Quest for subsalt steep dips Chu-Ong Ting*, Lei Zhuo, and Zhuoquan Yuan, CGGVeritas

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1 Chu-Ong Ting*, Lei Zhuo, and Zhuoquan Yuan, CGGVeritas Summary Advances in wide azimuth (WAZ) acquisition, TTI model building, and Reverse Time Migration (RTM) have brought significant improvements in subsalt imaging. However, imaging steeply dipping subsalt such as the potential three-way subsalt traps underlying a large salt sheet is still difficult. To understand this issue, we propose a new regional illumination method called illumination eye-balls. This method does not require any subsalt structural assumption, or surface interpretation, and can be efficiently applied to a large scale regional illumination study. Our results confirm that even full azimuth acquisition geometry is insufficient to illuminate steeply dipping subsalt reflectors such as faults and 3-way traps. On the other hand, 3D VSP could have provided better chance to illuminate these steeply dipping targets by placing the receivers below the salt. Introduction During the last few years, marine WAZ acquisition has gained acceptance for subsalt exploration in the deepwater Gulf of Mexico (GOM) where complicated salt bodies distort wave field propagation. The benefits of wide azimuth data for subsalt imaging have been widely discussed and reported, (Michell et al., 2006; Corcoran et al., 2007; Ver West and Lin, 2007; Lin et al., 2008; Huang et al., 2009). Advances in WAZ acquisition and imaging technology have produced subsalt images that are superior to images from narrow azimuth (NAZ). Figure 1 compares a NAZ one-way wave equation migration (WEM) image from 2005 with a WAZ RTM image from 2009 near the Jack discovery in Walker Ridge, GOM. They are migrated with their respective velocity model built by the available PSDM technology at the time. WAZ image (Figure 1a) shows improved structural continuity and image clarity, particularly near the crest of the Jack 4-way closure (within the green oval). On the other hand, the image of a potential 3-way trap (within the red ovals in Figure 1a & b), where subsalt reflectors truncate against a dipping salt keel, has not improved appreciably. The lack of coherent events within the red ovals indicates a possible illumination shadow zone. (a) 2005 NAZ one-way WEM image (b) 2009 WAZ RTM image Figure 1: Evolution of the subsalt image at Jack discovery in Walker Ridge. Overall, WAZ image has better continuity and S/N ratio, particularly near the 4- way closure at Jack (indicating by green oval). However, WAZ fails to improve the image within the red oval, where the dipping subsalt reflections are missing. The subsalt reflectors to the left of the potential shadow zone have dips up to 25. If we can only image subsalt dips up to 25, then any subsalt 3-way closure with dips greater than 25 poses significant risk and uncertainty in structural interpretation, prospect identification, and well planning. To mitigate the risk, an illumination study can be useful to verify the existence of a shadow zone.

2 (a) Regional view of 3D half shells populated over 120 OCS blocks (b) view of one single half-shell (i.e. eye-ball) Figure 2a shows the distribution of a single layer of 3D half shells at 9km depth. Figure 2b indicates the eye-ball represents the dips from 0 to 90, and a full azimuth range from 0 to 360. Regional illumination study In the past, a wide range of subsalt illumination had been studied, from a simple ray-based hit-count on single target horizon, to a sophisticated full-blown 3D acoustic wave equation modeling with fine layered reflectivity. These approaches require extensive structural interpretation and surface editing to build a set of 3D consistent geological surfaces free of artifact (Kabir et al, 2006). For a regional illumination study covering several thousand square kilometers, the required interpretation effort becomes time-consuming and impractical. Furthermore, the interpreted surfaces have to be extrapolated into the no data zone (i.e. shadow zones) based on certain geological assumptions. If the extrapolation is wrong, then the illumination results are not trustworthy. As a result, the conventional illumination approach is not suitable for a regional-scale illumination study, especially for areas with shadow zone issues. Alternatively, we propose a new approach that utilizes illumination eye-balls for a regional illumination study. The details of this approach are described in Ting, Basically, it involves three key steps. First, we populate 3D half-shell surfaces throughout the entire area in a 2x2x2km grid, with a radius of 500m. This is illustrated in Figure 2a, where only one layer of these 3D half-shells, at 9km depth is shown. Since half-shell surfaces contain full dip and azimuth angles, the resulting illumination is not limited to a specific subsurface interpretation, and they are well suited for studying shadow zones where the dip and azimuth are uncertain. Figure 2b shows the oblique view of one half-shell. The four iso-dip contour lines correspond to a dip of 10, 20, 40, and 60, respectively. Due to its appearance and functionality, we named these half-shelled surfaces illumination eye-balls. Second, we perform 3D acoustic two-way waveequation forward modeling to generate synthetic prestack data for a given acquisition geometry using the final salt model from existing project, and the density model with illumination eye-balls inserted. In the final step, the synthetic data is migrated using RTM and the resulting amplitudes of each eye-ball are extracted and displayed as 3D surfaces (eyeballs). The extracted amplitudes represent the illumination strengths. Examples of illumination eyeballs are in Figures 3-5. We apply illumination eye-balls for an area of 120 OCS blocks in Walker Ridge that centers at the Jack discovery. A NE-SW WAZ acquisition geometry is used to simulate an existing WAZ survey that has 4km crossline offset and +/-8km inline offset. The source spacing is 150m by 500m. We also generate NAZ illumination eye-balls by limiting the crossline offset to 1km. Figure 3 shows illumination eye-balls at 9km depth for NAZ geometry (Figure 3a) and for WAZ geometry (Figure 3b). Most of eye-balls are beneath complex allochthonous salt bodies with few located in the deep salt withdrawal mini-basins. The outlines of the mini-basins are marked with solid white contours. The illumination results are displayed within the range of -30dB to 0dB, from low illumination in red, to good illumination in green (~0dB). An inner circle that marks a dip of 40 is also shown for each eye-ball. From Figure 3a and 3b, we observe the following: Within mini-basins (inside the solid white contour in Figure 3a and b), the illumination eyeballs are mostly green, indicating good illumination for all azimuth and dip angles. Because the overburden velocity is simple,

