Reevaluation of the Majunga and Morondava Basins, Madagascar

Eisenstadt, Gloria¹, Dave Bushnell², Barry Rodgers³, Jackie Reed⁴, Alfred Berroteran⁴ (1) Weinman Geoscience, Dallas, TX (2) Weinamn Geoscience, (3) Madagascar Oil Limited, (4) Weinman Geoscience,

Exploration opportunities exist in the Majunga and Morondava Basins of western Madagascar. Reinterpretation of seismic and well data shows that many existing wells did not test valid structural traps. Our study highlights several untested stratigraphic and structural plays.

Structural analysis shows that development of the western passive margin did not follow distinct phases of rift, breakup and drift. Rather extension occurred along major basement faults intermittently from Permo-Triassic to late-Jurassic times, with local unconformities reflecting fault movement. In the Majunga Basin the margin was controlled by a long-lived basement fault that controlled both depositional facies and structural deformation. At least one episode of inversion occurred during the Late-Cretaceous to early Tertiary. This inversion appears to be coincident with eruption of terrestrial flood basalts. Regional tilt of margin to the west occurred sometime in the mid-Tertiary.

The impact of a regional Upper-Jurassic/Lower Cretaceous bathyal shale has not been fully recognized. The presence of this seal may have prevented vertical charge into younger reservoirs from older source rocks, effectively horizontally partitioning the stratigraphy into separate petroleum systems. Hydrocarbons from the dominantly lacustrine, Permian Sakemena shale would be available to laterally charge reservoirs in the deeper, older section. Hydrocarbons for the shallower system could come from the organic rich upper Jurassic or shales within the Cretaceous. The latter have been observed in outcrop and the former are known regionally. Clastic and carbonate deep-water fan deposits within the seal facies may constitute a new exploration opportunity that can be developed using modern seismic technology.