2012 SEG DOI SEG Las Vegas 2012 Annual Meeting Page 1

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1 Full-azimuth towed-streamer seismic: An exploration tool for pre-salt hydrocarbon exploration offshore Brazil Alex Cooke*, Franck Le Diagon, Ricardo De Marco, Daniela Amazonas, Timothy Bunting, Nick Moldoveanu, Stephen Klug and Elaine Mattos, WesternGeco Summary With a string of recent discoveries in the pre-salt areas in deep-water offshore Brazil, this hydrocarbon province has proven to be of global importance. The pre-salt reservoirs, however, present many challenges for exploration. In this paper, we describe the acquisition and processing of a fullazimuth towed-streamer seismic survey designed for presalt exploration offshore Brazil. We show that this solution overcomes many of the pre-salt exploration challenges to provide high-quality reservoir information in an efficient manner. Introduction This study was carried out in the Santos basin, with the objective to characterize potential microbial oil reservoirs in pre-salt Aptian carbonates. These reservoirs are deeply buried, up to six thousand meters below the sea surface and vary in thickness from tens to hundreds of meters. The reservoirs are structurally bounded by tectonic faults associated with the separation of the South Atlantic. The overburden consists of a complex dipping salt layer up to two thousand meters thick, consisting of both homogeneous halite bodies and layered evaporates. The salt is, in turn, overlain by Albian carbonates and interbedded sands and shales. The complex propagation of the seismic wavefield within this geological environment provides a challenge, not just to acquire data which adequately illuminates the reservoir events at depth, but also to process the data such that the data are free from noise and are represented in their true geological context. The majority of seismic surveys acquired offshore Brazil to date have used narrow-azimuth (NAZ) geometries, where the shooting vessel acquires data along straight lines. Wideazimuth (WAZ) geometries employ several seismic vessels sailing in parallel, and have been used successfully for subsalt imaging in the Gulf of Mexico in recent years (Kapoor et al., 2007). In other areas of the world, multiazimuth (MAZ) surveys, where a single vessel acquires multiple conventional straight-line surveys at different azimuths over the survey area, have proved successful (Keggin et al., 2006). Most recently, an acquisition technique was developed in which a single towed-streamer vessel acquires data in a circular or helical pattern; this is known as coil shooting (Moldoveanu et al., 2008). The relative cost/benefit ratios of these enhanced-azimuth acquisition techniques compared to NAZ geometries are still uncertain in the Brazilian pre-salt environment, and this study was undertaken using coil shooting for its characteristic full-azimuth illumination, high fold of coverage, and efficient single-vessel acquisition. This is the first coil survey acquired offshore Brazil. Data acquisition The survey was acquired from November 2010 to January 2011 in water depths ranging from 2000 to 2300 m over a survey area of approximately 600 km 2. The survey design and acquisition was described in detail by Le Diagon et al. (2011) and Table 1 shows the key parameters. Streamers 12 x 8000m Sources 2 x 5085 in 3 Shotpoint interval 37.5 m Streamer depth 12 m Source depth 10 m Streamer separation 120 m Source separation 60 m Table 1: Key acquisition parameters. Figure 1: Pre-plot (left) and post-plot source positions (right). Figure 1 shows the pre-plot (planned) and post-plot (acquired) source positions, which show high similarity, even in the presence of a drilling rig in the southwest part of the survey area (denoted by a black circle). Although the rig did prevent acquisition of contiguous coil data over the whole survey area, it was mitigated by was a spiral 3D vertical seismic profile (VSP) data set, whose additional measurements were used to validate the earth model as it was being derived. Acquisition was expected to last 33 days, but due to the combination of several operational factors (time share with SEG Las Vegas 2012 Annual Meeting Page 1

