Joeli V. Lalaharisaina
Office des Mines Nationales et des Industries Strategiques
Antananarivo, Madagascar
Nonie J. Ferrand
Oil & Gas Consultants
Palaiseau, France
Recent drilling in the Majunga basin of northwestern Madagascar revealed unexpected light oil shows in excellent quality reservoir sands of "Mid" Cretaceous age.
Regional reconstructions show the development of a prograding clastic shelf from the Aptian until the Mid-Turonian that extended laterally from the northwest coast of Madagascar into Northwest India and Southeast Pakistan.
In Southeast Pakistan this prograding clastic system forms excellent reservoirs producing light oil and gas. Potential exists in the Majunga basin of Madagascar for oil and gas from the same reservoir levels.
Uplift of the shelf margin in the Turonian caused partial erosion of the shelf and canyon formation on the shelf margin. Deep sea clastic fans associated with these canyons offer as yet untested potential for significant petroleum accumulations in the Majunga basin of Madagascar.
Six untested play concepts have been identified in Cretaceous reservoirs of the Majunga basin. These plays offer multiple objectives in the depth range of 800-2,500 m within a well defined area. Further untested plays exist for Tertiary and Dogger objectives.
GEOLOGIC SETTING
In Permian times Madagascar was located at the heart of the Gondwana continent close to the Kenyan coast. An early phase of continental rifting in the Permo-Triassic led to the formation of a network of deep rift basins that were filled with Karroo formation clastics.
Remnants of these basins are located on the west and northwest coasts of Madagascar (Fig. 1).
Further rifting in the Middle Jurassic led to subsidence and a marine incursion leading to widespread platform carbonate development. Truly oceanic rifting between Africa and the Madagascar-Indian plate began in the Late Jurassic and continued into the Early Cretaceous.
A change in plate movements in the Barremian-Aptian caused the axis of rifting to shift from the Somali basin to the southern side of the Madagascar-Indian plate (Fig. 2).
A new system of prograding marine and deltaic sandstones and shales formed along the length of the now inactive passive margin from Madagascar through into Northwest India and Southeast Pakistan (Figs. 2, 3).
The deltaic system became more and more charged with sand until the rifting of India from the east coast of Madagascar in the Turonian. This rifting was accompanied by effusions of flood basalts, especially in the northern part of the Morondava basin and in the Majunga basin (Fig. 1).
Uplift of the Majunga basin led to partial erosion of the deltaic shelf and the formation of canyons on the continental margin (Fig. 4). Sedimentation became localized at the continental slope until the shelf was flooded completely in the Campanian.
Tertiary sedimentation is dominated by marine carbonates and marls located in coastal outcrops and on the offshore shelf.
EXPLORATION HISTORY
Since early this century surface hydrocarbon seeps and the giant Bemolanga tar sand accumulation have attracted numerous exploration campaigns to Madagascar.
Most of this effort has been directed at reservoirs charged by prolific Karroo formation source rocks. Despite frequent encouraging shows and the recent discovery by Shell Madagascar of light oil in Karroo reservoirs, all of these campaigns have been abandoned due variously to problems with the timing of charge and trap formation, biodegradation by surface water infiltration, reservoir quality, and logistical problems in remote areas.
The Karroo remains an objective with potential for giant accumulations, but a new evaluation of the existing data is required before realistic objectives can be defined.
Four main phases of exploration in the Majunga basin have resulted in five wells in the 56,000 sq km onshore area and two wells in the 10,000 sq km offshore area. Various parties have shot 8,830 km of seismic onshore and 13,200 km offshore (Tables 1, 2).
Light oil shows and cut-fluorescence have been observed in all wells that penetrated Cretaceous sands, but to date no wells have been drilled onshore to test a Cretaceous objective.
The most recent exploration campaign was carried out by Shell Madagascar in the onshore area from 1989-93. All blocks in the Majunga and Morondava basins were open as of early June 1994.
CRETACEOUS RESERVOIRS
The early Cretaceous is dominated by marine shales (Fig. 3). During progradation of the shelf from the Aptian until the early Cenomanian thin sandy beds and occasional carbonate horizons occur, but the sequence is dominated by shale.1 2 Reservoir quality of these thin, sandy intervals is generally poor with porosities rarely exceeding 15%.
In the Cenomanian a marked change in sediment supply occurred. In the subsurface, sandy beds 10-100 m thick alternate with marine shales, giving net/gross ratios of 35-85%. The sands have excellent reservoir characteristics with porosities of 20-37% with permeabilities up to several darcies.
By the Turonian the sands and shales are dominantly nonmarine, with sand being dominant. The porosity and permeability of these sands is excellent.
Uplift and erosion of the shelf in the Turonian will have swept sand off the shelf down into the deep sea fans (Fig. 4) where similar excellent reservoir characteristics can be expected. Locally, continental sandstone units up to 370 m thick are found in outcrop. The phase of uplift ended with volcanic flows that covered the shelf and flowed in part down the canyons (Figs. 6, 7).
From Coniacian through to the Maastrichtian marls and thin (tens of meters) carbonate horizons alternate with nonmarine sands and sandy shales. Only high stand deposits have been observed in the wells and outcrops. To date no well has penetrated low stand or shelf margin sediments that accumulated in the shelf front area during this period (Figs. 5, 8).
