Zaïre turbiditic channel studies shed light on area hydrocarbon systems

Geodynamic depositional relationships

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Subsea exploration tools deployed by Ifremer on the ZaïAngo project.
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Recent exploration offshore Congo and Angola, including the continental slope, has led to the discovery of huge oil and gas discoveries and some fields in water depths exceeding 1,000 meters. Many of these are turbidite deposits. The well-known Elf Girassol Field includes paleo-Zaïre meandering channel-levee complexes. The study of deep sea fans has generated abundant literature in the last 20 years, but the knowledge about giant meandering channel-levee systems that develop on passive continental margins is still sparse.

Studies conducted onshore on outcropping fossil systems are limited as outcrop conditions govern the quality of geometric, facies, climatic, and paleogeographic reconstructions. Simultaneously, the improvement of underwater exploration techniques, in particular high resolution multi-beam echosounders, sonar imaging, HR seismic profiling, and the development of multiple parameter core measurements (true logging on cores), allow studies for developing models based on the analysis of present systems.

A study of the present Zaïre turbiditic system is the only way to obtain information required to develop a reliable deposition model of this type of turbiditic system. At the same time, more detailed knowledge of the geodynamic setting of the Gabon-Congo-Angola margin is necessary to understanding how the hydrocarbon system worked (source rock, hydrocarbon migration, traps). Finally, problems of slope stability are still a major concern in the perspective of potential field development at great water depth, especially as the stability of the sedimentary cover could be strongly affected by circulating fluids or by the presence of gas hydrates.

The ZaïAngo project is an integrated study of the Gabon-Congo-Angola margin, that includes all these objectives and is based on several dedicated underwater exploration surveys in water depths of 400-5,200 meters. The study will include marine geoscience research activities conducted by Ifremer in Brest (France).

The ZaïAngo project, which has been ongoing since 1998, is funded 50/50 by Ifremer and Elf EP. The program involves a set of various subsea exploration tools deployed by Ifremer (EM12 multi-beam echosounder, multi-channel seismic acquisition, deep sea coring, heat flux measurements, deep-tow HR side-scan sonar, HR and THR monotrace seismic, ROV).

Powerful tools

The project involves some operations that are not routine in the oil industry and some of which will require adaptation by Ifremer, in particular 2D HR multi-channel seismic profiling. New tools were developed (400-meter-long digital seismic streamer; new OBS systems for seismic refraction). Thus, the project is a collaboration of a highly scientific nature, in which Ifremer and Elf cooperate as partners on a contractual basis.

A total of six ZaïAngo missions with various vessels were conducted over the area, representing 130 onsite days, working jointly with Elf's Gulf of Guinea subsidiaries. As a result, our present knowledge of the area has increased considerably over the last two years.

The recent Zaïre fan (latitude, 4-8 degrees S) is fed by a relatively rectilinear canyon, one that is highly incised, with a depression of more than 950 meters observed at a distance of some kilometers of the coast. The direct connection of the river mouth and the abyssal plain via the canyon is one of the original characteristics of the Zaïre system.

This could explain why the fan remains active in a period of high sea levels (contrary to the Amazon system, for example). Downslope, the Zaïre active channel is unique and highly meandering in its upper portion. It presents several changes of direction, but it flows globally from east to west. The channel becomes quasi-rectilinear down to the entry into a complex of distal lobes.

Despite the extended area covered during the ZaïAngo 1 and ZaïAngo 2 cruises (about 200,000 sq km), the Zaïre fan is far from having been explored entirely. More than 80 paleo-channels have been identified, south and north of the present channel, but also to the west offshore of the recent lobe complex. For most of the paleo-channels, the survey has not allowed us to reach zones of associated distal lobes. One can therefore estimate that the size of the Zaïre turbiditic system approaches that of the Amazon fan (330,000 sq km).

The last important discovery of the missions is the omnipresence of giant sediment-waves that are observed all along the present levees, and also along some of the most visible paleo-channels. The crests of the giant dunes are often curved and parallel to the layout of meanders, illustrating the direct link between overflow currents and these sedimentary figures.

Conclusion

On a regional scale, the ZaïAngo data set is unique. The Zaïre fan will become a useful case study for the industry, just as the Mississsippi and Amazon fans are to-day. The studies will allow us to appreciate, for the first time, the turbiditic Zaïre system in totality, and this, from the river mouth to the distal lobe along a direct distance of more than 760 km from the coast.

The coarseness of sediment indicated by cores in the distal zone, either in lobes or in distal levees, confirms the extrordinary grain-size segregation power of this type of turbiditic fan, which, although clay-dominated, may produce important sand accumulations.

Further studies of the present data set as well as the new cruise in the zone scheduled by the end of 2000 will help to specify the active process in this type of channel and more generally its evolution through time and space. The importance in obtaining details on specific morphological features with strong downstream reservoir implications has been stressed.

Present day geometries, as seen on the multibeam compiled charts, can be directly compared to seismic attribute maps of neighboring fields. Wherever possible, shallow reservoir geometries were enhanced by comparing the newly acquired profiles with existing 3D data.

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