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Seismic in Understanding a Geological Model: Exploration in the UK Southern North Sea Rotliegend Transition Zone*
By
Hubert J. M. Dejong1, Richard Knight1, Ray McClenaghan2, Franek Mrozek3
Search and Discovery Article #40114 (2004)
*Adapted from “extended abstract” for presentation, entitled “What if you need seismic to understand your geological model? Exploration in the UK Southern North Sea Rotliegend Transition Zone,” at the AAPG International Conference, Barcelona, Spain, September 21-24, 2003.
1Nederlandse Aardolie Maatschappij, Assen, Netherlands
2Shell U.K. Exploration and Production, Lowestoft, United Kingdom
3ExxonMobil International Limited, London, United Kingdom
Introduction
The Cleaver Bank High in Quad 49 in the Southern North Sea (SNS) is Shell Expro's and joint venture partner ExxonMobil's Rotliegend Transition Zone heartland and is located along the northern fringe of the conventional Rotliegend fairway (Figure 1). It contains several discoveries and fields that are being appraised and developed. The Carrack field is the JV's most significant discovery in the area and will be brought on stream in Q4 2003. Although the UK SNS Rotliegend creaming curve is flattening (Figure 2), new technology and closer cooperation between exploration and production continue to unlock relatively small but high-value volumes. The future Carrack hub and the recent success of the Cutter appraisal well (49/9a-7) have increased the attractiveness of the area for further exploration.
Our objective is to demonstrate that exploration in the Rotliegend Transition zone can be done more successfully by the application of depth migration followed by inversion since they can lead to a better understanding of the local intra-Rotliegend lithology.
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The Geological ChallengeSince 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.
Method to Better Intra-Rotliegend Lithology Prediction 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-
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
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 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.
Examples 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. 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 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.
Conclusions
Exploration and development in the Rotliegend Transition Zone in Quad
49 can be done more successfully by intra- Rotliegend seismic
interpretation and attribute
AcknowledgementThe 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.
ReferencesGeluk, 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. |