Structural imaging using PS seismic on the Kvitebjørn Field in the North Sea Chau Ao* and Edel K. Areklett, Statoil

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1 on the Kvitebjørn Field in the North Sea Chau Ao* and Edel K. Areklett, Statoil Summary Converted shear seismic (PS) from multi-component seismic surveys often contains features that are not obvious or visible on the streamer and the PZ seismic. To ensure that valuable information is captured, it is important to understand how PS data are related to lithology and well logs. The use of the shear-wave elastic impedance function (Duffaut et al., 2000) in combination with an improved multi-component interpretation tool, enables the interpreter to perform a more robust event correlation between well logs, PS and PZ stacks, and illustrates why PZ and PS reflectors appear differently. This is an important process in the PS interpretation workflow. This paper presents the interpretation results from the OBS (ocean bottom seismic) data on the Kvitebjørn Field, where the PS image of the Jurassic reservoirs is significantly different from the PZ image. Extensive modelling and calibration work has ensured that a reliable PS horizon interpretation could be carried out. Through detailed interpretation and correlation with well results, PS seismic is being used actively in the well planning. Large uncertainties are associated with the existing structural reservoir model and alternative interpretations have been defined in several areas. The results from the PS data have been used in the evaluation of the probability of different structural models, where more weight has been put towards the PS structural interpretation. In addition, interpretation of the PS data will be used when updating the reservoir geomodel for the Kvitebjørn Field, late Introduction The Kvitebjørn Field is a gas and condensate field located on the western flank of the North Viking Graben on the Norwegian Continental Shelf. It is a high pressure and high temperature (HPHT) field located at approximately 4000 metres depth. Production started in September This study focuses on the early to middle Jurassic strata that consists stratigraphically of the fluvial Statfjord Formation followed by the shaly Dunlin Group including the shallow marine Cook Formation and then by the shallow marine Brent Group. The Brent Group is the main reservoir in the Kvitebjørn Field consisting of deltaic sandstone interbedded with shale and coal stringers. The top seal consists of the overlaying offshore shale of the Viking Group. Figure 1 shows the Top Brent depth map with well locations. Streamer data acquired in 1998, and reprocessed in 2002 Figure 1: Top Brent depth map with the Kvitebjørn Field outline. and 2006, are very low frequency and noisy. The main reason for the poor seismic quality is a shallow gas accumulation above the crest of the structure in addition to several lithological effects in the overburden that distort the seismic signal. Seabed multiple is also a problem at the reservoir level. The interpretation of faults and reservoir reflectors is therefore very uncertain. A small 3D OBS test swath consisting of 4 x 6 km cables (total of 9.6 km 2 ) was acquired in 2003 in order to evaluate the feasibility of a full 3D OBS survey. The main objective was to obtain better structural imaging through PZ seismic. The result was very promising and led to a full field acquisition (total of approx. 80 km 2 ) in The final prestack time migrated PZ data was delivered from the processing contractor early 2005 and the PS data towards the end of Although the PZ results gave an improved image with less noise, the quality was still limited due to gas leakage and shallow lithological effects. Due to the many development wells planned to be drilled on the unappraised flanks of the field, all available seismic datasets had to be carefully investigated. Hence, extensive work on the PS seismic was carried out in order to determine whether it could provide important additional information. Calibrating and identifying PS reflectors In order to correctly identify PS reflectors on the seismic, 1034

