no it’s a bad idea
and you would be fooling yourself if you think it will
work…you might get lucky…but how lucky do you feel ???
Some survey design styles
All cases need on and off target data
Figure 12 showing some survey design types
Brief Technical Aside
A brief word about the last 3-4 years – The 2 or 3 main clients
who drove CSEM on in the early days were astute enough to
know that the method needed rigorous proving over a variety
of different geological cases. They carried out extensive trial
surveys in different global, geological and structural situations
and in situations varying from appraisal to surveys where the
rig was steaming over the horizon to the next location.
Over a period of time and analysis they came to better
understand what type of survey is appropriate for what sort of
target, where and when etc. These type of R & D surveys are
still going on and will do for many years as companies push
the boundaries of CSEM and establish what it can really
achieve. New dry wells are already being drilled as these
edges are found.
The key to success in any single line survey is knowing for
sure that when you cross each blob you are measuring the
maximum response possible from a hydrocarbon filled
reservoir that might or might not be there. Now in the case of
our six blobs (See figure 13) we decided that as they were all
the same we would run the line straight down the middle.
Keeps it simple, no deviations, if the AVO had varied maybe
we would have used that, but it didn’t so we had to make a
judgement call.
For the first 3 prospects with the blue line you are lucky, the
blue single line works well, the reservoir is there, its full of oil,
the response is good and hey its in the middle of each
blob…great. The first is the best, the second smaller and third
far too small and the CSEM response was actually pretty ratty,
but hey you hit them good, especially the first one.
Unfortunately for you the next three prospects turn out to look
bad, not because they really are bad, but because you missed
the reservoir in each one with your single red line. That’s too
bad as the seismic anomaly is ok, its just a shame that the
reservoir wasn’t centred on the seismic blob.
Now the word on CSEM’s capability is truly out. Other
smaller companies with less EM experience are realising that
CSEM can help them and so the last year we have been
getting a new type of enquiry that goes something like
…..”wow I wanted to talk with you because I have heard
CSEM is a real oil finder and that if I ran a single CSEM line
over my 10 prospects identified in my permit in Wogga
Wogga land it would tell me where the oil is and which is the
good one to drill P.S.….the rig will be ready in 3 weeks time
and I have budget approval for $200k to do the survey…
please can you help??” signed “desperate from Wogga
Wogga HQ”.
Now I can empathise sincerely with this class of inquirer, in
fact I am sure we have all been in similar situations…but their
expectations are unrealistic and they are setting themselves
Figure 13 One Line is Rarely Enough !
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So you missed all of the second three, and feel bad, but hey its
ok because you got a really good one on the first prospect so
you have a great drilling target.
Its only a couple years later after you have relinquished the
other 5 prospects and the next company comes in and drills 5
straight successes in a row that you feel maybe you cut corners
a bit too much?
The key point to this little tale is that even on the best of
seismic and AVO datasets we all know that the actual
reservoir outline is not always easy to see, and frequently
changes shape as new more detailed information becomes
available. By just running a single line over a series of similar
prospects you run a very high risk of missing the best part of
the target and in doing so missing the very thing you are
seeking.
Please don’t get me wrong here, these Yes-No prospect
ranking surveys are an excellent use of the CSEM method
once you know that high resistivity = hydrocarbon reservoir in
an area. But you do need to make the CSEM survey big
enough to give a representative response from over most of
the target otherwise you are setting the scene for failure. If the
blobs scenario didn’t convince you then consider how more
difficult it becomes in a more complex area where several
channel systems run side by side over a shelf margin…what
are the odds of getting a single line to sample the sweet spot in
each of those ??...not very good we think.
SO please design in enough lines to adequately sample your
targets.
Another reason for acquiring more data earlier is that it allows
more information to be gained about the reservoir that can be
very useful once a specific target has been drilled and proven
hydrocarbon bearing (See Processing and Interpretation 10 ).
Understanding Regional Resistivity Variations - USE
MMT !!
Knowing the background regional resistivity of your target
area is essential to be able to both place the CSEM results into
a local context and to be able to remove or adjust for any
significant local variations in
resistivity across the target area.
It may be that differential sedimentary and compaction factors
across the area have resulted in a resistivity slope across your
target that need’s to be taken into consideration. One of the
earliest CSEM surveys done had a very large salt feature
located next to the survey area that wasn’t pointed out to the
CSEM survey crew before the survey. When some rather
spectacularly large anomalies started coming in it was clear
that there was a huge resistor in the area that wasn’t the target.
