uIntroduction
uFigure
captions
uGeological
challenge
uLithology
prediction
uMethodology
uDepth
migration![Next Hit](/images/arrow_right.gif)
uExamples
uConclusions
uAcknowledgement
uReferences
uIntroduction
uFigure
captions
uGeological
challenge
uLithology
prediction
uMethodology
uDepth
migration![Next Hit](/images/arrow_right.gif)
uExamples
uConclusions
uAcknowledgement
uReferences
uIntroduction
uFigure
captions
uGeological
challenge
uLithology
prediction
uMethodology
uDepth
migration![Next Hit](/images/arrow_right.gif)
uExamples
uConclusions
uAcknowledgement
uReferences
uIntroduction
uFigure
captions
uGeological
challenge
uLithology
prediction
uMethodology
uDepth
migration![Next Hit](/images/arrow_right.gif)
uExamples
uConclusions
uAcknowledgement
uReferences
uIntroduction
uFigure
captions
uGeological
challenge
uLithology
prediction
uMethodology
uDepth
migration![Next Hit](/images/arrow_right.gif)
uExamples
uConclusions
uAcknowledgement
uReferences
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Figure Captions
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Since
Tertiary charge is abundant and the Rotliegend reservoir is capped by a
thick sequence of Zechstein evaporitic seals, the major risks and
uncertainties are "structure" and the occurrence of "producible
reservoir". Unlike some parts of the Dutch offshore sector, Top
Rotliegend in Quad 49 is a fairly low-relief surface. Directly north of
the Permian erg (Figure 1, the "100% sand
line"), the Upper Rotliegend sediments (Silverpit Formation) form an
interplay of fine-grained sabkha deposits and coarser grained sands of
predominantly fluvial origin with minor aeolian input. Farther to the
north this unit shales out and becomes interbedded with halite. There it
forms a seal to the underlying Basal Leman Sandstone, examples of which
can be found in Cutter (49/9a-3) and Markham (49/5-2) (Myres et al.,
1995). The presence of this sandstone seems to be controlled by paleo-topography
and/or syn-sedimentary faulting (Maynard et al., 2001; Geluk et al.,
2002) and is therefore difficult to predict. In the area several wells
that have been drilled have failed due to the absence of economic
reservoir. For the Basal Leman play, in the case of low-relief
structures, there is an additional risk: in the Transition Zone the
Silverpit Formation can act as a potential waste zone. The combination
of these risks makes it critically important to understand the
depositional model and to have an accurate time-to-depth conversion.
Although the general geological model, based on well data (Figure
3), is fairly well understood (Glennie, 1990), the prediction of
economic reservoir presence on a field and prospect scale is
challenging.
Below we
will discuss our approach to better local intra-Rotliegend lithology
prediction through the use of acoustic impedance data derived from
depth-migrated seismic. Due to significant overburden complexities in
this area, like the high- velocity -gradient Chalk and salt tectonics,
imaging based on (anisotropic) velocity models is the preferred
methodology. The results of this approach and impact on exploration
potential will be illustrated on the basis of three examples.
Depth Migration
Methodology and Inversion
To obtain
optimal data quality and minimise costs, pre-stack data of different
seismic surveys have been preprocessed and merged in order to have
pre-stack input gathers available for depth migration . In parallel a
large-scale 1600 km2 initial anisotropic velocity /depth model has been
generated on the basis of stacking velocities, well data, and horizons
picked on various time-migrated data sets. Using this anisotropic
velocity model, a post-stack depth migration (PostSDM) has been
performed using a stack of the PreSDM input gathers. This approach has
the following advantages:
-
In areas of relatively
simple geology the imaging and positioning is better than that of the
post-stack time migrated data.
-
The data allow a consistent
interpretation on one seismic data set in depth that generally matches
the wells (unlike any isotropic depth migration ).
-
A calibrated structure map
can be generated by interpretation of Top Rotliegend on the PostSDM in
depth (no overburden horizon interpretation required) and by
application of an error correction grid (the use of one anisotropy
factor per layer in the velocity model prevents the interpretation in
depth to match the wells exactly).
In case
the imaging on the basis of the post-stack depth migration is not of
sufficient quality, iterative pre-stack depth migration is required.
Residual move-out analysis on common image gathers not only validates
the correctness of the velocity model for imaging, but, in combination
with the well matches, it can also improve the confidence in the
anisotropic velocity model for depth conversion (assuming mild lateral
variations in anisotropy within a layer and a target that is not too
deep). If the model is of sufficient quality, selected 2D lines can be
3D migrated, and imaging enhancements can be evaluated prior to
performing a full 3D PreSDM.
