Th D North Sea Case Study for Enhanced Imaging Using a New Multimeasurement Towed Streamer

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1 Th D North Sea Case Study for Enhanced Imaging Using a New Multimeasurement Towed Streamer C. Cunnell* (WesternGeco), M. Francis (Schlumberger), C. Abu (Schlumberger), M. Paydayesh (Schlumberger) & A. Imamshah (Schlumberger) SUMMARY We present a case study over the Bruce Field area in the North Sea of a seismic survey acquired using a new multimeasurement towed-streamer system to provide a broadband, isometrically sampled seismic dataset. Images, with qualitative and quantitative interpretation comparisons are made against an equivalent broadband hydrophone-only dataset derived as a subset of the original measurements. Both datasets are processed through matching fast-track processing sequences. Inspection of the two volumes indicates uplift in the multimeasurement dataset at multiple levels from seafloor to the reservoir. High-resolution images in the shallow section show the multimeasurement data fidelity, and indicate the potential for seismic data suited for geohazard identification. In the Palaeogene overburden, understanding of the intricate structural geometries of turbidites and sand injectites are enhanced using the multimeasurement data, which demonstrates good spatial imaging of fine-scale structures with complex three-dimensional orientations. Across the reservoir interval, improved continuity in the prestack inversion cube from the multimeasurement data suggests better separation of signal from noise, coupled with low-frequency content to deliver close to absolute impedance without low-frequency model input.

2 Introduction A new multimeasurement towed-streamer system acquired a seismic survey in the North Sea. The system comprised point-receiver multimeasurement sensors to record scalar pressure and both vertical and horizontal crossline components of particle acceleration (Robertsson et al., 2008). These measurements enable joint 3D deghosting and reconstruction of the seismic wavefield to mitigate high-order spatial aliasing in the sparsely sampled crossline direction between streamers (Özbek et al. 2010). The aim of the technology is to provide a broadband, isometrically sampled seismic data set. Isometric refers to equal sampling in all directions, and is derived from the Greek isos equal and metrica measuring. Broadband refers to a high spatial resolution in three dimensions, vertically and horizontally - both inline and crossline to the acquisition orientation. This case study of the Bruce Field area survey focuses on the uplift in image quality from the full multimeasurement data versus a more conventional hydrophone-only equivalent. We place particular emphasis on the impact to geological understanding in the area, including both qualitative and quantitative interpretation. Examples demonstrate the differences seen from seafloor to the reservoir. Geological setting and imaging challenges The Bruce Field area is located in the UKCS sector of the North Sea, approximately 340km northeast of Aberdeen. It is situated in a historically prolific part of the North Sea, close to the Shetland and Beryl basins, in average water depths of 120 m. The area has a geologically complex overburden as well as at the reservoir level. The geologic sequence comprises Palaeogene and Neogene formations overlying Cretaceous-era calcareous limestones and marls. These exhibit significantly higher seismic velocities than the underlying Jurassic sediments, creating a strong acoustic impedance (AI) contrast at the base Cretaceous unconformity (BCU). Reflectivity is weaker in the target Jurassic reservoir interval, below which lies a Permo-Triassic sand sequence, with stronger AI contrasts, including thin coal beds. Significant faulting is observed below the BCU, including large-scale listric bounding faults, in addition to smaller-scale tilted fault blocks and grabens. These conditions combine to create a wide range of geophysical imaging challenges that hinder generation of reliable volumes for structural and stratigraphic interpretation at the reservoir level. Shallow channels result in a complex near-surface environment, which is expected to cause significant distortion of the recorded seismic wavefield at scale lengths smaller than the separation between streamers. The hard seafloor interface also generates significant short-period multiple contamination throughout the section, including the reservoir interval. Significant loss in highfrequency signal content is observed at and below the BCU, and suggests attenuation by absorption or wavefield disruption in the Tertiary or Cretaceous overburden. Complex faulting and low AI contrasts prevent reliable interpretation of connectivity in the reservoir units, with corresponding uncertainty in well placement. Data acquisition and processing The survey was acquired using a marine isometric seismic technology. The acquisition geometry comprised eight streamers of 3 km length and 75 m separation, towed flat at a depth of 18 m. A single source configuration was employed with 37.5 m separation between consecutive shots. The deghosting capabilities of the multimeasurement streamer on the receiver side were complemented by a multilevel source array to mitigate the source side ghost and provide a broadband seismic dataset. Initial processing steps were applied to mitigate noise modes associated with the forces experienced by the streamer during towing (Teigen et al. 2012), plus additional steps to attenuate residual waterborne noise modes that are not consistent with the wavefield reconstruction process. A generalized matching pursuit algorithm used the multimeasurements to perform wavefield reconstruction in the frequency-space domain using gradient information as described by Özbek et al. (2010). The output