2 the Top Etive reflector is not expected to be clearly imaged on the PZ data. Similar observations were also made in other wells on the field. Overall, the strong PS reflectors were consistently clear throughout the PS seismic dataset. Based on these well analyses, the interpretation of the Top Etive and the deeper Top Statfjord reflector was carried out on the PS data. Figure 2: Well tie on well In blue is the PS impedance log and synthetic seismogram tied to the PS stack. In red is the equivalent P impedance log and PZ synthetic seismogram tied to the PZ stack. Good PS responses can be seen below 4120 m TVD (Top Etive and Top Cook), which are results of high PS impedance contrasts. synthetic modelling and well calibration was carried out. There are three vertical wells with recorded shear logs covering the reservoir interval on this field. The calibration from PS to PP time was done using Hampson Russell s ProMC software. At the reservoir level the PS seismic has a slightly lower vertical resolution than the PZ seismic. The peak frequencies are 8 Hz and 15 Hz respectively, where the contrast is mostly due to the difference between PZ and PS velocities. Even so, the modelling reveals important seismic PS responses that show a good tie to the seismic stack. Figure 2 shows the PS and PZ synthetic modelling and seismic calibration for well On the right side of the figure is the shale log (VSHDRY) showing the Brent sands from approximately 4120 to 4180 metres. The curve to the left of the synthetic seismograms (SYN) is the computed impedance based on measured velocity and density logs. There are significant amplitude differences between PS and PZ synthetic seismograms from the Viking Group ( m) to the Statfjord Formation. Due to the lack of PS impedance contrasts between layers, the Top Heather and Top Brent reflectors are not imaged on the PS stack. However, these reflectors have stronger P impedance contrasts, and consequently are being better imaged on the PZ stack. Further down in the stratigraphy we observe very strong PS responses corresponding to the large PS impedance contrasts at the Top Etive and Top Cook sands. The calculated P impedance within this interval is relatively constant which results in weaker PZ responses. The explanation to why the Top Etive seems to be strong on the PZ synthetic seismogram is due to the influence of the strong lower impedance at the Top Drake reflector. Hence, Mapping the overburden gas cloud area was necessary to understand the limitation of the primary seismic. This was done by calculating a V p /V s attribute map directly from PS and PP travel times. The relationship between PS time and PP time is based on a simplified geometry between downgoing primary and upgoing secondary wave propagation (Equation 1). Lower values are indicative of overburden gas. Figure 3 shows the migrated gas coverage between the Top Balder and Top Svarte event, an interval in the overburden believed to be highly affected by the gas leakage from the underlying reservoir. As expected, the PZ image below this gas cloud area is severely deteriorated by frequency attenuation and low amplitudes. Outside the gasinfluenced area, PZ structural imaging of the reservoir was a problem in many areas as it is only possible to track events for short distances. The PS image, on the other hand, contains stronger and more continuous reflectors at the reservoir level. This also led to an improved and simpler fault interpretation. V p /V s = (2 Tps Tpp) 1 Equation 1 To investigate the reliability of the faults observed on the PS seismic, the entire PS dataset was scanned for major fault trends (visible more than 800 m in length). Comparison with faults from streamer data interpretation indicated that the latter was over interpreted due to very noisy data. However, the PS fault pattern correlates good with the updated fault interpretation (base case) based on information gained from producing the field, newly drilled production wells, and the PZ and reprocessed streamer data. In addition to this, there are still significant differences in the fault interpretation. Well results support the PS interpretation as we will show in the next section of this paper. Comparison of PS and PZ interpretation The synthetic modelling has verified that there are differences in the amplitude responses between PS and PZ data and that the PS is better than PZ image in the lower part of the stratigraphy. These differences are also apparent on the recorded seismic. Figures 4, 5 and 6 show random seismic lines through different areas on Kvitebjørn with the PS and PZ interpretations (PS interpretation has been carried out independently of the PZ). These seismic cross- 1035

