Spectrum’s Paolo Esestime*, Milos Cvetkovic, Jonathan Rogers, Howard Nicholls and Karyna Rodriguez discuss a learning case study from 3D Seismic data offshore Gabon in this issue of First Break.
We present an integrated geological and geophysical study conducted during the acquisition and processing of extensive 3D Multi-Client seismic campaigns offshore Gabon. These campaigns resulted in two distinct surveys, the first of 11.500 km² in the southern shelf, and a second of 5500 km² to the north, offshore Libreville (Figure 1). The acquisition parameters and survey design were planned with seismic illumination studies, long offset streamers were utilised and the data was processed with a modern broadband sequence (Esestime et al., 2017).
The surveys presented various imaging challenges, from shallow water multiples to shallow high velocities, carbonate and salt. The Gabonese Authorities for Hydrocarbon (DGH) provided a comprehensive well dataset, including geological markers, logs and checkshot data. These penetrated both pre-salt and post-salt sections, and provided valuable data during all stages of the velocity model building, especially for the southern part of the area. Well data was more limited for the northern survey, where previous exploration stopped mainly in the post-salt shallow section and velocity logs are limited to a few wells.
The continuous iteration of geological information at every stage of the processing allowed an integration of the geological information at multiple stages. In particular, pre-stack and post-stack analyses allowed for determining regional petro-physical properties, such as ‘recurrent velocity trends’ which are useful to interpret at multiple scales from post-stack to single gather data.
We obtained accurate seismic velocities, regionally consistent with the geology from sparse wells. Excellent seismic-to-well tie was achieved on the final depth data in South Gabon, from wells 50-70 km apart, in a highly variable structural-stratigraphic setting, dominated by tectonics and salt movement. The results provided a strong learning curve for analysis and QC of several intermediate velocity models obtained from the tomographic updates during the depth migration exercises (Kirchhoff PreSDM).
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Combined pre-stack and post-stack interpretation for velocity model building and hydrocarbon prospectivity: a learning case study from 3D seismic data offshore Gabon