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PSExploring the Jurassic Carbonate Platform Margin, Majunga Basin, Madagascar*
By
Robert E. Webster1 and Paul S. Ensign2
Search and Discovery Article #10128 (2007)
Posted June 25, 2007
*Adapted from poster presentation at AAPG Annual Convention, Long Beach, California, April 1-4, 2007
1Hunt Oil Company, Dallas, Texas, 75202 ([email protected])
2Formerly Hunt Oil Company, Dallas; currently, Nexen Petroleum USA, Dallas, Texas, 75251
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 would have 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 Belobaka-1 was drilled to 9520 ft (2901.7 m), encountering 183 ft (56 m) of
water-bearing Dogger oolitic 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
.
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Figures 1-1 – 1-2
Majunga Exploration History (Onshore/Nearshore) • 1950-1968: S.P.M. § Aeromag and Gravity
§
§ 2 Wells- Ihopy 1 & Tuiliere 1 • Invalid prospects • Mesozoic tests • 1968-1975 • Conoco
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§ 1 Well (Sophia 1) - Mid-Jurassic and Karoo test. • AGIP (shallow offshore)
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§ 2 Wells: Mariarano 1 & Mahajamba 1 • Mesozoic tests • 1975: Nationalization. OMNIS created • 1983-1985: AGIP
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Coastal zone • 1988-1992: Shell § Reprocessed 1585 km gravity
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§ 2 Wells (Marovoay 1 & Ankara 1) • Unsuccessful; Karoo objectives • 1998-2002: Hunt Oil Co.
§
Reprocessed >1000 km § 1 Well (Belobaka 1), Jurassic test.
In
1998, Hunt began an extensive
Petroleum Geology of the Contract Area
Subsequent to Triassic rifting, the Majunga Basin consisted of a passive margin from Jurassic through Cretaceous time. Potential reservoirs include the carbonates of the Jurassic Dogger formation, and sandstones of the Triassic, upper Liassic and Cretaceous. Regional seals exist in Upper Jurassic and Cretaceous shales.
Excellent source rocks were identified in the Upper Liassic Beronono
Shale, which has known TOC values in excess of 10% in outcrop.
BASINMOD
In
order to identify a location for a deep test of the Dogger-age
Bemaraha Limestone, it was essential to determine the most likely
areas of reservoir development within this carbonate section. On
A
map of the relative amplitude of the
In
an attempt to better correlate the By
comparing the low impedance zones to the amplitude map of the trough
(top porosity) near the top Dogger, a direct correlation is found to
exist between the anomalous high amplitudes and the low impedance
values. Based on the results of the
Drilling Results: Belobaka 1
No four-way structural closures or fault traps were identified in the basin at the Dogger or shallower horizons. However, a large stratigraphic trap was delineated corresponding to the area of high amplitude and low acoustic impedance along the paleo-shelf edge of the mid-Jurassic Dogger section, the Bemaraha Limestone. The Hunt Belobaka 1 well was drilled to test this potential porosity trap, spudding on August 16, 2000, and reaching TD of 9520 ft (2901.7 m) on September 21, 2000. The well spudded in Danian or Maastrichtian limestone. Below 218 ft (66.4 m) is 7670 ft (2338 m) of Cretaceous-age clastic rocks, both continental and marine. The upper 2000 ft (610 m) is dominated by poorly consolidated sandstone with interbedded claystone of probable deltaic to nearshore marine origin. The claystone increases downward and becomes 100% of the section between 5400 and 7115 ft (1646 to 2168.8 m), where it is a slightly overpressured plastic shale that forms an excellent seal for any underlying traps. Between 7115 ft (1646 m) and the Bemaraha Limestone at 7888 ft (2404.3 m), the rock is mostly glauconitic siltstone with minor marl and sandstone layers. The Bemaraha (Dogger) Limestone was the objective of the well. The top part of the limestone is oolitic and intraclastic grainstone, tightly cemented by sparry calcite. Between 8113 to 8442 ft (2472.8 to 2573.1 m), however, there are beds of poorly cemented oolites with a net thickness of 183 ft (56 m) of reservoir quality rock, with porosity in excess of 12% that reaches as high as 22%. Below the porous oolitic grainstones, the lower Bemaraha is again tightly cemented, composed mostly of intraclast wackestone with some low porosity dolomite beds below 9000 ft (2743 m) and some apparent pisolitic beds near the base. Below the limestone are sandstone, siltstone, and claystone believed to be the Isalo Formation. No
oil shows were observed, although minor fluorescence was present
continuously from 5300 ft (1615 m), and the porous oolite section
showed brown staining. Total gas was quite low throughout the well,
and the Bemaraha reservoir was filled with salt water. Although
excellent porosity was encountered, validating the
Thanks to Hunt Oil Company for permission to present this paper. Thanks to many current and former Hunt employees and consultants who contributed to the evaluation of this basin, including George Roth, Gary Mitchell, Tony Gonnell, Gerhard Martin, and Fritz Fischer.
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