Feature Article

Review of the Exploration Potential of Madagascar

by David N. Clark , Clark Researcis Ltd. Uh'

Introduction The island of Madagascar comprises a

central area of basement rocks surrounded by four extensive sedimentary basins on the western side, and a large fault linea- ment and one small basin on the astern side (Fig. 1). The western basins are the Ambilobe, Majunga, Morondava and

Cap Sainte Marie basins, and the east- em basin is the ne Sainte Marie graben. Explodon W n g has been concentrat- ed mainly in the ~ o r o n d a i a basin,

David and to a lesser extent in the Majunga basin. The remaining basins are virtually unexplored.

Exploration Hiptory Interest in petroleum exploration in

Madagascar started in the early 1 W s , with the dscovery of large accumulations of tar and heavy oil at Bemolanga and Tsimiroro, in the northern Morondava basin. Modem exploration did not begin until the early 1950s. Seventy-two exploration and appraisal wells have been drilled. Despite this effort, only three small hydrocarbon discoveries have been made. The Eponge- 1 (Copetma, 1971) and West Manambolo- 1 (Petro Canada, 1987) both tested gas from Cretaceous sandstone reservoirs, and Manandaza-1 (Shell, 1991), produced oil from a Lower Sakamena reservoir.

The lack of success in findmg commer- cial accumulations of petroleum can be amibuted to some extent to an incomplete understanding of the geology of Madagas- car. Thls, in turn, is thought to be a conse- quence of the poor quality of much of the earlier seismic and well data, together with inaccurate surface geology maps.

A review of the various data suggests that only twelve of the modem exploration

wells can be regarded as tests of valid prospects. The remaining wells appear to have tested features that are either lacking in closure, or were based on invalid con- cepts. If the results of all of the wells are taken into account, the chance of finding producible hydrocarbons in Madagasw is 1 in 23 (4%). If only the valid tests are con- sidered, however, the chance of future success may be as high as 1 in 4 (25%).

Permian to Late Cretaceous ~ a o s l ~ ~ r y

The geological history of the sedimenta- ry basins of Madagascar I by three events; Permo- Triassic rifting (Fig. 2), bate Liassic rifting and separation from Africa, and Late Cretaceous sep aration from India.

In the Early Pennian, a failed inhacontinental rift propagated through west- em Madagascar. Limited sedimentation occurred in half-grabens which were filled with fluvial and marginal marine sedi- ments of the Sakoa (Early Permian) and Lower Sakamena (Late Permian) formtins. Clustal exten- sion continued into the Early Triassic, accompa- nied by symmetrical rift- ing and the uplift of the graben shoulders. This phase commenced with the deposition of marine shales of the Middle Sakamena (Early Triassic), followed by deltaic sandstones of the Upper Sakamena (Mid Triassic). Thermal subsidence (i.e. basin sag) started in Late Triassic times and fluvi- atile sandstones of the

is dominated

Isalo (Late Triassic-Early Liass~c) were deposited. The sandstones rest uncon- fomably on the Upper Sakamena and overstep onto the adjacent basement shoul- ders, where they rest directly on metamor- phlc rocks. Salt was also possibly deposit- ed at this time in the Ambilobe basin.

In the Late Liassic, a new rift formed to the west of the failed Permo-Triassic rift. A series of half-grabens developed that were filled with the marine shales and sandstones of the Andaf~a/Beronono for- mation (Toarcian-Aalenian). Madagascar

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Figure 1 . Tectonic elements of Madagascar

Houston Geological Society Bulletin

started to drift southwards away from Africa in the Bajocian, and a passive mar- gin developed along the western coast. Drifting wac accompanied by thermal sub- sidence and a carbonate platform was formed in a harrier-lagoonal environment. These carbonates now form the Bemaraha limestone (Bajocian-Late Bathonian). The passive margin continued to develop in the Cretaceous and Tertiary. A thick, cyclic succession of off-lapping, deeper marine shales (Duvalia marls), base-of-slope and basin-floor fans. and shallow marine and fluviatile sandstones (the Sitampiky and Tsiandava sandstones) were deposited.

Renewed tectonic activity occurred in the Late Cretaceous as Madagascar sepa- rated from India. The break-up appears to have resulted from a shearing movement rather than a rift, and a pull-apart basin formed in the vicinity of Ile Sainte Marie, on the eastern coast. Wrench faulting and associated compressional folding also appears to have occurred in the western basins from the Late Cretaceous onwards. The folds have previously been interpreted as extensional features, but they are now tentatively associated with lateral move- ments either along the Davie fracture zone, or the Ile Sainte Marie lineament.

From an exploration viewpoint, the western coastal basins can be divided into an eastern failed rift system and a western passive margin. The structural, deposition- al and burial histories of these two provinces are quite distinct. Hence, the potential reservoirs, seals and source rocks that occur in each province are different. Trap formation and hydrocarbon genera- tion also took place much earlier in the failed rift than on the passive margin.

Eastern Area: the Failed Rift The failed rift passes down the eastern side of the Ambilohe. Majunga and Morondava basins. It forms a narrow zone between the passive margin to the West and the Central Highlands in the east.

Potential reservoirs can be recognized in the sandstones of the Lower Sakamena, Upper Sakamena and Isalo. The Lower and Upper Sakamena display reasonably good porosity and permeability at outcrop. In the subsurface. however, the sandstones are generally lacking in porosity as a result of burial, compaction and cementation (e.g.

Manandaza- I , and Vohibasia- I). The porosity of the Isalo is excellent at outcrop and good reservoir quality is widely pre- served in the subsurface because the depth of burial is generally much less than that of the Sakamena, the heavy oil fields of Tsimiroro and Maroaboaly.

