FWI and RTM 3D Seismic Reprocessing of Vintage data from Carbonate Basement

Abstract

SUMMARY Reprocessing of vintage seismic data was conducted to address reservoir uncertainties in a fractured and karstified carbonate basement. The objectives were achieved by employing advanced processing technologies that included Full Waveform Inversion (FWI), TTI Reverse Time Migration (TTI TRM), Adaptive Curvelet Domain Multiple Separation (ACDS) and azimuthal anisotropic NMO. FWI is a data-fitting method designed to deliver high-quality, high-resolution velocity models critical for PSDM imaging. Currently, FWI concentrates on low frequency (3 - 10Hz) transmitted energy (head waves and diving waves) as well as some reflection energy. The resulting FWI velocity model displayed very high resolution and good geological correlation. Forward modeling through the FWI technique generated synthetic shots that corresponded to the real seismic shots accurately. TTI RTM was applied to complement the high resolution FWI model and to achieve the best imaging around the target area where velocities were complex. Prior to the imaging Shallow Water Demultiple (SWD) and Surface Related Multiple Elimination (SRME), multiple separation was as critical to multiple prediction. A conventional Least Squares separation was applied in the 2D X-T domain. However, it was unsuccessful in separating crossing primaries and multiples with similar dips. Therefore, ACDS was applied in the 4D curvelet domain and achieved successful representation of linear and curved events with sparse coefficients. Finally, azimuthal anisotropy was observed in the CDP gathers and in the crossline sections. Azimuthal anisotropic NMO based on a scanning method corrected this effect and aligned the data thoroughly. These technologies covered pre-migration processing, velocity modelling and migration. The combined application of these advanced technologies delivered significant improvement of the overall seismic quality, with reduced multiple contamination, sharper fault definitions, a crisper top basement and superiorfracture imaging within the basement. INTRODUCTION The study area was located in a block with water depths varying between 28–35 metres. Early exploration in this area focused on Tertiary plays. Subsequently a discovery within a pre-Tertiary carbonate reservoir, with thick shales below the carbonate, provided new exciting exploration opportunities (Figure 1). With this development, the seismic reprocessing needed to address the reservoir uncertainties in the fractured and karstified carbonate in order to better evaluate the hydrocarbon potential of the deeper sections. Hence, these advanced processing technologies were proposed during the planning stage. Low seismic frequencies are a key to achieving good FWI results by avoidingcycle skipping and realizing better low frequency velocity updates. Extended seismic streamers achieve deeper penetration for the transmitted energy used in FWI. But for this survey the low frequency signal to noise ratio was poor and seismic streamer was limited to 4500 metres. Therefore, additional efforts were required to attain good FWI results.