3 was a densely sampled shot grid of virtual streamers equivalent to a 6.25 m by 6.25 m surface increment. Spatial resampling was performed to reduce the data to a 6.25 m by 6.25 m subsurface bin. For the purpose of fair comparison, we derived a reference dataset from the hydrophones of the same point-receiver sensors (scalar pressure measurements). A single-streamer deghosting technique addressed the receiver side ghost notch. Due to the 75 m streamer separation and single-source configuration, the natural crossline common midpoint (CMP) bin size was 37.5 m. Cable infill was performed using a matching-pursuit Fourier interpolation technique to generate an interpolated, broadband, hydrophone-only volume with a m subsurface CMP bin size in the crossline direction. Both datasets were taken through a matching fast-track processing workflow. This included shallow-water demultiple and prestack depth migration. In this abstract, the results represent preliminary, fast-track analysis. We plan for revised interpretations, resulting from the final, optimized workflow, to be available at the time of presentation. After imaging, the two volumes were evaluated based on both qualitative and quantitative interpretation attributes, including prestack inversion. Results: Near-surface geology, Palaeogene sand injectites, and reservoir compartmentalization Figure 1 compares shallow slices from a depth of 174 m below the sea surface (approx. 50 m below seafloor). The volumes are displayed after a 3D interpretation attribute was derived based on structurally sharpened continuous color processing and analysis (SRGB), as described by Laake and Fiduk (2013). 1km 1km Figure 1 Colored SRGB depth slice at 174m showing enhanced resolution of shallow geology from multimeasurement streamer data (right) versus the hydrophone-only equivalent (left). Channels and fine-scale faulting are observed across the entire area. The multimeasurement volume shows good fault clarity, and also definition of the edges and internal characteristics of channels, in a full 3D sense. By contrast, the hydrophone-only data shows image quality that varies depending on orientation of the geology relative to the inline and crossline directions. The multimeasurement images demonstrate the fidelity of the acquisition system, and broad spatial resolution of the smallscale features in the shallow geology for example, resolving channel tributaries of widths less than 20 m. This indicates potential for identification of geohazard zones prior to drilling, and ahead of any detailed site survey work; for example, to aide identification of shallow water flows. Improved imaging of near-surface features may also enable more detailed velocity modeling in the shallow section, which is important for accurate imaging of the deeper structures. The Palaeogene section includes turbidite systems, with evidence of channels on the seismic section and related frequency attributes (Laake, 2014). Injectites resulting from dewatering of sand formations through the overlying muds in earlier times are also expected to be prevalent in formations from the Palaeocene to Eocene. Figure 2 illustrates one example from the Palaeocene Rogaland Group at a depth of approximately 1.7 km, and shows the edge of the graben filled with turbiditic fans and evidence of sand injectites in the deeper areas. The multimeasurement data enable more reliable

