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1 A broadband full azimuth land seismic case study from Saudi Arabia: interpretation and inversion lessons learned Brian P. Wallick*, Luis Giroldi, and Husam Mustafa, Saudi Aramco; Ayman Suleiman, Dominic Lowden, Karim Elsadany, Mukarram Ahmed, and Roberto Fainstein, WesternGeco. Summary Interpretation of conventional wide azimuth 3D seismic data over deep reservoirs in Saudi Arabia has reached its limits. Impedance calculated from these data and long used as a predictive tool is low in both resolution and fidelity from the standpoint of reservoir properties. Further seismic reprocessing and recalculation of inversion is considered to yield limited upside. A state-of-the-art, high-density full azimuth seismic dataset was acquired over a producing field in eastern Saudi Arabia to attempt to address these issues. The southern one-third of the total volume was selected with well control to develop, optimize, and quality control processing workflows. This pilot volume was processed through anisotropic prestack time migration followed by poststack spatially-adaptive wavelet processing and spectral balancing. Analysis of the new reflection seismic data volume shows an increase in bandwidth with improved signal-to-noise ratio when compared with legacy 3D data. Colored inversion was prepared from the pilot volume and compared with the legacy colored inversion conventional 3D volume. The increase in signal bandwidth significantly improves the relative impedance resolution when compared with the legacy inversion. Use of colored inversion simplifies the process of horizon picking and was used to generate horizon control for use in calculation of absolute impedance. Seismic data-driven relative inversion compares favorably with the well-log-derived low frequency model. Acoustic impedance inversions were computed from the new data and benefited significantly from the increase in low frequency signal. In the final absolute acoustic impedance inversion, a portion of the bandwidth typically reserved for well-generated models was replaced by seismic low frequencies. As the methodology for incorporating seismic low frequency into the inversion process improves, so will the prediction of reservoir properties and geometry. Extraction of geobodies at this time varies among the several inversion results and this variability reflects uncertainty in methodology. Additional work will be required to determine which methodology produces the most accurate predictive capability. Introduction To address a need for improved reservoir property prediction, a high-density full azimuth 3D land survey was acquired in 2011 over a producing oil and gas field in the eastern province of Saudi Arabia. This area was selected because it is the continuing focus of both production and exploration activities, has multiple reservoir targets in the Jurassic and Permo-Carboniferous sections, and has a wealth of well-derived data. In the past, this area has presented significant imaging challenges particularly in the deep clastic Paleozoic section due to limited bandwidth, various forms of noise and contamination by multiples. The legacy narrow azimuth 3D volume for this area has a nominal fold of 486 (25 m x 25 m common midpoint bins). It was recorded with a 12 s linear 8 to 80 Hz sweep and a maximum crossline and inline offsets of 1.8 km and 5.4 km, respectively. These data have seen continuous use for development and exploration well placement, and have undergone a minimum of two reprocessing efforts over the past seven years. The new full azimuth volume has a nominal fold of 9,216, 6.25 m x 6.25 m common midpoint bins and offsets up to 6 km in both the inline and crossline direction. It was recorded with a broadband, 3.5 to 90 Hz at -3 db, maximum displacement 12 s nonlinear sweep (Bagaini, 2008). At the beginning of the project, the team was concerned with the significant increase in seismic data volume over the legacy dataset. The total volume of prestack source records was approximately 190 Tb. For quality control purposes, the team agreed to generate a fully-processed pilot volume from the southern one-third of the complete full azimuth 3D survey. These data were used to test and evaluate the processing workflow and provide interpreters the opportunity to suggest changes or improvements in the processing sequence. The data were migrated in the common offset vector tile (COVT) domain and stacked (Cary, 1999; Vermeer, 2000). The poststack data were further enhanced through spatially-adaptive wavelet processing and spectral balancing. Comparing and contrasting the new and legacy data sets is relatively straightforward. Figure 1 shows the representative frequency spectra extracted across a 2000 to 2500 ms window, centered on the primary Permian clastic SEG Las Vegas 2012 Annual Meeting Page 1

2 reservoir, for both 3D volumes. The new data have achieved an extension in bandwidth and an increase in the signal-to-noise ratio. Recoverable high frequency signal is now up to approximately 55 Hz and the lower end of the frequency spectrum has extended to around 4 Hz. The significance of this low-end uplift is seen in a 0 to 10 Hz band-limitedd data comparison of both the new full azimuth and narrow azimuth legacy datasets (Fig. 2). The new dataa are fully interpretable, while the legacy data displays a monochromatic appearance caused by a lack of low frequencies in the 8 to 80 Hz sweep design. relative impedance. Figure 3 shows a comparison across a singlee merged seismic line of the original model-based inversion from the legacy data, the colored inversion from the new 3D survey and a previously calculated colored inversion from the legacy data set. The new colored inversion offers a higher frequency result but also clearly benefits from the extended low frequencies. Similar to the previously shown band-limited seismic data (Fig. 2), the monochromatic lookk of the legacy colored inversion is thought to be a function of limited low frequency signal. The new colored inversion compares favorably with the original model-basedd inversion but appears less noisy and richerr in detail. Figure 1. Frequency spectra from the legacy (left) and new broadband (right) seismic data extracted over a window of 2000 to 2500 ms. Red is signal and blue represents the noise floor. Figure 2. Identical transect through both data setss showing only the 0 to 10 Hz contribution. All improvement in the new data is due to the broader bandwidth with the addition of low frequencies (right). The legacy data (left) looks very monochromatic and does not have frequencies below 8 Hz. White horizontal bar is approximately 5 km and the vertical time scale is in 100 ms increments. The low frequencies promise to provide the means to better image deep targets and to deliver improved inversionn results by providing that portion of the spectrum typicallyy absent in conventional land reflectivity data and usually provided solely by well log data (Dragoset and Gabitzch, 2007). Figure 3. Adjacent inversion results along the same traverse: a) the legacy model-based inversion, b) the colored inversion of the new data, and c) the colored inversion of the legacy data. Warm colors represent low impedance. Horizontal and vertical white bars are 5 km and 5000 ms, respectively. Generating horizons from the new dataset as input data for full acoustic impedance inversion was less complex and generally faster thann horizon picking on the legacy data. The new full azimuthh data can be used to produce coherent interpretation over large portions of the 3D survey while still preserving structural detail in comparison with generally poorer continuity and tracking in the legacy 3D. Acoustic impedancee inversion The deterministic poststack acousticc impedance inversion workflow can be simplified to four basic steps: Preliminary interpretationn While conventional interpretation of both datasets has been accomplished to some extent, preliminary horizon work was done by first computing the colored inversionn (Lancaster and Whitcombe,, 2000) for the new full azimuth data set and subsequently tracking major changes in Space-adaptive wavelet processing Relative acoustic impedance inversion (RAI) Low-frequency model building (LFM) Combiningg RAI and LFM to generate absolute acoustic impedance (AAI) SEG Las Vegas 2012 Annual Meeting Page 2

3 Space-adaptive wavelet processing provides two main benefits prior to inversion: it produces a near zero-phasee data set and normalizes the seismic wavelet to a spatially stable and well-defined wavelet for use in the inversionn process. This approach is widely used as an objectivee means of nverting for reservoir properties (Huyen et al, 2011). Iterative discrete sparse spike inversion is computed for these wavelet-processed data that produces a synthetic reflectivity sequence for each trace. The data are then filtered to remove frequencies outside the sweep range, integrated to RAI and converted to zero mean amplitude. seismic contribution in 1 Hz steps starting at 10 Hz and goingg down to 2 Hz. For each step, impedance results were compared with thee well log data and a correlation coefficient was calculated showing the quality of fit for each of the results. Figure 6 shows the result for each of two wells indicating an increasee in correlation with increasing seismic input. The correlation with the log data reaches a peak at 3 Hz and below this value, begins to decline. This suggests that the useful seismic bandwidth extends as low as 3 Hz and these low frequency data are real signal and can add value to the inversion process. Figure 4 compares the RAI from the legacy and new fulll azimuth data, where a 2 Hz low cut filter has been applied to both datasets. Similar to the results seen in colored inversion computation, the full azimuth data show the contributionn of low frequency signal to the volume. The resultant relative impedance layering is continuous and well definedd in contrast to the legacy data. The typical low-frequency model is constructed using welll log data alone and accounts for the 0 to 12 Hz portion of the bandwidth missing from land seismic data. However, the new fulll azimuth data has recovered signal above 3 to 4 Hz and can be used to assist in model construction between 4 and 12 Hz. Compared with the model generated from two wells from the field, the seismic-only RAI is distinctly similar (Figure 5) in the zone covered by logs, but in addition, extends above and below well data. This comparison supports the hypothesis that low frequency seismic dataa can contribute significantly to the quality of impedance estimates. Figuree 4. Relative acoustic impedance calculated from legacy (left) and new broadband dataa (right) showing the importance of the low frequency contribution. Trace spacing is 12.5 m. Discussion Ultimately, the goal of both interpretation and inversion is to provide an