In similar terms, we don’t always know as much about the
local or regional geology and its resistivity as we would like.
In a project a couple of years back in NW Europe it was
explained to us that recent Tertiary section was 1-5 ohm mtrs,
and Mesozoic was 10-20 ohm mtrs. The survey found a much
thinner Tertiary than expected, while the Mesozoic was much
thicker than expected and not of the same structural style…ie
it was flat and sedimentary rather than having a large fault
block appearance. The EM data didn’t fit at all well with the
seismic data in the area.
At the time of the survey we were given a very hard time
about this, but couldn’t explain it other than saying that there
was another unknown higher resistivity layer present where
the lower Tertiary should be. It was only a year or so later
when this deeper section was drilled for the first time that it
was discovered that the deeper part of the Tertiary had a
different sand provenance and structure and was indeed much
higher resistivity at 10-20 ohm mtrs. The EM data were fully
correct and vindicated as an excellent predictive tool.
Also, the most cost effective and time effective period to
acquire a larger data set is when you have the boat there the
first time. It is much more expensive and takes longer to
remobilise a new CSEM crew for a prospect infill survey after
it has departed the area.
Example MMT Results
Block one
Block 2
Sub
Salt
?
Salt
?
Salt
Rank Exploration Applications – More difficult Geology
Figure 14
More Complex Geology is More Fun to Unravel ?
Example of regional MMT data results with an accuracy of around
5-10% of depth which in the Faeroe Islands is about 250-300 mtrs
Figure 15
Showing MMT Regional Resistivity Results
CSEM surveys are different to MMT. In CSEM surveys we
are from the start specifically targeting the geometry of the
method at one or two selected reservoirs. The method is in EM
not MT mode and so we gain none of the required regional
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background resistivity data. For this reason we always record
about 10-50% of our CSEM receiver locations as MMT
(Passive MT) sites. This ensures we have adequate knowledge
of the regional resistivity gradients and variations.
This is a VERY important part of the survey and should not be
omitted.
Regional 3D MMT volume draped over
3D seismic volume
Figure 17 Showing CSEM receivers ready to deploy
4. An “A” frame on the stern to facilitate the deployment of
the source and source cables, plus sufficient deck space for the
source power equipment.
Image courtesy
of
Figure 16 Regional MMT Resistivity Data can be draped over 3D
seismic
Survey Boats & Operational Considerations
This is not a section on how to use survey boats, as I am sure
you all know how to do that. But rather a few points on the
specific needs and requirements of current day CSEM
operations
The survey boat should ideally have the following capabilities:
5. A navigation spar hole in the ship’s bottom
6. Sufficient dry scientific laboratory space for the instrument
loading-downloading and initial data processing.
7. Sufficient deck or scientific space for the system navigation
equipment. This is often carried in a separate container system
to facilitate moving from ship to ship.
8. Scientific crew space for around 15 people.
To date several different vessels have been used for CSEM
surveys from the early surveys on the NERC vessel the “RRS
Charles Darwin” through to long term projects on the
“Polarbjorn”, a superb vessel.
1. A large open back deck that will allow the assembly and
deployment of the EM receivers. Ideally this would be partly
covered so the receivers can be assembled and tested in the
dry.
2. A crane large enough to lift and deploy the receivers far
enough over the ships side so that the receiver legs do not get
fouled on the ships side.
3. Storage space for some 40-100 EM receivers and their
related backup kit. Together with rough storage space for the
concrete anchors which weigh around 200 kilos each. Each
receiver recording point will require a separate anchor so that
if you are recording 100 points, then you will need 100
anchors plus a few spares. Anchors can be held in a suitable
transport container to prevent movement in heavy weather.
Figure 18
RRS Charles Darwin
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E, Processing and Interpretation of Results
Again this section is not intended to be a full description of all
EM data processing and interpretation methods as that would
take many papers in their own right. The following are few of
the more important points to be aware of when you start
processing and interpreting your dataset.
1. When compared with present day seismic processing, EM
processing is relatively simple and quick to do. The raw
CSEM data are downloaded immediately when each receiver
is recovered to check that the data is ok before abandoning any
receiver site. This data is quickly run through an early
processing sequence to also confirm that the data are correct
and meaningful.
2. This early on-board processing allows first hand quality
control and also means that the data can be quickly evaluated
for CSEM response content. It helps allow real time changes
to the survey design and plans which can surveys right up to
the wire in tight drilling situations, in some past cases we have
literally had the rig steaming over the horizon to spud as we
were doing surveys.