4 identification of irregular, localized sand bodies not easily seen on the hydrophone-only equivalent, indicating great variability in composition. Geobodies extracted from the hydrophone-only data show small-scale structural elements of the turbidites and injectites that are clearly orientated normal to the shooting direction. This suggests that asymmetric sampling of seismic data throughout the processing flow may introduce interpretational bias. By contrast, the multimeasurement data set resolves the same structures in all directions, demonstrating that their geometry represents a full range of azimuths and dips. Improved understanding of these structures and potential pore pressure regimes in similar formations may support more confident well planning and efficient drilling. Furthermore, it is possible that these complex structures may also cause distortions in the seismic wavefield, in turn contributing to the attenuation of high frequencies observed in the deeper sections. Figure 2 Geobody extraction of turbidites and sand injectites above the Sele formation, demonstrating resolution of fine structural elements with all orientations from multimeasurement streamer data (right) versus the hydrophone-only equivalent (left). Figure 3 compares geobodies extracted from prestack inversion cubes generated at a test well location in a depth range from 2.8 to 3.1 km below the surface. The strong AI contrast at the BCU and deeper Permo-Triassic sands and coal beds are easily resolved from P-data alone (yellow/red events). However, across the reservoir interval, shear impedance contrast is required to define the reservoir units. This requires prestack inversion, which, in turn, relies on good signal-to-noise characteristics and broad bandwidth. Inversion of the multimeasurement data set demonstrates more reliable geobody extraction for discrete Vp/Vs units in the low AI reservoir zone. There is clear evidence of fault compartmentalization within the reservoir interval. This suggests improved wavelet extraction and stability across different angle ranges; whereas, the hydrophone-only data show evidence of the receiver-side ghost reappearing at the far angles. Relative AI and density attributes were inverted, with the well logs used only for wavelet extraction. No low-frequency model was used from either well or seismic velocity data, indicating the inherently broadband nature of the input data. Conclusions A new multimeasurement towed-streamer system delivered a seismic dataset over the Bruce Field area in the North Sea, and was taken through a fast-track processing sequence. For comparison, an equivalent data set derived from the hydrophone-only measurements followed a matching sequence. High-resolution images in the shallow section show the multimeasurement data fidelity. This indicates the potential for seismic data suited for geohazard identification, while also demonstrating the importance of characterizing near-surface wavefield distortions in imaging the deeper section.

5 Comparison of multimeasurement versus hydrophone-only interpretation of turbidites and sand injectites at the intermediate Palaeogene level demonstrates good spatial imaging of fine-scale structures with complex three-dimensional orientations. This also highlights the importance of uniform sampling in all directions, prior to attribute generation, to avoid interpretational bias. Figure 3 Geobody representation from prestack inversion cubes, indicating more coherent extraction of Vp/Vs intervals (blues, purples, browns) in the low-acoustic-impedance reservoir interval from multimeasurement streamer data (right) versus hydrophone-only equivalent (left). At the depth of the reservoir, low AI contrasts indicate the need for shear impedance from prestack inversion to help delineate reservoir compartments. Improved continuity in the inversion cube from the multimeasurement data suggests better separation of signal from noise, coupled with lowfrequency content to deliver close to absolute impedance without low-frequency model input. Acknowledgements We thank BP and Schlumberger for permission to publish these results, the many contributors to this study, including Andreas Laake, Peter Wang, Pete Watterson, Dominic Lowden, Margaret Leathard, Alan Minney, and the Schlumberger data processing team in Houston. References Laake, A. [2014] Mapping of sand injectites from colour-processed multi-measurement seismic data. submitted to, Amsterdam. Laake, A. and Fiduk, J.C. [2013] Seismic processing and interpretation in colour. First Break, 31, Özbek, A., Vassallo, M., Özdemir, K., van Manen, D.J. and Eggenberger, K. [2010] Crossline wavefield reconstruction from multicomponent streamer data: Part 2 Joint interpolation and 3D up/down separation by generalized matching pursuit. Geophysics, 75(6), WB69-WB85. Robertsson, J.O.A., Moore, I., Vassallo, M., Özdemir, A.K., van Manen, D.J. and Özbek, A. [2008] On the use of multicomponent streamer recordings for reconstruction of pressure wavefields in the crossline direction. Geophysics, 73, A45-A49. Teigen, O., Özdemir, K., Kjellesvig, B.A., Goujon, N. and Pabon, J. [2012] Characterization of noise modes in multicomponent (4C) towed-streamer. 74 th EAGE Conference & Exhibition, Extended Abstracts.