ABSTRACT

Detailed seismic stratigraphic analysis, tied to available well control, has revealed that the Morondava Basin of west Madagascar holds considerable hydrocarbon potential. The area contains a thick sequence of riftrelated sediments with abundant evidence of source and reservoir intervals, hydrocarbon shows and multiple periods of structuring.

The study focused on the Manambolo-Serinam area in an attempt to evaluate the exploration implications of the PCIAC-OMNIS West Manambolo well drilled in 1987.

A total of 25 high-side fault closures were mapped in rifted sediments of the Permo-Carboniferous Karroo level. To the north, burial depths to the Karroo are less and larger structural traps are present with favourable source, reservoir and seal conditions.

The Albian-Aptian represents a major potential reservoir horizon in the study area. It was deposited following a fall in sea level that incised large valleys into the pre-existing shelf margin. This sand-prone valley-fill forms a lowstand wedge that was oil stained in wells in the study area. In the Manambolo-Serinam area it shows a number of stratigraphic closures. Areas of significant structural deformation to the south would enhance the prospectivity of this play type.

Coniacian turbidites make up the reservoir unit at West Manambolo. Geochemical evidence indicates that the gas at West Manambolo results from the thermal cracking of an oil at burial depths in excess of 3,800 m. The gas migrated vertically into the Coniacian, possibly aided by vertical fractures and faults associated with igneous intrusion.

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