PGS Mega Surveys - the key to new discoveries in mature areas?

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A Publication of PGS Geophysical Vol. 2 No. 3
August 2002
PGS Mega Surveys - the key to
new discoveries in mature areas?
PGS is now creating some of the
worlds largest ever 3D seismic data
sets. Hundreds of 3D surveys from the
Central North Sea (CNS), the Southern
North Sea and West of Shetlands are
now being merged into three separate
mega surveys.
The largest merge project PGS has
taken on is the CNS Mega Survey,
covering more than 60,000 sq km. For
comparison, Denmark is 43,000 sqkm
and the state of Louisiana 113,000 sq
km. The CNS Mega Survey merges
more than 150 separate 3D surveys, a
task that is far from trivial. The amount
of data that has to be gathered, merged,
quality controlled and interpreted is
almost incomprehensible. The first
35,000 sq km is comprised of 3
terabytes of migrated seismic, the
world's largest single 3D dataset. Only
a couple of years ago such a project
would not have been possible due to a
lack of capacity to handle the data.
Continued on next page
Figure 1: Location of the Central North Sea Mega Survey. Data from the area on the UK side
(potential data outlined by orange) are merged and interpreted, while 25,000 sq km on the
Norwegian and Danish shelves (potential data outlined by pink) are ongoing.
Summary
In mature oil and gas areas like
the North Sea, it is time-critical to
find remaining hydrocarbon
resources as well as to develop small
finds before existing big field
installations and their associated
infrastructure are closed down.
Otherwise, the resources in minor
fields may never be developed.
Finding the remaining smaller
fields with subtle geophysical
expression is a challenge. A
thorough understanding of the
geology is a good start. To achieve
that, the depositional systems must
be analyzed in a regional context, to
reveal transport directions and
depositional patterns of reservoir
rocks.
Although prospective areas on
the UK and Norwegian side of the
North Sea are almost completely
covered by 3D seismic, most 3D
surveys have been acquired for field
development and production.
Consequently, the surveys are
relatively small in size. In addition,
the 3D surveys are of different
vintages, orientations and quality.
This makes it difficult to extract, use
and compare information from
adjacent surveys. To address this
problem, PGS initiated the
ambitious Mega Survey Project,
merging a large number of 3D
surveys from the mature areas of the
North Sea into consistent 3D data
sets covering large areas. These data
sets make it possible, for the first
time, to evaluate prospects and fields in
their true regional context.

TechLink August 2002 Page 2
The Central North Sea Mega Survey (CNSMS)
Continued from Page 1
Data from the UK shelf, covering
50,000 sq km, are already merged and
interpreted. Merging data from the
Norwegian part of the CNS (15,000 sq
km) is ongoing and data from the
Danish waters (10,000 sq km) are next
(Figure 1).
The mega surveys are based on
seismic that has been released by oil
companies, PGS owned seismic, and
non-exclusive seismic available from
other geophysical contractors. For the
most part, the data are available as final
zero-phased 3D time migrated seismic
surveys. These are first QC'd and
loaded to the UK PetroBank.
Following the initial QC, the data are
output from PetroBank, and loaded
onto the PGS processing system to start
the merge process. Quality control of
the merges is vital and is performed by
inline/cross-line analysis of both
amplitude and time data in an
interactive workstation review process.
time matching (bulk shifts);
merging; and
reload to PetroBank.
After the reload, the PetroBank
contains a common regional final
migration 3D dataset of consistent
orientation and line / trace numbering
that can be delivered in a variety of
workstation formats.
Interpretation,
Visualisation and Analysis
At an early stage in the
MegaSurvey project development, it
was recognised that interpreting the
very large MegaSurveys could stretch
the resources of today's more assetfocused
oil companies. Therefore, PGS
also provide a complete regional
interpretation of the MegaSurvey
dataset, tied to released well data. This
enables oil companies to concentrate on
the more detailed 'search for the subtle
trap' to find, understand, and develop
remaining reserves.
An initial feasibility study was
carried out to determine a range of
project parameters including data
loading, auto tracking, gridding and
The different vintages and data
acquisition methods that have been
used imply that the seismic varies in
quality. Thus, some adjustments of the
data are necessary to achieve the
optimal merge of all the different
surveys. The main objective has been to
preserve the original character of the
seismic data while maintaining the
overall quality of the final product.
The merge sequence consists of:
input final migration (SEG-Y);
interpolation, rebinning to standard
grid;
polarity checking;
phase analysis;
amplitude matching;
Figure 2: Timeslice through the CNS Mega
Survey (35,000 sq. km) showing the start of the
extensions into South Norway and Denmark.
Lower left corner: example of UK well tie line.
Altogether 1600 wells are tied to the survey.