The Middle Sakamena shale forms a thick seal that is developed throughout the

failed rift. Small accumulations and shows of light oil occur immediately beneath the shale in Lower Sakamena at several places in the Morondava basin (Manandaza-I, Sakaraha-I and Vohibasia-I). A locally developed but important seal is provided by the Mokara shale (Late Triassic), which occurs within the Isalo.

cnt1rit7ued or7 page 27

LATE LlASSlC RIFT 1

4 Lata Liassic Rift Failed Permo-Trlassic R i f t - - - - - - +

lWRstl BerononoIAndaf~a BemarahaiMarovoay high r-. A-* P.. /I.. --% / . -. .

PERMO-TRIASSIC GRABEN

MID JURASSIC DRIFT AND MARGINAL SAG

U/.LIAS?

LATE JURASSIC SAG AND OFFLAP Late Jurassic Sandstone?

FIGURE 3.4 .iurassic paqsive margin

Figure 2: Permo-Triassic failed rifr, Jwassic passive rnayqin m d Cretnceolts passit3e margin

June 1998 Houston Geological Society Bulletin 25

Madagascar continuedfiorn page 3.7 - This shale is responsible for trapping large accumulations of heavy, biodegraded oil and tar at Bemolanga, Maroaboaly and Tsimiroro, in the northernmost part of the failed rift of the Morondava basin. Elsewhere, no seals are found above the Middle S-na and shows are generally lacking in the Upper Sakamena and Isalo.

The Middle Sakamena shale also forms the principal source rock in the failed rift. It is thought to have generated the oil found at Manandaza, Bemolanga and Tsimiroro and is probably responsible for the oil shows and seeps that occur widely within the rift. Oil maturity was probably reached in the Late Jurassic or Early Cretaceous, and gas maturity was locally attained in the deeper parts of the rift by Middle Cretaceous times.

Three different trapping styles are developed in the failed rift, namely tilted fault blocks, drape anticlines and roll-over anticlmes. Tited fault blocks are problem- atic because they are dependent on updip fault seals, which appear to be ineffective as seen in the Manandaza-1 and Vohibasia- 1 wells. Drape anticlines, in contrast, form very effective traps provided that a sealing shale is peas& Such anticlines are nor- mally found at relatively shallow levels in the subPurfroe, above large tilted fault blocks and tmd to be associated with heavy, biodagrpded oil (Tsimiroro). Roll- over anticlines have yet to be tested but they may contain lighter, unmodified oil

because they occur at deeper levels, away from the influence of meteoric water. These anticlines are found on the down- thrown sides of some of the larger fault blocks, where dipclosure can be mapped at the base of the Middle Sakamena, the hanging wall of Tsimiroro.

Western Area: Passive Margin The passive margin extends along the

entire length of western Madagascar. It forms a wide zone occupying the central, coastal and offshore areas of the Ambilobe, Majunga and Morondava basins.

Possible reservoirs can be identified in the sandstones of the "Aalenian" and Cretaceous, and in the limestones of the Bernaraha. The most attractive reservoirs are provided by Cretaceous sandstones such as the Tsiandava and Sitamplky. They are predominantly fluvial or shallow marine in origin on the eastern parts of the passive margin. Further to the west, how- ever, deposition took place in deeper water. Here, the sandstones tend to be thinner and more argrllaceous, and reservoir quality is commonly very poor. At some localities, nevertheless, base-of-slope and basin-floor fans are developed; and made up of sand- stones with much better reservoir quality, provided that they have not been buried too deeply. Porosity of the Bemaraha lime- stone is generally very low in the subsur- face but a belt of porous oolitic-grainstones may be present at the edge of the shallow marine carbonate platform. This belt has

not been penetrated by any of the existing boreholes. The Isalo is also widely devel- oped on the passive margin but it does not constitute a potential reservoir because porosity and permeability are always very low.

Seals are provided by the Andafia shale (Late Liassic), the Beboka marl (Callovian) and various Cretaceous and Tertiary shales. The Andafia shale suc- ceeds the Isalo and onlaps the tilted fault blocks that formed in the Late Liassic. This shale does not always completely cover the crests of the fault blocks, so it is unreliable as a top seal. Sporadic shows of heavy oil or bitumen have been noted at the top of the Isalo. The Andafia also encases the "Aalenian" sandstones, thereby possibly constituting a stratigraphic trap. The Beboka marl rests upon and onlaps the Bemaraha limestone. It appears to be present across the whole of the passive margin and forms an excellent top seal to the limestone. Strong gas shows are pre- sent in the limestone at some localities. Several thick units of shale are present in the Cretaceous provide excellent top seals to the interbedded reservoir sandstones. S n d accumulations of gas have been trapped beneath these shales in some places as seen in the Eponge-1 and West Manambole1 wells. Elsewhere, the sand- stones appear to be stratigraphically enclosed by shale and enwuraging oil shows have been observed. This suggests

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Rgum 3. Relationship between the failed Penno-Triassic Ankara Graben in the south-east and the hter tilted f a d blocks of Liassic age to the north-west of the Mamvoay High 7his seismic line passes across the boundary of the Ankara graben and shows the overstep of the Isalo onto adjacent basement shoulders. The transition f m m the m r l s and limestones of the Sakaraha formation, with good intermi reflections, into the more massive plarfonn carbonates of the Bemaraha limestone in the north-west is clearly evident. Clinofom are also well developed in the Lower and Middle Cretaceous.

. .- .-

June 1998 Houston Geological Society Bulletin

Madagascar ...r,,rr,,,..,l.~,,p,tr ,w-,. .-7 .- -

th:lt g~xx l 1;tter;d seals msly be present in somr xe;n t \4 .mamh) l~~l and Marnvoay-I wells).

I'otential source rocks have k e n itlenti- lied in thc T