3. Most of this early processing will be on laptop based
computers, although in the future we expect more
comprehensive computing capability will be available on
board as well.
4. Another processing aspect to be aware of is the fact that
everything is so new. Most processing programmes have their
origins in academia somewhere and as such are not necessarily
the best or most flexible for working with complex present
day structural shapes. While much progress is being made,
don’t expect the same level of choice or capability as with
present day seismic processing.
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8. That said there are also literally only a handful of EM
experts who are capable of making sensible CSEM
interpretations. Unfortunately it also has to be said at present
of these, even fewer have any experience of working in an Oil
Company environment or with the pressures that are present in
the run up to most drilling operations. Thus you need to ensure
they work very closely with experienced oil company
geologists and geophysicists to ensure their work is properly
guided and validated with all other forms of geological and
geophysical data available in the prospect area.
9. Iteration and integration are keys words for the best overall
risk reduction using CSEM. Integration….with all the other G
& G data to ensure the best possible constraints can be placed
on the CSEM inversions and models, and iteration….around
the improving CSEM model…using that to improve the
seismic model…using that improved seismic model to
improve the CSEM model again….and so on.
Use all data-sets, reiterate and reinterpret….re-invert and
reprocess until the data has converged as far as seems sensible
to a “Best Earth Model”.
10. Often for simple YES-NO type answers this rigorous
approach is not required as one or more prospect responses
will truly stand out clearly from the rest even on early
processed data. In such cases, that might be sufficient for a
first cut for a drilling location. However after the well has
been drilled and proven, then depending on the survey design,
the CSEM data can later provide more information about the
reservoir shape and size and edge locations. This is another
reason for acquiring more CSEM data early. Having more data
coverage over the entire prospect can lead to detailed reservoir
edge shapes, reservoir sizes, locations and relative volumetric
sizes. If you only record one line then this capability is greatly
restricted.
5. Of the three main contractors offering EM processing
capability only one has more than 2-4 years experience of
processing modern commercial EM data sets. Also, current
CSEM data processing is really in its early days despite the
fact that the technique has been known for around 25 years.
6. Thus the 3 present contractors are rather paving the way and
rather making it up as they go along in true pioneering spirit.
Each has their own way of showing and displaying the data
with pro’s and con’s for most of the processes. Care has to be
taken with some of the displays in these early days as the
limits of present day modelling and inversion techniques are
being pushed all the time especially when talking about true
3D capability versus various alternative imaging techniques.
7. It has to be said that at present there are NO really true
expert CSEM data interpreters around. The method as used at
present has only been around for 4 years and of that the first 2
years were very experimental. While many surveys have now
been done and results interpreted the actual number of proven
results…I.E. drilled and proven correct is numerically very
small.
Figure 19 above shows an example of regional contour data
draped over a seismic time slice. If you had this type of data set
showing the high resistivity variations as red would it make you
more confident about drilling several of them ?
F, Summary…and Good Luck
To sum up, if you have gone through this simple set of
thoughts and applied them to your targets then you will have
started down the right path to a successful first CSEM survey.
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It for sure will make you think hard about some old
geophysics we have until recently largely ignored. This to me
is a wonderful new example of what we know as the explorers
serendipity biting us again when a fringe black box
geophysical technique, ridiculed and abused by most of us can
suddenly with a little progress blossom into a new technique
that maybe we cant do without in the future.
As you do your survey, you are probably treading new ground
in your area of interest as although now some 100 or more
surveys have been acquired, not that many geological
provinces have been sampled and even less wells drilled on
CSEM results.
Acknowledgements
I would very much like to acknowledge the help, advice,
comments and experience of all my colleagues at AGO and
Sclumberger including :
Arnold Orange
Ransom Reddig
Lionel Fray
Kambiz Safinya.
From wider CSEM viewpoint, I also feel that we should all
acknowledge the main people who have over the last 3-4 years
and in some cases the last 10-20 years contributed and brought
the CSEM method to where it is today.
These individuals include :
Chip Cox of Scripps Institution of Ocenaography
Steve Constable also of Scripps
Len Srnka and others unknown of ExxonMobil
Terje Eidesmo of Statoil
Svein Ellingsrud of Statoil
Lucy McGregor of Southampton University
Martin Sinha of Southampton University
Thank you also to the OTC for both this special CSEM session
and for accepting this paper
As we sail off to our next CSEM survey……..
Maybe it will be for you………??