Page 3
A Publication of PGS Geophysical
mapping. This work developed both a
data management and interpretation
methodology.
The aim of the interpretation of the
Mega Surveys is to provide a consistent
regional digital interpretation across the
entire area, tied to released well data
(Figure 2). The interpretation is focused
on digital deliverables that will support
all the leading workstations including
Landmark, GeoQuest, SMT-Kingdom
Suite, Geographics and Tigress. The
interpretation focuses on eight principal
regional horizons to create a consistent
stratigraphic framework.
The well data have been supplied
by the IHS Energy Group (IHS).
Formation tops and two-way travel
times for more than 1600 wells were
made available for both the UK and
Norwegian sectors.
Geological results
of the Mega Surveys
interpretation
The PGS Mega Surveys allow
visualization of the subsurface on a
scale and resolution that has never been
available. There is no better example
than the Lower Tertiary sand
distribution in the Central Graben as
predicted from seismic amplitudes.
When looking at a typical individual
3D survey of 250 sq km (Figure 3) the
result of such amplitude work is
inconclusive. If the extraction is
extended to cover a large survey of
1000 sq km (Figure 4), again the result
is inconclusive. When the same
amplitude extraction is carried out over
25,000 sq km (Figure 5 shows a subset
of 15,000 sq km of the 25,000 sq km
used for the amplitude extraction) the
result is amazing: an entire submarine
tertiary fan system is clearly outlined.
This example highlights the need to
combine many 3D surveys to
comprehend the geology of an area like
the Central North Sea. Similar
methodology can be used to define
Fulmar sand, Triassic pods (Figure 6)
and Zechstein salt.
Continued on next page
Figure 3: The RMS
amplitude display
over one UK block
covering 250 sq km
shows (orange
colors) there is a
distinct sand unit
present within the
sequence.
FIG. 3
25 Km
Figure 5: The RMS amplitude display
covering approx. 15,000 sq km clearly
shows an entire sub-marine tertiary fan
system and details of its depositional
morphology.
FIG. 4
Figure 4: The RMS amplitude display over
four UK blocks covering 1000 sq km shows
the sand unit is more extensive and contains
thicker sections. Little can be inferred as to
depositional direction or extent.
The fan is coming from the North West to South East flowing round the flanks of the Forties-
Montrose structural high, before terminating in the deepest part of the Central Graben, where
its forward progress is impeded by the Josephine High.

TechLink August 2002 Page 4
Oil company use of Mega
Surveys
The seismic files have been loaded
to a number of visualization systems
including holoSeis, deskSeis and Magic
Earth. The horizon files and mapping
grids from the interpretation have also
been loaded to visualization systems
such as ER-Mapper and ARK. These
allow shaded relief (Figure 7) and
illumination displays of individual
horizons that greatly enhance the
standard contour map displays.
Typical oil company use of the
Mega Surveys include:
Evaluating complete basins
The powerful quotes below are from some of the customers of the UK Mega Survey
Project and illustrate how oil companies are starting to use these "Ultra large 3D
seismic surveys" (Mega Surveys) to create a true competitive advantage position.
“The PGS Central North Sea and Southern Gas Area Mega Surveys allow a step
change in our understanding of the petroleum geology of the North Sea.”
“For new entrant companies, such as ourselves, the UK Mega Projects have leveled
the playing field allowing us to compete on a more equal basis with the established
companies.”
“Our recent license application was made very much easier and more efficient as we
were all participants in the PGS Mega Survey Project.”
“The PGS Mega Surveys are likely to become industry de facto standard data sets for
exploration within the UKCS.”
- Re-evaluating held acreage
- Evaluating open acreage
- Re-evaluating producing fields
- Re-evaluating abandoned fields
- Defining well locations
- Evaluating regional play fairways
- Evaluating complete basins
modeling, potential field modeling and
seismic attribute analysis. The PGS
Mega Surveys push current technology
to the limit, but are already providing a
basis for a new era of hydrocarbon
exploration and development.
The PGS Mega Survey Project also
exemplifies a very successful integrated
project between various parts of the
PGS organization: PGS Reservoir
Consultants, Exploration Service and
Data Processing.
The PGS Mega Survey Projects
will likely form the basis for a number
of further studies that will include basin
CONTACT
PGS Geophysical
London
Tel: 44-1932-260001
Fax: 44-1932-266465
Oslo
Tel: 47-67-52-6400
Fax: 47-67-52-6464
Houston
Tel: 1-281-509-8000
Fax: 1-281-509-8500
Singapore
Tel: 65-6735-6411
Fax: 65-6735-6413
© 2002 Petroleum Geo-Services. All Rights Reserved
Figure 6: Triassic pod identification based
on Mega Survey seismic attribute analysis.
For more information about this project, contact:
Interpretation Project
Huw Edwards
PGS Reservoir Consultants
Tel: 44-1628-641000
Email: huw.edwards@pgs.com
Figure 7: Relief of Base Cretaceous
reflector; Central Graben.
Seismic Data Licensing
Jerry Witney
PGS Exploration
Tel: 44-1932-260001
Email: jerry.whitney@pgs.com
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