3 WAZ illumination is only marginally better than NAZ illumination within mini-basins. In subsalt regions, illumination eye-balls for NAZ geometry show generally poor illumination. In contrast, WAZ yields better overall illumination, especially for dips below 40 (inner circle of the eye-ball). Subsalt dips greater than 40 are not illuminated by either NAZ or WAZ. Moreover, a few eye-balls (those to the right of the blue oval in Figure 3) show poor illumination at all dips. To better examine subsalt illumination, a NE-SW line across the Jack area (blue oval in Figure 3) is extracted from the existing WAZ data and shown in show good illumination up to dips of 20 to 40. As a low relief anticline with dips generally less than 20, Jack structure should be well illuminated. In this case, seismic image is consistent with the results of illumination eye-balls. The extracted line also shows a subsalt truncation against a dipping salt keel at the far right hand side (indicated by a blue arrow, notice the same eye-ball is enlarged in Figure 5a). However, the subsalt events dim and disappear before reaching the salt. The lack of illumination is confirmed by the corresponding eye-ball that shows poor to no illumination, (a) WAZ Eye-ball (a) FAZ Eye-ball Figure 5: Illumination eye-ball from a NE-SW WAZ geometry (left) and from a full azimuth geometry (right). Illumination eye-balls are from the location indicated by the blue arrow in Figure 4. FAZ geometry produces marginally better illumination than the WAZ geometry does. (a) Regional illumination eye-balls for a NAZ geometry (b) Regional illumination eye-balls for a WAZ geometry Figure 3: Regional illumination eye-balls at 9km depth in the Walker Ridge area. Illumination color scales range from -30dB as red to 0dB as green, with -15dB as yellow. The inner circle within each eye-ball represents a dip of 40. The white contours outline the extent of deep minibasins and the blue circle marks the Jack discovery. Figure 4, together with WAZ illumination eye-balls. The eye-balls in the vicinity of the Jack anticline Figure 4: WAZ RTM image with illumination eye-balls along a NE-SW (SW to the left) profile across the Jack discovery (left). The illumination eye-balls are at 9 km depth and the contours within each eye-ball correspond to dip angle of 10, 20, 40, and 60 respectively. The same color scheme as in the previous figure is used to represent illumination strength. The blue arrow points to a potential shadow zone where the illumination drops to less than - 24dB for any SW dips greater than 25 degree. particularly for SW dips (dip to the left side of Figure 4). Apparently, the existing NE-SW WAZ is not sufficient to image this subsalt truncation. In fact, the lack of images near subsalt truncations is common in GOM and it has hampered the identification and delineation of potential 3-way traps. The challenge is to find an acquisition geometry that can illuminate the subsalt truncation. The most complete acquisition is to acquire a fullazimuth (FAZ) survey that has the same offset dimension (10 km) in both inline and crossline directions. Figure 5 compares illumination eye-balls at the location of subsalt truncation between a NE- SW WAZ geometry and full-azimuth geometry. Generally, there is no appreciable difference between the two geometries. Illumination is the poorest for SW dips, with no illumination for dips greater than 25. The results show that this steep dip cannot be imaged with surface seismic data.

