Hopes high for more giant fields off SW Africa

Elephant hunting is back in Africa and across the Atlantic in Brazil.

Chevron's good news confirmed the trend. On Jan. 5, the company's D14-6x (Landana) new field wildcat drilled on Block 14 off Cabinda to TD 3,281 m in 438 m of water reportedly tested a combined 7,300 b/d of 36° gravity oil from the Miocene.

Production is expected to start in 1999 at 50,000 b/d, later reaching 200,000 b/d.

Chevron estimated recoverable reserves at around 500 million bbl. This would be the second major field discovered in the area, the first, D14-2x or Kuito, being of the same magnitude. This discovery is in shallow water compared with huge finds to come, according to South Atlantic African Basins-A Study of Deepwater Potential, just published by Petroconsultants SA, Geneva.

Petroconsultants predicted that drilling will yield several new oil provinces in the study area, even in countries where results have been disappointing to date, and include multiple discoveries of giant oil fields in deep water the next few years. They will not be discovered using the classical concepts derived from shallower exploration; shelf concepts cannot be extrapolated to the slope, nor will the slope reflect the abyssal plain.

What is certain, however, is that a major new oil province is emerging in the deep South Atlantic, flanked by "elephants" such as Girassol and Dalia off Africa and Marlim, Albacora, and Albacora-Leste off Brazil.

Petroconsultants foresees exploration extending from 500-2,000 m of water to ultradeep areas in 2,000-4,000 m targeting giant turbiditic sandstone bodies on the plain of the ocean abyss. Massive investments, high risks, and high rewards will characterize this phase of exploration.

Essential ingredients

Pre-requisites for success are thick, well-defined, continuous reservoirs with high flow rates; unfaulted simple traps; and thick, oil prone source rocks being at present-day in the peak oil generation stage. Expulsion and accumulation of large amounts of gas and discontinuous reservoirs could ruin the economics.

This analysis is based on the geological interpretation of more than 60 unpublished, regional seismic sections, of which 15 are from off Brazil. It integrates a broad knowledge of Brazilian deepwater geology, which shows a marked resemblance to the African side due to comparable processes in similar geologic settings.

In addition, 16 sites distributed from the upper slope to the abyssal plain have been modeled to assess the timing of hydrocarbon charging. Deep Sea Drilling Project and Ocean Drilling Program reports provided further information.

For each of the 16 locations, Petroconsultants conducted a maturation simulation, including burial, heat flow, and generation histories, leading to play and prospect delineation.

Generally it found that the deep offshore South Atlantic is characterized by the association of huge rafted structures and extensive turbidite systems providing good quality large combination or stratigraphic traps. Many of the traps are considered to be giants because of the size and sealing of the turbiditic sand bodies.

Reservoir types

Three main reservoir types were studied and found similar on both sides of the Atlantic:

• Distal turbidites on the lower slope facing large rivers (e.g. Girassol in the Congo fan),
• Very distal turbidite bodies on the abyssal plain, associated with big fans, and
• Shelf-fed turbidites derived from medium or small rivers on the middle to lower slope.

It is surprising to note that various oil prone source rocks exist even over the oceanic crust in the abyssal plain. These source rocks are presently at their peak of oil generation in various areas where an optimum overburden was provided by late Cretaceous and Tertiary sedimentation.

Exploration frontier

The Africa side of the South Atlantic-Cameroon (Douala basin), Rio Muni (Equatorial Guinea), Gabon, Congo, Cabinda, Angola, and Namibia-has to be considered as a huge and complex frontier. Most of it is unexplored.

To define the exploration perspectives the only available tool is the interpretation of nonexclusive and frequently old seismic, often with no well tie. The seismic has to be combined with geological and exploration concepts drawn from Africa and Brazil. Outside of the Congo fan, nonexclusive surveys provide the only real data.

Only 46 new field wildcats have been drilled in the 3,000 km long study area, yielding at least four giant fields and several other discoveries 200-1,500 m of water. Almost all of the wells and discoveries and most of the new field wildcats have been drilled in the Congo fan area, where various areas of differing potential can be distinguished:

• The outer Congo fan crown, proven by Girassol field and the Dalia discovery.

It covers the lower slopes of northern Angola, Cabinda, and part of southwestern Congo and represents an area of prime interest: fairly high risk, very high potential.

• The central Congo fan, an area with excessive sediment thickness (high gas, overpressure) and strong trap compartmentalization. Bengo is representative of the interesting discoveries to be made despite higher risk.
• The upper Congo and Cabinda slope, proven by D-14-2 and N'Kossa fields: a high-reward, high-risk area.

OGJ author's guide available

The Oil & Gas Journal exploration editor is pleased to receive manuscripts on all exploration related subjects from new or experienced authors worldwide. OGJ accepts exclusive articles and manuscripts condensed from oral presentations. A brief author's guide outlining our needs and answering other questions is available from: Exploration Editor, Oil & Gas Journal, P.O. Box 1941, Houston, TX 77251; Ph. (713) 963-6292; Fax 713/963-6282; E-mail [email protected]

Copyright 1998 Oil & Gas Journal. All Rights Reserved.