GEOLOGICAL EVOLUTION AND HYDROCARBON HABITAT OF THE MAJUNGA BASIN AND KARROO CORRIDOR, MADAGASCAR.

by Peter J.F.Jeans and Gustave L.E. van Meerbeke (Shell Internationale Petroleum Maatschappij B.V. The Hague, The Netherlands)

Abstract
In November 1988, Shell Exploration BV signed three Production Sharing Contracts with OMNIS in Madagascar. The contracts covered one license in the Karroo Corridor and two licenses in the Majunga Basin (Fig. 1), in which two latter areas Shell farmed out a 50% interest to Amoco in July 1990. A total of 6400 km of 2D seismic data was acquired, extensive field studies undertaken, and 4 wells were drilled, at a total cost of some $110 million. These efforts unfortunately proved unsuccessful in finding commercial hydrocarbon reserves and the last of the licenses was relinquished in February 1994. The data acquired have, however, enabled an enhanced understanding of the geological evolution and hydrocarbon habitat of Madagascar.

The stratigraphic succession and tectonic history of Madagascar (Fig. 2) are directly related to the progressive break up of Gondwana, and the subsequent dismemberment of Eastern Gondwana.
Uppermost Carboniferous to Upper Permian (Karroo Supergroup): Extensional block faulting, possibly related to the closure of the Cape Fold Belt to the south, resulted in horst and graben formation and the deposition of thick sequences of fluvio deltaic, coal swamp, and immature terrestrial clastics of the synrift Sakoa and Lower Sakamena Formations. Differential tectonic movement slowed in Upper Permian times, coincident with the only significant marine incursion in the whole of the Karroo sequence (Middle Sakamena Formation). Post rift, though still terrestrial, clastic deposition continued until Lower Jurassic times (Upper Sakamena and Isalo Formations; dominantly sandy red beds, though with locally significant thicknesses of flood plain shales),
Middle Upper Jurassic: During Lower Jurassic times, the old NE SW and N S Karroo trends were re activated by the onset of rifting of the Somali Basin, as East Gondwana started to separate from West Gondwana. Initially restricted conditions are indicated by the development of local salt basins and source rocks. These were succeeded by a regionally extensive Dogger (Middle Jurassic) carbonate platform which, with its sandy littoral equivalents, unconformably overlies the block tilted and eroded Karroo sequence (Fig. 3). Following a major Upper Jurassic transgression, the carbonate platform was extinguished by the progradation of Upper Jurassic marine clastics, reflecting the onset of post rift subsidence and the development of the Majunga Basin passive margin and the Morondava Basin passive/transform margin as Eastern Gondwana started to drift south. Cretaceous Tertiary: Much of the Lower Cretaceous corresponds to a period of slow or non deposition, co incident with the separation of Seychelles/India from Eastern Madagascar. Post rift uplift resulted in the emergence of the cratonic hinterland and the start of a long period of deltaic fluviomarine clastic progradation. Plate motion changes in the Turonian, consequent on the separation of the Seychelles Bank from India, are possibly the cause of the widespread intrusive and extrusive basaltic magmatism which affected

Madagascar at this time, and of the renewed uplift and seaward tilting of the island. This uplift and tilting (see Fig. 3) has continued throughout much of the Tertiary and has resulted in subaerial erosion of the updip margin of the Cretaceous wedge. It has also caused partial or complete loss of landward closure on all pre existing structures; and the progressive elevation of the Tsimiroro and Bemolanga palaeo oil accumulations to near surface levels and their consequent inspissation.

The hydrocarbon habitat of Madagascar is dominated by its two large heavy oil / tar accumulations: the Bemolanga Tar Sands, reservoired in Isalo sands, have been known since the early 1900s; they cover an area of >400 sq km and have been appraised by over 250 shallow coreholes. Estimated STOIIP ranges from 2 20 billion barrels of which about 1 billion barrels are thought recoverable by open pit mining. the Tsimiroro Heavy Oil Field, also known since the early part of the century, and appraised by over 45 shallow wells, contains an estimated STOIIP ranging from 0.5 5 billion barrels of 11 15 deg. API oil in Isalo sand reservoirs folded over a shallow north south striking basement horst.

Numerous other surface oil impregnations have been recorded, especially along the eastern, faulted, margin of the Morondava Basin and over the plunge of the Cap St. Andre nose.

A total of over 70 exploration wells have been drilled since 1950 in Madagascar, most of which recorded some, albeit weak, indications of oil and gas throughout the stratigraphic column. However, only two wells recovered significant, though non commercial, flows of hydrocarbons to surface: West Manambolo 1 (1988) flowed 17 mmscfgpd plus some condensate from thin Lower Cretaceous turbiditic sandstones, Manandaza 1 (1991) recovered 10 barrels of waxy 41 degree API oil from tight, fractured Lower Sakamena sandstone.

Source rocks are present at several stratigraphic levels (Fig. 2), though none have yet been demonstrated to be of regional extent. Locally significant, coally, gas prone source rocks are present in the Sakoa Formation and the lower Isalo II ('Lignitic Shale'). Locally significant marine shale, oil prone source rocks have been proven in the Middle Sakamena of the Manandaza Basin Karroo Corridor, and in the Jurassic Beronono Shale of the southern Majunga Basin (outcrop only). Oil and source rock extract analysis suggests that the Tsimiroro/Bemolanga heavy oils, and the Manandaza light oil are derived from Karroo source rocks, probably the Middle Sakamena shale. Though rich in outcrop, the Beronono shales have not been encountered in the subsurface, and none of the oils analyzed have a composition suggesting origin from this source rock. The traces of oil and impregnations recovered from the Cretaceous sands do not resemble either Karroo or Bemolanga extracts; they have a mature crude like biomarker distribution and are thought to have been generated from, as yet unproven, intra formational pro delta source rocks.

Three principal hydrocarbon plays are recognized,, corresponding to the tripartite subdivision of the stratigraphic column (see Fig. 3). In the Karroo, the target is fault/dip structures in the Sakamena and Isalo I/II, draped over basement horsts and charged by co eval source rocks in the adjacent grabens

(Tsimiroro model). In the Dogger carbonate, no dip closures have been found to date, and trapping would depend on updip pinchout or occlusion of porosity, either on the platform or associated with the shelf margin. In the Cretaceous clastic wedge, there is a trend of large growth faults associated with the underlying Dogger shelf margin, and stratigraphic trapping possibilities in slope or basin floor fans. Shell's drilling results indicate that hydrocarbons have migrated through the Cretaceous sequence and that traps are the critical factor. In the Karroo sequence, the critical aspects are reservoir and seal quality.

Recognizing the unique richness and variety of Madagascar's fauna and flora, a minimum impact environment policy was in place throughout all phases of Shell's operation. Seismic lines were cut by hand (no bulldozers) in slalom fashion and all drilling locations and access routes were fully restored and replanted. Performance was monitored by both internal and outside experts, including audit by World Wildlife Fund representatives, with wholly satisfactory results.

Acknowledgements
This review is distilled from the work done during 1989 93 by the explorationists of Shell Exploration and Development Madagascar, under the direction of Drs Maarten Weimer, to whom recognition is gratefully accorded. Thanks are due to Shell Internationale Petroleum Maatschappij BV, The Hague, for permission to give this presentation.

List of Figures
1. Madagascar Overview Map
2. Madagascar Stratigraphic Summary
3. Madagascar: Majunga Basin Cross Section.