--> Abstract: Exploring the Jurassic Carbonate Platform Margin, Majunga Basin, Madagascar; #90063 (2007)

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Exploring the Jurassic Carbonate Platform Margin, Majunga Basin, Madagascar

Webster, Robert E.1, Paul S. Ensign1 (1) Hunt Oil Company, Dallas, TX

 

The onshore Majunga basin covers about 35,000 km² in northwest Madagascar and extends offshore. Above the Permo-Triassic Karoo rift complex, basin sedimentary fill includes a Jurassic section with Liassic organic-rich shale and a Dogger carbonate platform, overlain by Cretaceous marine sandstone and shale. Seven deep wells, including two offshore, had failed to test the Dogger limestone paleo-shelf edge. In 1998, Hunt Oil Company obtained a PSC covering the onshore basin and carried out field work, photogeology, Landsat mapping, and seismic reprocessing of 1983-1992 data.

 

Excellent Jurassic marine source rocks updip of the platform margin contained TOC's of 10 to 23%. Modeling of downdip shale of unknown source quality showed maximum oil generation occurred in the Late Cretaceous. Dogger shelf-margin facies, capped by Upper Jurassic and Cretaceous shale, were in structurally high positions at that time, providing excellent trap geometry. Reprocessed seismic showed an area of anomalous high amplitude running parallel to the Jurassic shelf edge, and acoustic impedance inversion suggested high probability of significant porosity development. Post-Jurassic basinward tilting precluded structural closure, but a large porosity-pinchout trap was indicated. In 2000, the Belobabka-1 was drilled to 2,900 m, encountering 56 m of water-bearing Dogger limestone with 12 to 22% porosity. A technical success, the well demonstrated the presence of reservoir-quality porosity at the bank margin, but the absence of hydrocarbons suggests either poor source facies downdip or an inadequate porosity seal in the carbonate rocks.

 

AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California