In this article, originally published in the March 2018 edition of First Break magazine, Spectrum’s Geoscience team describe an integrated geological and geophysical approach applied to time and depth seismic imaging in shallow water offshore Gabon.
Introduction and 3D seismic plan
In 2017 Spectrum conducted a major 3D seismic acquisition program of approximately 11,500 km2 in shallow water offshore South Gabon (Figure 1). The project was carried out in close collaboration with the Gabonese Authorities (Direction Generale des Hydrocarbures; DGH) and was driven by renewed interest in a proven hydrocarbon province. Continuing exploration since the 1960s has proven all the key elements: source rock presence and maturity, reservoir presence and effectiveness, seal and oil charge into structural and combined structural-stratigraphic traps. Indeed, many oil fields, such as the Olowi Field, have been discovered and are producing in the onshore and nearshore.
Like other Central Atlantic Margins, the geological evolution created two completely different geological settings offshore Gabon, subdivided by a mobile layer of evaporites, which separate the syn-rift section from the post-rift. Each section has a different paleo-geographic evolution, both with oil-prone source rocks. Carbonate and clastic reservoirs have been proven in the post-salt; good quality sands (Gamba and Dentale Fms.) are also proven in the very late syn-rift.
In the past, hydrocarbon exploration in this region has mainly been limited by poor subsurface imaging. Spectrum sought to address this issue through a new seismic acquisition program, planned through the analysis of data from previous exploration campaigns. This allowed for a better understanding of the geological and geophysical complexities behind the seismic imaging challenges. Survey specifications and historical wells were made available from the DGH, the latter resulting in an essential source of information used to build and refine the velocity for both time and depth imaging.
The pre-acquisition study anticipated three main imaging problems: 1) strong velocity contrasts in shallow carbonates (single and interbedded multiples); 2) presence of salt bodies (complex ray path and edge effects (Jones and Davidson, 2014); and 3) small contrasts of acoustic impedance in the sub-salt units (reduced signal-to-noise ratio). To optimize the acquisition parameters, the operational requirements were accurately tested to obtain a good control on energy recovered, in order to penetrate below the salt, in the syn-rift section (Esestime et al., 2017).
A 3D velocity model was adopted to understand ray paths and illumination under different variables: 1) several geometries and orientations of the salt bodies, 2) recurrent dips and strike directions in the sub-salt structures. The study proved that a sailing direction almost parallel to the coast was the most efficient for both operation and imaging purposes. In fact, primary energy from the sub-salt emerged since the single cable-shot stacks made on board the vessel, and the salt flanks were well defined by the initial migration tests (Figure 2).
The data were collected with two vessels; both provided with 8km-long offset streamers, with 15 seconds record length and continuous recording. The nearshore configuration had six streamers, while the deeper areas were covered using 12 streamers. Nominal shot spacing is 25 m in a flip-flop manner, with receivers every 12.5 m and cables 100 m apart. Water depths vary from 30 m to almost 1000 m, with more than 50% of the survey acquired in water depths of less than 100 m.
The most recent advances in seismic processing have been applied to enhance the modern broadband imaging techniques. Geological inputs have been used to carefully evaluate the geophysical outputs at all stages.
Further on we describe the processing methodologies and the main steps undertaken to create a velocity model for time and depth images. The imaging approach was made consistent with the geological complexities, honoring the geophysical observations made during the several steps of velocity analysis.
Click to read the full article here:
Shallow water Gabon 3D: focused processing images pre- and post-salt prospectivity