Abstract: Origin and Maturity of Oils from the Congo Coastal Basin

Levache, Denis and Daniel Dessort - Elf E&P

This fluid synthesis of the Congo basin includes the study of about 90 oils and oil shows from more than 50 wells. These wells mostly belong to the offshore blocks.

This synthesis has two main objectives:

- To locate source rocks of the discovered crude oils in this part of the Guinea Gulf.

- To estimate the maturity of the oils using molecular ratios.

All parameters indicate clearly that all these crude oils (in Post-salt reservoir for the most part) obviously originate from the lacustrine source rocks of the Pre-salt Series. But, among these Pre-salt formations, which one is at the origin of such or such oil? Using Cluster and Principal Component Analyses, six oil families have been defined. The interpretation in term of origin is given on Fig. 1:

I. oils from the Sialivakou source rock are located on the Kaba and Likouala structural highs

II. oils from the Lower Pointe Noire source rock are located around the Likouala High in the more external position

III. oils from the Upper Pointe Noire source rock are settled in the North part of the basin.

IV. oils from the Djeno source rock are located around the Likouala High but in intermediate location and also in the Emeraude Trough.

V. oils from the Haute-Mer block seem to be originated from a mixture of Pointe Noire and Djeno source rock in variable proportion. So longer migration pathways than in the East part of the basin are inferred. Above all, it supposes the presence of these source rocks in this part of the basin

VI. the Litela oil seems originated from Chela source rock

Finally the oil origins are closely linked to the spatial distribution of the various Pre-salt facies in the basin. The prediction of the oil type is closely dependent on the good knowledge of the Ante-Aptian paleo-geographical framework.

In terms of maturity, the oils can be pooled into 4 families covering a 0.6 to more than 1.0 eq %VRo. Interval as shown in figure 2.

Oils from the Upper Pointe Noire source rock, in the Northern part of the basin, show a great variation in terms of maturity in good match with the source rock maturity just below (-Yanga Sendji area with a low maturity 0.5 to 0.7 eq %VRo and Libondo Maseko area with an higher maturity 0.7 to 1.0 eq% VRo).

Oils around the Likouala High display high maturity over 0.9 eq %VRo Particularly, oils from the Sialivakou source rock in Vandji reservoir and oils from Haute-Mer block have reached an important maturity around 1.2 eq %VRo. This maturity corresponds to the maturity presently attained in reservoirs. These results imply an evolution of these fluids in reservoir because this kind of source rock expels its hydrocarbons earlier, near 0.6 eq %VRo as it is recognized in Yanga or Sendji fields.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil