A Hybrid Petroleum System on the Western African Margin, an Example in Offshore Angola

B. C. Duval, G. Choppin de Janvry, J. C. Heidmann, J. L. Pition, H. L. Ten Haven

Since 1975, twenty commercial discoveries have been made on Block 2, offshore Angola, in the Pinda carbonate formation. Gas was often found together with oil, and gas content generally increases westwards.

In the shelf area, Pinda structures are related to early halokinesis due to Aptian salt. These salt movements were responsible for the segmentation of the Pinda platform and the development of allochtonous blocks (rafts). The Pinda formation is a generally transgressive sequence and the main reservoir developments are related to high energy carbonates accumulated on structural highs.

Three main source-rocks are identified. Firstly, strong source potential related to synrift filling of grabens is observed in the Pre-salt Bucomazi equivalent formation. It is presently in the gas window in the westernmost part of the area whereas the grabens, present in the shallower offshore are appear to be at least partly in the oil window. Secondly the upper part of the Iabe formation (Senonian and Paleocene) plays a major role as a source-rock. The maturity map shows a rather wide oil kitchen over most of the western part of the area, and the gas window is reached only to the far west below the Tertiary depocenter. However, most of the eastern zone is immature. Finally, the Tertiary Malembo section, above the Mid-Oligocene unconformity, shows very homogeneous but low geochemical characteristics.

Oils tested in the Pinda formation belong to two main groups. Oil from the northernmost part of the block are of pure lacustrine origin interpreted to be generated from the Pre-salt section. All the remaining oils are thought to have a mixed origin with evidence of more or less important marine contribution from the Iabe source-rock. 1D and 2D basin modelling confirmed a twin sourced petroleum system at the Pinda level. Generation from Pre-salt source-rocks in the deepest grabens may have started quite early after Pinda deposition while generation from the Iabe and Malembo, and the final mixed charging of traps is very recent.

In conclusion, the Pinda appears to mainly belong to a hybrid petroleum system charged by at least two sources : the marine Iabe (oil) and the lacustrine Pre-salt (oil and gas), with some probable additional contribution from the Tertiary Malembo (gas and condensates). This hybrid system is well demonstrated in the rafted Pinda sector where vertical migration, including gas, is dominant, because of basement faults and large open windows in the Aptian salt.

This integrated study allowed for a better understanding of a complex petroleum system with multiple migration paths and a reliable evaluation of the gas risk for the numerous prospects remaining to be drilled in the area.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995