Time vs depth in a North Sea case study of the ugly truth Abstract

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1 Time vs depth in a North Sea case study of the ugly truth Thomas Massip 1, Lauren Braidwood 1, Juergen Fruehn 1, Owen Isaac 1, Jonathan Denly 1, Robert Richardson 2, Phil Mollicone 3 1 ION Geophysical; 2 INEOS Breagh; 3 Sterling Resources Abstract In an industry where exploration is declining and budgets are decreasing, reprocessing of vintage data is of upmost importance. Vintage data processed with the most up to date demultiple techniques, broadband processing and denoise practices can produce enormous uplift in the quality of the data used for velocity model building. In many reprocessing projects both prestm and presdm products are generated. However, the right choice of these products should be driven by the complexity of the geology which is to be imaged. The Breagh case study is a prime example of how presdm is necessary to properly image the target formation. The aim of this paper is to show how presdm products in complex areas may be the more cost and time efficient solution.

2 Introduction In the current economic climate, re-processing can be an important and cost-effective way of maximising the value of vintage seismic data, providing new insight into an area without the costly addition of acquisition. During 2015, re-processing of a 1995 survey over the Breagh field in the North Sea was carried out, in conjunction with processing of a new survey acquired by Polarcus in The main challenges of the Breagh field are the data quality of both datasets, the strong multiple energy at target level and the complexity of the geology in the area. Outcropping chalk in the region produces strong multiple energy within the data which can be difficult to remove due to very short offsets in the shallow section. Geological challenges are: outcropping shallow chalk (high velocity contrasts in short offset parts of the gathers), severe faulting in overburden, salt structures and detached salt intrusions. The two surveys have been reprocessed to produce dual-azimuth (DAZ) prestm and presdm products (the 1995 dataset was previously processed in 2008). Developments in processing techniques between 2008 and 2015 lead to significant improvements in data quality with the most noticeable uplift coming from demultiple techniques and workflows, including 3D SRME and SPMA (wavefield extrapolation short period multiple attenuation). Broadband de-ghosting was also applied to remove source and receiver ghost components and improve the useable bandwidth of the data. In this case study, seismic displays will only show the vintage 2008 survey (due to data ownership restrictions). The 1995 data were acquired by GecoPrakla using four cables of 3000m length, source and receiver depths were 5m and 7m respectively, giving source and receiver notches at ~149Hz and ~106Hz. The zero-frequency notch combined with a low cut acquisition filter of 3Hz (6dB/Octave) also resulted in a significant reduction of the very low frequencies. The enhanced, cleaner gathers resulting from this reprocessing, with useful information at their longest offsets, were input to an iterative velocity model update using non-parametric picking in conjunction with high resolution TTI tomographic inversion. In this paper, an overview of the broadband processing and uplift derived from advanced demultiple is presented, leading into a comparison of time and depth migration, to show the necessity of anisotropic depth velocity model building in the Breagh area. Demultiple and Broadband Workflow Enhancements in demultiple techniques and strategies resulted in significant improvements in data quality between 2008 and 2015 processing. Broadband deghosting technology was applied to the data to remove source and receiver ghosts and increase the bandwidth of the data (Zhou et al., 2012; O Driscoll et al., 2013). Extensive testing was carried out to find the optimal deghosting result: the most effective result was achieved when deghosting was applied after multiple suppression. Results for the 2015 processing are shown in Figure 1. Figure inner track 2D stacks, before (left) and after demultiple (right) processing.

