Ocean Bottom Node Acquisition and Processing Techniques Provide New Insights into the Exploration Potential of the Gulf of Suez

Abstract

The complex geology of the basin makes the Gulf of Suez one of the most challenging areas for seismic imaging. Strong impedance contrasts between lithological boundaries, such as the top of the layered evaporite sequence (LES) and various intervals within the heterogeneous LES, generate strong multiple reflections that mask the subsalt target at the reservoir level. Additionally, the halokinesis creates illumination effects in the pre-salt section. Existing legacy seismic data in the area, primarily from towed-streamer acquisitions, suffers from poor imaging, heavy multiple contamination, and a lack of illumination as the complex subsurface scatters much of the wavefront energy before it penetrates deeper pre-salt targets. We performed extensive survey design and modelling (SDM) studies accounting for the challenges in the Gulf of Suez and completed simulations of different acquisition programs to evaluate the best parameters to solve these challenges. SDM studies confirmed that the long-offset, wide-azimuth, and high-fold attributes of an ocean-bottom node (OBN) acquisition coupled with a short, towed-streamer acquisition for near offsets would overcome these challenges to deliver improved images of the pre-salt target. Hence, in 2020, we executed the first ever combined OBN and towed-streamer acquisition in the Gulf of Suez, acquiring suitable data in the presence of the various obstructions including shipping lanes, platforms and pipelines that are common in this area. We present the challenges faced and our innovative use of existing seismic data processing technologies to maximize the potential of the inherent benefits of the full-azimuth, longer offset, higher fold and multi-component data recorded by the OBN acquisition. The up-down deconvolution (UDD) technique was used to attenuate surface multiples. As almost all the layers from the water bottom to the salt body are considered as strong multiple generators, internal multiples were also a significant challenge, and hence both short towed-streamer and OBN data were used to predict the internal multiples using all major multiple generators in the prediction process. The earth model building was performed using a cascaded top-down approach. Full-waveform inversion updates were interleaved with common-image point tomography updates, to define the shallow sediments, evaporite section and pre-salt target. Integration of the data available; OBN, streamer, gravity, magnetics, near field and well data was key to the success of the earth model building process. Comparing the final results with the legacy seismic image available, we observed that the OBN data substantially improved the imaging of pre-salt structure, stratigraphy, rotated fault blocks and dipping strata of the pre-rift and syn-rift reservoirs. Dip-meter data in the surrounding wells validated the steeper dips. The OBN image, combined with the short towed-streamer data, provides new insights into the pre-salt target and opens avenues to re-explore mature and prolific areas of the Gulf of Suez.