3 sections are good examples of how the PS and PZ images compliment each other; better PZ reflectors definition of the upper stratigraphy and poorer of the lower and vice versa. Figure 4 shows a seismic crossline (SW-NE) of PS and PZ seismic located across the south-western segment. Faults seen on the PS data are clear rotated fault blocks with large displacements, suggesting that these blocks belong to a different pressure regime. Visible breaks seen on the BCU (Base Cretaceous unconformity) reflector coincide with the location where the faults terminate. This can not be seen on the PZ seismic. An exploration well (5 S) was drilled in this area and a different pressure regime was measured. It confirmed the suspicion that this segment is connected to Valemon, a discovery adjacent to the Kvitebjørn Field. Hence, the PS interpretation in this area is verified through this well. During this work, a well (A-2) was drilled through the south-eastern segment where the reservoir communication was uncertain. The base case (PZ) interpretation had a large fault between A-2 and the neighbouring well A-4 as shown in Figure 5 (right), but the throw was very uncertain. The drilling result from the A-2 well proved that this area has a similar depletion to the producing A-4 well. A cross-section from A-4 to A-2 from PS seismic is shown in Figure 5 (left). Based on the PS cross-section between A-4 and A-2, no clear large faults can be observed that support a no communication model. This is clearly seen on the Top Etive to Top Statfjord Formation on the PS data, which has strong continuous reflectors and minor depth variations. As a result of this interpretation, focus was turned to another well (A-13) that was about to be drilled in the northern area of the field. An extensive PS interpretation in this area was undertaken. Comparison between PS and PZ cross-sections (Figure 6) shows similarity to the case around well A-2 with continuous PS reflectors with minor throws. The structural PS interpretation from this area has contributed to evaluating probabilities for different outcomes of the well. There are large uncertainties associated with the interpretation in the northern area due to poor seismic data quality and lack of well control. The interpretation of the PS data has been used in the weighting between the different models. Drilling of this well is ongoing, but it is anticipated that the information from this well will provide a good calibration point for the PS versus PZ data. understand and explain these differences. This process should be undertaken on all PS data prior to interpretation. Investing time and effort on PS modelling and well tie, to form the basis for a separate PS interpretation, have proved to be crucial in providing additional information to the structural framework of the Kvitebjørn Field. This has been verified after drilling production wells. Explanations to why the faults are better defined on the Kvitebjørn PS data are not yet fully understood. One simple, but relevant explanation is that PS data enables us to successfully track events across the field and hence also achieving a better fault delineation. In some areas images of fault planes can be seen, and this is believed to be due to large fault displacement creating a high impedance contrast across the fault. Even though a better structural image can be obtained on the PS data, it has still poorer vertical resolution at some parts of the reservoir interval. The ideal approach is therefore to interpret several cubes and datasets at the same time; the PS data for structural and fault imaging, and the PZ and streamer data for reservoir details that are not resolved by the PS data. Acknowledgments We would like to thank our partners in the PL193 license for permission to present this work, and our colleagues Einar Magerøy and Tore Odinsen for valuable discussions and suggestions. Conclusions The PZ and PS images on the Kvitebjørn Field look very different. For instance the strong Top Brent reflector on the PZ data is not identifiable on the PS, whereas the Top Etive reflector is much stronger on PS than on PZ. It is important to carry out detailed modelling with well ties, in order to Figure 3: Vp/Vs map between Top Balder and Top Svarte; values below 2.2 indicate presence of shallow gas accumulations. 1036

4 Figure 4: Crossline 2642 through the major Graben faults towards an adjacent discovery. Valemon. Faults are better delineated on the PS image (left). BCU is weaker on PS but reveals more structures compared to the PZ image (right). Figure 5: Seismic cross-section through A-4 and newly drilled A-2 well. Left picture shows the PS image with 2 interpreted target events that can be easily tracked between the wells. Right picture shows the PZ image including the base case interpretation prior to drilling. Drilling results proved that there is communication between the wells and consequently the throw is smaller than anticipated. Figure 6: Seismic cross-section through A-15 and A-13 wells in the northern area. The images seen here are smiliar to cross sections from the southern area (Figure 5). 1037

5 EDITED REFERENCES Note: This reference list is a copy-edited version of the reference list submitted by the author. Reference lists for the 2007 SEG Technical Program Expanded Abstracts have been copy edited so that references provided with the online metadata for each paper will achieve a high degree of linking to cited sources that appear on the Web. REFERENCES Duffaut, K., M. Landrø, H. Rognø, N. F. Al-Najjar, 2000, Shear-wave elastic impedance: The Leading Edge, 19,