2 another seismic vessel, acquisition taking place in the worst barnacle season for Santos Basin, and periods of very bad weather), the survey was extended by an additional 30 days. Table 2 summarizes the main operational statistics. Line change time (% of production) ~13% Infill (% of prime data acquired) ~0.4% Technical downtime (% of survey duration) ~1% Number of shots per km 2 ~130 Number of circles 77 Number of shots Note that time spent during line change and infill is a fraction of that for conventional NAZ acquisition, making coil acquisition a highly efficient technique for acquiring high-density seismic data. Figure 2 shows the coverage zones that are characteristic of surveys acquired with coil geometry and Figure 3 shows the offset/azimuth coverage achieved at the center of the survey. Central area: 360 az. coverage, up to 1000 fold Table 2: Key acquisition statistics. Target area: 180 az. coverage, 400 fold (average) approx 250 km 2 WG Cook later in Table 3 illustrates the relative trace densities achieved with these different acquisition techniques. Acquisition type Trace density (traces/km 2 ) 2D and exploration NAZ (nominal) < 250,000 High-density reservoir NAZ (nominal) 1,024,000 Coil 180 target area (average) 1,280,000 Coil 360 central area (average) 2,560,000 Table 3: Comparison of trace densities between different acquisition techniques employed in the area. Data processing As Buia et al. (2010) describe in their case study of a coil survey offshore Indonesia, coil data sets provide both challenges and opportunities to the data-processing geophysicist. Many of the lessons learned from that effort apply to offshore Brazil, but we have both different challenges due to a different geophysical environment and new opportunities due to the availability of new technologies. Noise attenuation: The continuous turning acquisition spread, which is unique to this acquisition type, tends to increase the noise levels due to the increased flow of water across the cable. Figure 2: Survey coverage zones. Figure 3: Offset/azimuth distribution in center of survey. Initial interpretation was done on a grid of 2D data that covers the area. An exploration NAZ survey covers only a small part of the area and a high-density NAZ program is planned for acquisition by the WesternGeco seismic vessel Figure 4: Raw point-receiver data (top) showing high levels of noise. and (bottom) after infield digital group Forming including singular-value decomposition. SEG Las Vegas 2012 Annual Meeting Page 2

3 A workflow combining noise attenuation techniques and including singular-value decomposition, which was specifically designed for this application by Moldoveanu (2011), was tested and applied in near real time onboard the seismic vessel. This workflow was highly effective against high-amplitude turn noise, whilst having no apparent impact on signal amplitudes (Figure 4). Further noise attenuation processing was applied later in the sequence to attenuate the remaining noise. Figure 5 shows the effect of regularization on a common offset; holes are filled and jitter between traces due to azimuthal variation is reduced. Water-velocity correction: It is well known that variations in the velocity of sea water during acquisition of marine seismic surveys leads to differences in the traveltime of reflection events between different sail lines. This leads to errors in all aspects of the data-processing flow, from velocity analysis to stacking. The rapidly changing current regimes characteristic of offshore Brazil ensure that such variations in water velocity are of a significant magnitude and vary rapidly in both space and time. This problem becomes even more critical to solve for coil data, where many different coils (each with its own water-column characteristics) contribute data to each midpoint. In this case, we used the approach of Carvill (2009). As a single coil sail line with a radius of 6.25 km can sample significantly different current regimes in this area, each coil was split into separate segments before an estimate of the true water velocity was made for each part, and a dynamic correction made to a single water-velocity function. Multiple attenuation: In this deep-water environment, the first-order surface multiple from both the seafloor and complex top of salt reflectors coincide with the weaker base of salt and pre-salt reflections. It was, therefore, critical to attenuate this energy without corrupting the primary amplitudes beneath. One of the characteristics of coil acquisition is that it has a higher shot density than conventional NAZ surveys and more near-offset information than multivessel WAZ surveys. This provides data that more closely meets the requirements of multiple attenuation techniques such as true-azimuth 3D SRME (Dragoset et al., 2008), which was applied here. Regularization: Data coverage in terms of offset, azimuth, and midpoint is inherently irregular over the area of a coil survey, as can be seen in Figure 3. For some dataprocessing algorithms, such as tomographic velocity model building, it is desirable to have coverage within CMPs, which are regular in terms of midpoint, azimuth, and offset. There are several different methods to achieve this, and different methods were used as appropriate for different parts of the data-processing sequence. To build the velocity model in the sediments, compact Fourier interpolation (Moore et al., 2008), recently extended to efficiently interpolate in multiple dimensions, was used to produce fully regularized data in offset, midpoint, and azimuth. Figure 5: An inline from a common offset/azimuth volume (left) before and (right) after regularization. Inset: fold of coverage for the offset/azimuth volume. Imaging: In complex areas such as these, the development of an accurate velocity model for depth imaging is essential to correctly represent reflections in their true subsurface positions. With full-azimuth acquisition, multiazimuth tomographic methods can be used for velocity model updating. Dazley et al. (2007) showed how the introduction of additional information from multiple azimuths in reflection tomography reduces uncertainty and gives more confidence in more detailed velocity model updates. In our case, the data were split into three azimuth volumes for multiazimuth tomography (0-60, and , and their reciprocal azimuths). Anisotropic tilted transverse isotropy Kirchhoff depth migration was used to update the velocity model, which was performed in three zones: sediment, intra-salt and pre-salt. Validation of the earth model included 3D VSP traveltime analysis, where measured and modeled arrival times are compared to produce a measure of confidence in the model. Figure 6 shows a map of the residuals after completion of sediment tomography. The final migration image will be produced using anisotropic reverse-time migration. SEG Las Vegas 2012 Annual Meeting Page 3