After
availability of the depth migrated data set, of which the imaging can be
improved further by application of post- migration image-enhancement
filters, an inversion has been done. A good seismic-to-well match is
critical for the inversion to be successful.
In our
first example we show how an assumed Basal Leman prospect identified at
Top Rotliegend level becomes less attractive after better imaging. Over
this particular prospect the overburden geology is relatively simple,
and as a result the imaging after the PostSDM has improved sufficiently
to establish reliably the presence of the Basal Leman. Analysis of the
fault based on the new Top and Base Basal Leman interpretation shows
sand-sand juxtaposition (Figure 4). Although
small-offset faults with sand-sand juxtaposition can seal via cataclasis,
one would expect to find encouragement from Basal Leman amplitude
anomalies, either related to gas fill or better preserved porosities due
to early preserved charge, if the fault would be sealing. However,
unlike at the gas-filled 49/9a-3 structure (Cutter), we do not see any
amplitude anomaly. This example demonstrates the importance of intra-Rotliegend
interpretation and attribute analysis when being in the Transition Zone.
This can only be done when the imaging is of sufficient quality.
In our
second example, a very low-relief structure, we show how a 3D-in-2D-out
PreSDM test, a quick and low-cost exercise, has led to critical imaging
improvements (Figure 5) and to more
confidence in the velocity model for the depth conversion. On the
PostSDM, and also on the post-stack time migration , the Basal Leman
Sandstone is not well visible. It is unclear whether this is caused by
an artifact or whether this is true geology. Looking regionally, it
could well be possible that we are dealing with a Carboniferous paleo-high
on which no Basal Leman Sandstone was deposited. The results of the
3D-in-2D-out PreSDM test based on the PostSDM model tell a completely
different story: although faulted, the Basal Leman Sandstone does seem
to be present. As a result the local depositional model has been refined
in between the well locations, and the probability of finding reservoir
has been increased. In this case the flat common image gathers have also
resulted in more confidence in the model for depth conversion over this
low-relief structure.
The third
example is from the 49/9a-3 (Cutter) area. After 3D PreSDM, a sparse
spike inversion has been performed, the results of which are shown in
Figure 6. Detailed examination of the
inversion data set reveals a lot of information about the depositional
model. In the top part of the Silverpit Formation a soft shale can be
seen. Directly above the Base Permian unconformity, the Basal Leman
Sandstone is present quite distinctly as a soft layer. Also, thickness
variations that can be picked-up have been confirmed at the Cutter
appraisal well. Above the Basal Leman there is a hard shale, most likely
the equivalent of the Dutch "Ameland Shale". This shale can be
interpreted over longer distances, and its continuity reduces the risk
of having a thief zone in the Silverpit Formation.
Exploration and development in the Rotliegend Transition Zone in Quad
49 can be done more successfully by intra- Rotliegend seismic
interpretation and attribute analysis, both on depth migrated
reflectivity and acoustic impedance data. The analysis of the different
reservoir units in turn critically depends on the quality of the seismic
image. The use of a combination of post- and (3D-in-2D-out) pre-stack
depth migrations, preferably based on an anisotropic velocity model, has
proven to be a cost effective and flexible means to address critical
issues quickly for a sizeable exploration area. Once an interesting
prospect has been identified, a full 3D PreSDM and inversion can further
de-risk the local geological model. The approach requires very close
cooperation between the geological and geophysical specialists in order
to be most effective.
The
authors would like to thank Shell and ExxonMobil for permission to
publish this paper. We would also like to express our gratitude towards
John Verbeek and Steve Fryberger for sharing their experience with the
authors and initiating this work in Quad 49. Leo Moonen and Folkert
Hindriks are thanked for the processing of the data.
Geluk, M., Haan, de, H., and Swie-Djin, N., 2002, The
Permo-Carboniferous gas play, Cleaver Bank High area, Southern North
Sea, The Netherlands, in Canadian Society of Petroleum
Geologists, Memoir 19, p. 877 - 894.
Glennie, K.W., 1990, Lower Permian – Rotliegend, in
Glennie, K.W., ed., Introduction to the petroleum geology of the North
Sea, Blackwell Scientific Publications, Oxford, p. 120-152.
Maynard, J.R., and Gibson, J.P., 2001, Potential for
subtle traps in the Permian Rotliegend of the UK Southern Sea: Petroleum
Geoscience, v. 7, p. 301 - 314.
Myres, J.C., A.F. Jonmes, and J.M. Towart, 1995, The
Markham Field: UK blocks 49/5a and 49/10b, Netherlands Blocks J3b and
J6: Petroleum Geoscience, v. 1, p. 303-309.
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