3 The corresponding deghosted results are shown in Figure 2, and amplitude spectra for the demultiple and deghosting results are shown in Figure 3. Figure 2. 2D stacks, inner track, before (left) and after (right) deghosting for 2015 processing. Figure 3. Left: Amplitude spectra before (blue) and after (green) demultiple for 2008 data and before (red) and after (purple) 2015 processing (spectra are not normalized). Right: Amplitude spectra before (blue) and after (red) deghosting for 2015 processing Model Building and Migration A prestm product is a solution to enable initial observations of potential target formations in a short time frame. However a comparison of prestm vs presdm final migrated stacks reveals that this is not always a solution worth pursuing. In a standard prestm workflow, the velocity model is generated typically using 2 iterations of parametric automatic picking (Figure 4A). The time velocity model does capture some of the lithological units but the model remains non geological at target level. The geological complexity of the Breagh area requires a highly detailed, interpretative and iterative velocity model building workflow to properly image the target reservoir (Jones, 2010; Jones 2012). The model used for depth migration is a highly interpretative geological model that was derived from five iterations of TTI anisotropic depth velocity model building using non-parametric hybrid-gridded tomography (Figure 4B). The velocity field is both conformable to the structure across the section and correlates with the well logs available in the area. Joint velocity and epsilon tomographic inversion was used to derive anisotropy fields in the over burden. Much effort was put into picking the Top Zechstein event. In this area this event is not necessarily visible and as a result several iterations of Top Salt picking were required to converge towards the true position. This is visible in Figure 5 which shows a depth slice through both prestm and presdm velocity models and migrated stacks. The difference in detail in the velocity model is obvious. More importantly, Figure 5B shows the level of detail required to properly image the geology in complex areas such as Breagh (Fruehn et al. 2014).

4 Figure 4. PreSTM (A) and presdm (B) velocity models overlaying respective final migrated stacks; pink: depth slice locations for Figure 5. Sections are displayed in depth with prestm data stretched to depth. Figure 5 PreSTM (A) and presdm (B) velocity model depth slices (1600m) overlaying respective final migrated stack slices (depth location indicated by pink dashed line on Figure 4.) The Breagh project is a prime example where effort and time is essential to produce a good velocity model and in turn properly image the geology. Although the final prestm stack does show good imaging of the complex faulted overburden, the quality of the presdm stack is notable everywhere, and produces higher quality in layer resolution, fault definition and positioning. Most importantly the resolution of the target Carboniferous section is greatly improved compared to the prestm on which it is almost non-existent. This fact puts into question the use of a prestm within an area of such geological complexity. Depth slices at 1600m, through the salt and overburden, show significant differences between prestm and presdm final migrated stacks (Figure 6). Layer continuity and resolution is much superior in the presdm. This is to be expected but the more important realisation is that the prestm velocity model is already significantly inferior which degrades the imaging at the target.

5 Figure 6. PreSTM (A) and presdm (B) final migrated depth slices (1600m) (depth location indicated by pink dashed line on Figure 4.) Conclusions The Breagh case study is a good example of how the entire seismic reprocessing workflow should be tailored to the complexity of the environment. It demonstrates the uplift in data quality that can be achieved through reprocessing of vintage data. Enhancements in demultiple technologies and strategies allow for significant improvements in the quality of existing data, providing a better platform on which to base further processing and imaging. This case also illustrates the effectiveness of broadband processing on vintage data, which can be successfully deghosted and the useable bandwidth of data increased even for older data not acquired with broadband in mind. This leads to cleaner data, more accurate model building and ultimately superior final products and subsequent interpretations. In many cases, prestm products are produced for initial observations of target formations and comparisons with vintage prestm data. Concerning these two points we can conclude the following: - A presdm in a complex area such as this will better image the target formation and provide more reliable information. - In the case where a prestm product is required to compare to vintage, a more accurate comparison would be a presdm of the new and the vintage data. The point of this case study is to highlight the need for a seismic reprocessing workflow that takes the geological complexity into account. In complex areas presdm products may be the more cost and time efficient solution. Where a comparison of new versus vintage data is required, depth migration of the vintage data is a far superior comparison than a time migration of the new data. Acknowledgments We thank INEOS Breagh, Sterling Resources and ION for permission to present this work, John Chamberlain for his contributions to the project and Ian Jones for help in preparing the material. References Jones, I.F, [2012], Tutorial: Incorporating near-surface velocity anomalies in pre-stack depth migration models. First Break, 30(3), Jones, I.F. [2010] An Introduction to Velocity Model Building. EAGE, Houten. Fruehn, J., K., Greenwood, S., Valler, V., and Sekulic, D., [2014] Resolving small-scale near-seabed velocity anomalies using non-parametric autopicking and hybrid tomography, CSEG Recorder, 39, (10) O'Driscoll, R., King, D., Tatarata, A., and Montico, Y., [2013], Broad-bandwidth data processing of conventional marine streamer data: An offshore West Africa field case study. SEG Technical Program, Expanded Abstracts. Zhou, Z. Z., M. Cvetkovic, B. Xu, and P. Fontana, [2012] Analysis of a broadband processing technology applicable to conventional streamer data. First Break, 30(10),