4 2D data courtesy of TGS and WG Figure 6: Areal map of traveltime residuals (ms) plotted at the 3D VSP source locations, using an intermediate velocity model. Results Figure 7 shows a line from the existing 2D depth-migrated multiclient data in the area (left) compared with a depth migration of the coil data, using an intermediate salt flood velocity model. It can clearly be seen that the full-azimuth 3D data provide a step change in imaging improvement over the 2D data, and that the pre-salt reflections are well imaged. Conclusion Coil acquisition is an efficient tool for acquiring fullazimuth towed-streamer seismic data offshore Brazil, and a data-processing sequence was developed for the coil data in the deep-water Santos basin that both overcomes the challenges and exploits the benefits associated with the technique to provide high-quality information for pre-salt hydrocarbon exploration. Acknowledgments Figure 7: Existing 2D depth migration (top) and full-azimuth TTI anisotropic Kirchhoff depth migration of the coil data (bottom). Pre-salt reflections (highlighted) are well imaged The authors thank WesternGeco for permission to publish this paper. SEG Las Vegas 2012 Annual Meeting Page 4

5 EDITED REFERENCES Note: This reference list is a copy-edited version of the reference list submitted by the author. Reference lists for the 2012 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 Buia, M., R. Vercesi, M. Tham, S. L. Ng, A. T. Waluyo, and S. Chen, 2010, 3D coil shooting on Tulip field: Data processing review and final imaging results: 80th Annual International Meeting, SEG, Expanded Abstracts, Carvill, C. V., 2009, A new approach to water velocity estimation and correction: 71st International Conference and Exhibition, EAGE, Extended Abstracts, U027. Dazley, M., P. J. Whitfield, B. Santos-Luis, A. Sellars, P. Szabo, F. Nieuwland, and L. Lemaistre, 2007, Solving short-wavelength velocity variations with high-resolution hybrid grid tomography: 69th International Conference and Exhibition, EAGE, Extended Abstracts, C001. Dragoset, B., I. Moore, M. Yu, and W. Zhao, 2008, 3D general surface multiple prediction: An algorithm for all surveys: 78th Annual International Meeting, SEG, Expanded Abstracts, Kapoor, J., M. O'Brien, D. Desta, I. Atakishiyev, and M. Tomida, 2007, Subsalt imaging The RAZ/WAZ experience: 77th Annual International Meeting, SEG, Expanded Abstracts, Keggin, J., M. Benson, W. Rietveld, T. Manning, B. Barley, P. Cook, E. Jones, M. Widmaier, T. Wolden, and C. Page, 2006, Multi-azimuth towed streamer in the Nile delta, Egypt: 76th Annual International Meeting, SEG, Expanded Abstracts, Le Diagon, F., J. Soldo, E. C. Mundim, C. E. Lemos, D. C. Garcia, A. Z. Nunes de Barros, N. Moldoveanu, A. Fenwick, T. Bunting, A. Cooke, and R. De Marco Centeno, 2011, First full-azimuth towed-streamer survey offshore Brazil An acquisition and survey design case study: Presented at the 12th International Congress of the Brazilian Geophysical Society. Moldoveanu, N., 2011, Attenuation of high-energy marine towed-streamer noise: 81st Annual International Meeting, SEG, Expanded Abstracts, Moldoveanu, N., J. Kapoor, and M. Egan, 2008, A single-vessel method for wide-azimuth towedstreamer acquisition: 78th Annual International Meeting, SEG, Expanded Abstracts, Moore, I., R. Ferber, and R. Vauthrin, 2008, Quality control and bandwidth optimization of compact Fourier interpolation operators: 78th Annual International Meeting, SEG, Expanded Abstracts, SEG Las Vegas 2012 Annual Meeting Page 5