Exploring the
Jurassic Carbonate Platform Margin,
Webster, Robert E.1, Paul S.
Ensign1 (1) Hunt Oil Company,
The onshore Majunga basin covers about
35,000 km² in northwest
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