GEOLOGICAL EVOLUTION AND HYDROCARBON HABITAT OF
THE MAJUNGA BASIN AND KARROO CORRIDOR, MADAGASCAR.
Peter J.F.Jeans and Gustave L.E. van Meerbeke (Shell Internationale Petroleum
Maatschappij B.V. The Hague, The Netherlands)
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, consequent upon 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 terrestrial immature
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). 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 (eg. Tanzania) and source rocks, and 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
Madagascar/India from Australia/Antarctica. Syn-rift uplift resulted in
the emergence of the cratonic hinterland and the start of a long period
of deltaic – fluviomarine clastic progradation. A major unconformity
and widespread intrusive and extrusive basaltic magmatism mark the
Turonian, coincident with the separation of India/Seychelles from
Madagascar. Magmatism continued into the Campanian, whilst the
progressive uplift and seaward tilting of the island (see Fig. 3) has
continued throughout much of the Tertiary. This has resulted in
subaerial erosion of the updip margin of the Cretaceous wedge; 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 STOIIP variously estimated to range from 0.2-5 billion barrels
of 11-15 deg. API oil in Isalo sand reservoirs folded over a shallow
north-south striking basement horst. Reported field data imply that
migration of oil into these traps occured prior to Turonian magmatism
Numerous other surface oil impregnations have been recorded, especially
along the faulted southeastern 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-l (1988) flowed 17 mmscfgpd
plus some condensate from thin Lower Cretaceous turbiditic sandstones,
and Manandaza-l (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 lsalo 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
tri- partite 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.
This review is distilled from work done in 1989-93 by the staff of
Shell Exploration and Development Madagascar, under the direction of
Drs Maarten Weimer, to whom this recognition is gratefully accorded.
Thanks are also due to Shell Internationale Petroleum Maatschappij BV,
The Hague, and to OMNIS, for permission to publish this paper.
List of Figures:
1. Madagascar Overview Map
2. Madagascar Stratigraphic Summary
3. Madagascar: Majunga Basin Cross Section.