CGG: State-of-the-Art Seismic for Pre-salt Hydrocarbon Exploration Offshore Gabon

Map showing available license blocks and coverage of new broadband 25,000-km2 multi-client survey and additional data being acquired in blocks F14 and F15.

June 2, 2016

Despite depressed oil prices, the Gabonese 11th deepwater licensing round has generated considerable interest from both International Oil Companies (IOC’s) and newcomers alike, encouraged by the success of exploration in the conjugate margin offshore Brazil, as well as recent pre-salt discoveries such as Ruche, Tortue, Diaman and Leopard in nearby Gabonese waters. CGG has worked directly with the Direction Generale des Hydrocarbures of Gabon (DGH) to acquire and process over 25,000 km2 of new 3D BroadSeis™ multi-client seismic data over available and licensed blocks in the South Basin to enable evaluation of this prospective area. A fast-track pre-stack time-migrated dataset for this survey is available now, along with sample pre-stack depth reverse time-migrated (RTM) data in one area. The final RTM for the whole of the survey area will be available this summer.

The objective of the survey is to image potential prospective structures at base salt level without compromising the shallower post-salt image quality. The data do not disappoint. Even a preliminary ultra-fast-track dataset produced onboard was described as “way beyond expectations” by a major oil company interpreter. On the full fast-track dataset, more detail below the salt is being revealed than has ever been seen before, revolutionizing the understanding of the geology of the area. Early seismic imaging results indicate the presence of thick syn-rift and sag sequences below the salt, which are the key intervals of a pre-salt petroleum system and indicate many exciting prospects, some of which continue beyond the borders of the survey so that their full extent cannot be gauged.

This survey is being processed using the latest high-end imaging technology to produce the clearest images. The fast-track PSTM volume shows clear uplift over the existing data, while the fast-track PSDM shows the full benefits of advanced velocity modeling and depth-migrated modern broadband 3D seismic data. The uplift in subsalt imaging that will be achieved in the final dataset is dramatic; the RTM fully depth-migrated example line (which can be seen in the lightbox on the CGG booth #1250) shows clearly defined tilted fault blocks and horst features as well as the highly complex nature of the thrusted and distorted salt and sediment overburden.

Multi-layer tomography (TomoML) and full waveform inversion (FWI) are being used to create the final velocity model, with RTM being used in the velocity model building iterations. The high-quality, low-frequency data acquired using the BroadSeis solution is particularly beneficial for FWI as the low frequencies prevent cycle-skipping. The imaging improvement from the FWI velocity model can be seen both in the shallower section, where pull-up/push-down image distortions are precisely corrected, and in the deeper salt bodies, where salt flanks and subsalt reflectors show improved focus and continuity due to better overburden description. A booth theater presentation describing details of the imaging of this dataset is being given daily on our booth #1250, and these were also presented by Andrew Ratcliffe in the technical sessions on Tuesday (Tu SRS2 07).

Although this dataset delivers unprecedented images of the subsurface in this