GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 3.3 735 NEW FRONTIERS IN MARINE SEISMIC ACQUISITION: AN EXAMPLE OF OCEAN BOTTOM NODE SURVEY DESIGN B. Castelbarco, S. Baudo, M. Buia, F. Garofalo, J. Panizzardi Eni Upstream and Technical Services S.p.A., AESI (Seismic Acquisition and Processing) department., Italy Introduction. In the last decades a new technology for marine seismic acquisition is getting more and more popular in the hydrocarbon exploration world: the ocean bottom receiver systems. Contrary to a more conventional towed-streamers acquisition, these devices lay on the sea floor while on the sea surface the source vessel is shooting. Among the several advantages of this layout, the most significant is the capability to guarantee very long offsets with a full azimuthal range that helps to solve some critical issues as the lack of seismic illumination in case of structure with complex geometry as salt diapirs. Some of the first examples of ocean bottom technology see the use of cables laying on the floor both for exploration purposes and for 4D seismic reservoir monitoring, because this layout ensures a high receiver repeatability especially in those areas heavily populated by infrastructures and obstacles. However, the operational water depth of the ocean bottom streamers is limited to few hundred meters. A further development in this sense is represented by Ocean Bottom Node (OBN) that exploits all the advantages of the ocean bottom technology also in deep water. OBN surveys have been successfully conducted over some fields as in the Gulf of Mexico (Lewis et al. , 2016). In a recent feasibility study, this option was considered as the most reliable and cost effective to solve some geological challenges in an area characterized by the presence of salt diapirs and where the water depth ranges between 1400 and 1800 m. Geological framework and geophysical challenges. In the area of interest for this study, the sedimentation and reservoir structure are deeply affected by salt tectonics. After a large accumulation, up to 1000 m, of evaporites during the mid-Aptian time, the Early Cretaceous break of Gondwanaland and the sedimentation of Cenozoic caused the movement of the salt itself. The hydrocarbon targets are represented by the post Miocene units that consist of shale dominated sediments with localized channel-fill sandstones that pinch against the salt flanks. Specifically, in the area of interest, there are mainly two shallow salt diapirs: geometrically- complex bodies with sharp flanks characterized by petrophysical properties that strongly differ from the surrounding sediments. The geological framework leads to very specific geophysical challenges, due to the presence of sharp slopes in any direction and strong seismic compressional and shear velocity discontinuities. Thus, the two critical issues with velocity model building in presence of salt drops are the construction of an accurate salt model (in terms not only of identification of top and bottom but also the definition of the velocity inside) and the subsalt velocity field. Subsalt velocity estimation is a very challenging task when conventional narrow azimuth datasets are used, due to the limited illumination of the subsalt. A full azimuth dataset could easily increase illumination in difficult areas, dramatically improving the quality of the subsalt velocity field. Lack of illumination in the sub-salt area is particularly evident in the two vintage narrow-azimuth streamer datasets available in the area. As a matter of fact, in order to properly illuminate, and hence image, the target horizons below the salt drops, a full azimuth or at least a wide azimuth acquisition is required. Another key point is related to the high elastic impedance contrasts and hence the consequent generation of converted waves that contaminate the data. OBN: technology and main related novelties. In an OBN acquisition layout, nodes are deployed on the sea floor while a source vessel is shooting on the sea surface. With respect to conventional marine seismic towed-streamers acquisition, together with reduced costs and time, especially for small areas, OBN ensures also full azimuth, long offset and dense shooting geometries. Thus, the availability of this information allows to improve multi-dimensional interpolation, multi-azimuthal velocity analysis and stacking response, and ultimately the improvement of the sub-salt illumination and imaging. Furthermore, four components are