4 3D synthetic modeling To further confirm the conclusion drawn from the illumination eye-balls, we conducted a full-azimuth 3D acoustic modeling with a fine layered density model covering 30 OCS blocks located within the regional illumination study area. In contrast to the regional illumination eye-balls method, this approach requires extensive interpretation of 3D geological surfaces in order to build a fine layered density model incorporating layered sediment beddings, 4-way closure, 3D sinusoidal channels, steeply dipping subsalt faults (50 0, 60 0 and 70 0 ), and a three-way closure with up-dip (up to 45 0 ) truncated against salt. The final density model is shown in Figure 6a. Inspired by the successful cases of VSP to image surface seismic blind spots (Hornby et al, 2007; Burch et al, 2009), we also simulate a 3D spiral VSP acquisition to evaluate the VSP imaging capability in comparison to surface seismic. Eighty-one VSP receivers were displaced at the 4-way closure, from 6km to 10km with a 50m interval (red dots in Figure 6c). The overall areal coverage of the spiral sources is 10km radius (blue dots in Figure 6c). To limit our scope of the study to only illuminationrelated effects, surface multiples are excluded in the acoustic modeling by adopting an absorbing freesurface boundary condition. The modeled shot gathers are then migrated with RTM. Figure 6 shows the density models, migrated section for FAZ and 3D VSP modeling. While FAZ surface seismic is able to image the 4-way closure and the channels (blue arrows), it is not able to image the subsalt faults and the sediments (dips > 30 0 ) near the 3-way closure. The shadow zone marked by red oval is similar to what has been observed in the real data at Figure 1. The result is consistent with the regional illumination eye-ball that they both confirm the surface seismic is insufficient to image steeply dipping subsalt. On the other hand, Figure 6c shows a 3D VSP is able to image most of the subsalt steep dips, as indicated by the red oval and green arrows. The main drawback is its localized illumination to flat reflectors near the well location and steep dips in the adjacent area. Consequently, it requires both surface seismic and VSP image for a complete subsalt interpretation. Conclusions To summarize, we have presented an accurate and flexible method for regional illumination studies using illumination eye-balls. Our results show that while current WAZ acquisition yields much better illumination than NAZ acquisition, it is still insufficient to illuminate steep subsalt dips higher than 30 to 40. Furthermore, we have shown that even a FAZ geometry has limitations in imaging steep dips associated with subsalt truncations. On the other hand, a 3D VSP manages to image subsalt steep dips by placing receivers below the allochthonous salt. The success of 3D VSP in modeling subsalt steep dips demonstrates its effectiveness for specific target-oriented imaging objectives. A 3D VSP can be used as a good complement to surface seismic acquisition. However, due to its localized illumination, VSP imaging is highly dependent on the specifics of subsalt structures, wellbore location, and receiver depth. A modeling study is needed to understand the objectives and to design an effective VSP acquisition program for the intended subsalt targets. Acknowledgements The authors are grateful to CGGVeritas for permission to publish this work. We also thank Jerry Young, Bruce Ver West, and Scott Shonbeck for reviewing the paper. (a) Density model (b) RTM images of FAZ modeling (c) RTM images of 3D VSP modeling Figure 6: RTM images of FAZ and 3D VSP modeling. The FAZ image (b) shows good imaging quality for channels (blue arrows) and 4-way closure, while the 3D VSP image (c) shows steep dip structures associated with the faults (green arrows) and sediment truncations (red oval).

5 References Burch T., Hornby B., Sugianto H., Nolte B., Subsalt 3D VSP imaging at Deimos Field in the deep water Gulf of Mexico: 79th Annual International Meeting, SEG, Expanded Abstracts, Corcoran, C., Perkins, C., Lee, D., Cattermole, P., Cook, R., Moldoveanu, N., A wide-azimuth streamer acquisition pilot project in the Gulf of Mexico: The Leading Edge, 26, Hornby, B., J. A. Sharp, J. Farrelly, S. Hall, and H. Sugianto, 2007, 3D VSP in the deep water Gulf of Mexico fills in sub-salt `shadow zone': First Break, 25, Howard, M., Marine seismic surveys with enhanced azimuth coverage: Lessons in survey design and acquisition: The Leading Edge, 26, Huang, T., Zhang, Y., Zhang, H., Young, J., Subsalt imaging using TTI reverse time migration: The Leading Edge, 28, Kabir, N, Burch, T, Etgen J, Subsalt wavefield illumination and amplitude study on Mars field in the Gulf of Mexico: 76th Annual International Meeting, SEG, Expanded Abstracts, Michell, S., Shoshitaishvili, E., Chergotis, D., Sharp, J., Etgen, J., 2006, Wide azimuth streamer imaging of Mad Dog; Have we solved the Subsalt imaging problem?: 76th Annual International Meeting, SEG, Expanded Abstracts, Stork, C., Predicting subsalt dip-dependent illumination variations using density bubbles with RTM migration: 79th Annual International Meeting, SEG, Expanded Abstracts, VerWest, B. J., and D. Lin, 2007, Modeling the impact of wide-azimuth acquisition on subsalt imaging: Geophysics, 72 no.5, SM241-SM250