accurate description of any given subsurface reservoir. These results may be used in a spectrum of taskss that range from simple minimization of risk for a given development or exploration drilling location to providing reservoir property and geometry information for reservoirr simulation. Now that low frequency sweep designs can extend the lower frequency bandwidth we can anticipate further improvements in seismic resolution and impedancee prediction. We are only in the early stages of developing the fulll potential of extended low frequency seismic data and willl begin to address the question: at which frequency do we cease using the seismic dataa and begin to use the well data? A simple test was devised in which impedance was calculated for each case using cutoff frequencies for the Figuree 5. A comparison of low-frequency models as generated from data-only (left) and wells only (right) showing a very good matchh through the logged interval. Trace spacing is 12.5 m. Two separate AAI results are shown and compared with the colored inversion generated earlier ( Figure 7). Because the colored inversion does not rely on an a priori model but ratherr the inherent seismic bandwidth and well-derived acoustic impedance spectra, it provides an opportunity for comparison of resultss from AAI calculations. SEG Las Vegas 2012 Annual Meeting Page 3

4 volumetric geobodies in order to assess the possibility of reservoir compartmentalization and, hence, new drilling opportunities. As cann be seen in Figure 8, the acquisition methodology selected for inversionn calculation impacts both relative properties and reservoirr geometry. More work is required to determine which of these final products provides the best answer. Most likely, the best solution is somewhere in the middle. Figure 6. Graph showing goodness of fit between inversion and well data with increasing low-cut filtering. Figure 7. Comparisons of broadband colored inversion to AAI results from two methodologies. The model-based inversions are shown to the right of the well. The first comparison (a.) shows a horizon-limited (no data areas) 5 Hz crossover frequency while the second (b.) shows crossover at 4 Hz over the complete time window. Warm colors represent low impedance. White ovals mark areas for comparison and show how each approach creates a different reservoir solution. Horizontal and vertical white bars are 5 km and 5000 ms, respectively. The two results differ in crossover frequency (4 vs. 5 Hz) and calculation by slightly different algorithms. Focusing on one of the interpretationn problems at the reservoir thee distinction between productive reservoir and overlying non- productive clastic strata it is seen in Figure 7 that each solution shows a different answer and that each does not reproduce the more desirable result seen in the colored inversion. The former finds low impedance in the upper and lower clastic units but does not show the expected separation of the two. Extracted geobodies from this volume used to characterize the lower unit will likely fail. The latter method produces what appears to be uniform low impedance at the reservoir level but provides little informationn regarding the upper layers. It appears that scant geologic information can be gained from these data and some of the apparent reservoir detail is not as evident. Careful evaluation of all blind-test well results in comparison with each inversion product should provide a conclusion. Figuree 8. Geobodies extracted from broadband full azimuth AAI data with 5 Hz crossover (top) and legacy AAI data (bottom). Note the differences in both geometry and extent. Conclusion Results from this work indicate that the recovery of low frequency signal has a dramatic impact on both interpretation and inversion. While much work remains to be done with these data in the areas of prestack inversion, amplitude analysis, azimuthal anisotropy, and others, these results show an increase in impedance resolution and ease of interpretation. As recovery of broadband signal becomes more prevalent in the industry, we expect that techniques for interpretation andd inversion will improve as well. Acknowledgments The authors thank the Saudi Arabian Oil Company (Saudi Aramco), for permission to publish this paper. The authors are grateful to Peter Crisi, Ayon Dey, Aiman Bakhorji, and Mike Jervis for their constructive comments. In the visualization realm, the interpreter will likely look for trends in the data through the extraction of 3D 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 Bagaini, C., 2008, Low-frequency vibroseis data with maximum displacement sweeps: The Leading Edge, 27, Cary, P. W., 1999, Common-offset-vector gathers: An alternative to cross-spreads for wide-azimuth 3-D surveys: 69th Annual International Meeting, SEG, Expanded Abstracts, Dragoset, B., and J. Gabitzch, 2007, Introduction to this special section: Low-frequency seismic: The Leading Edge, 26, Huyen, B., J. Graham, S. Singh, F. Snyder, and M. Smith, 2011, Incorporation of geology with rock physics enables subsalt poststack inversion: A case study in the Gulf of Mexico: Geophysics, 76, no. 5, WB53 WB65. Lancaster, S., and D. Whitcombe, 2000, Fast-track coloured inversion: 70th Annual International Meeting, SEG, Expanded Abstracts, Vermeer, Gijs J. O., 2000, Processing with offset-vector-slot gathers: 70th Annual International Meeting, SEG, Expanded Abstracts, 5 8. SEG Las Vegas 2012 Annual Meeting Page 5