SOURCE ROCKS, CHARGE
The West Manombolo discovery made by Madagascar's Office Militaire National pour les Industries Strategiques (Omnis) in Cretaceous sandstone reservoirs in the Morondava basin (Fig. 1) demonstrates the presence of light oil to gas charge which is derived from an unknown source rock and suggests the presence of Jurassic or Cretaceous source rock horizons.
Oil shows encountered in Cretaceous reservoirs in wells in the Majunga basin also suggest the presence of an unknown source horizon. In outcrop the richest source rock encountered to date is in Liassic shales.
High potential exists for source rocks in Late Jurassic to early Cretaceous sediments in the shelf front area (Fig. 5). Listric faulting and pull-up on seismic lines suggests the presence of evaporites in this zone (Fig. 8) favorable for the development of source rocks.
Jurassic to early Cretaceous source rocks are thought to be present in the Seychelles,3 and Cretaceous source rocks have been proven in Southeast Pakistan in an identical setting 4 5 (Fig. 2). In Southeast Pakistan these source rocks generate light oil (37-510 gravity) and gas charge for reservoirs 1,000-2,000 m deep.
While the presence of source rocks in Jurassic and Cretaceous sediments in Madagascar remains unproven, much of the sequence remains unknown and undrilled. The ubiquitous presence of shows in nearly all wells suggests that there have been oil generation.
Preliminary modeling of source rock maturities suggests that late Jurassic or early Cretaceous source rocks in the Majunga basin enter the oil window as a result of burial in the Tertiary. Early to middle Jurassic source rocks will be in the gas window consistent with gas shows encountered in Dogger carbonates.
The presence of light oil shows in the wells Tuilerie and Marovoay at levels infiltrated by fresh water suggests that active hydrocarbon migration may be taking place.
CRETACEOUS CONCEPTS
The passive margin nature of the sedimentary basins of Madagascar means that stratigraphic trapping mechanisms form a dominant role in most plays. The stratigraphic traps in some localities have enhanced closure due variously to listric faulting, differential compaction, and drape over volcanics.
Listric faulting is most apparent in areas seaward of the stable Dogger shelf platform where the faulting is probably enhanced by movement on evaporate or soft shale horizons in the Dogger shelf front area (Fig. 5). Pronounced rollover structures are present in this setting close to Majunga (Figs. 5, 8). Several phases of listric faulting leading to several phases of trap formation can be recognized with the most recent faults going to surface.
The prograding clastic shelf margin from the Aptian through to the Turonian leads to numerous stratigraphic trapping possibilities. During the early phase of the prograding shelf from the Aptian to Albian basin floor fans will tend to pond distally in the unstable part of the shelf in the Dogger shelf front area.
Low stand wedges and slumps are most apparent in the more sand rich second phase of progradation from Cenomanian to Turonian. During this phase continuing uplift caused a forced regression with truncation of earlier sand prone foresets and progradation of the shelf margin. These truncated foresets form potential traps but are poorly sealed due to the dominance of sand in the overlying beds.
Perhaps the most attractive play with potential fox large volumes is related to the Turonian canyons and submarine clastics fans (Fig. 4). The canyons are clearly visible on seismic (Figs. 6, 7, and suggest significant relief (more than 1,000 m).
The clastic fans are difficult to define with the seismic available and are represented by a zone of confused. reflections dipping both seaward and landward, thought to represent super-imposed lobes of each fan, The fans are drawn schematically on Fig. 4.
Basalt fill in some of the canyons (Fig. 7) and onlapping marine shales provide seals to the canyon and fan sediments. A series of transgressions and regressions in the shelf front area (Fig. 8) following the canyon formation has caused low stand fans and truncated foresets to form in the same shelf front area.
From Campanian through to Tertiary a progressive onlapping of sediments onto the eroded shelf leads to a shoreline play in the shelf front area. Numerous small, often superimposed mounds and onlapping units are visible on seismic (Figs. 5, 8). Some of the mounds may represent carbonate buildups.
The lateral extent of the mounds is difficult to define due to the wide spaced nature of the seismic grid. The shallowest mounds are probably invaded by fresh water, but deeper mounds in the shelf front area offer small potential targets.
Evaluation of the above plays in the onshore area of the Majunga basin shows that the most prospective zone lies in the area close to the coast (Fig. 4) where multiple objectives can be evaluated during one exploration campaign, thereby increasing chances of success.
OTHER POTENTIAL PLAYS
Apart from huge potential in Karroo reservoirs mentioned previously, several other untested plays have been identified.
These include tilted fault blocks in Tertiary carbonates in the offshore (Fig. 5), stratigraphic or karst traps in the Dogger carbonate shelf margin (Fig. 5), and large fault bounded traps in the rapidly subsiding area identified in the northern part of the Majunga basin (Fig. 4).
CONCESSIONS
All blocks in the Majunga basin are open. Conditions are governed by the 1982 Petroleum Law together with the 1989 Investment Law.
The Petroleum Law is being updated to align it with standard international practice with a view to encourage foreign investment in the petroleum sector.
Contract terms are flexible and open to negotiation with Omnis. OGC has prepared a data package containing pertinent well, gravity, and seismic data for the onshore Majunga basin.
CONCLUSIONS
New indications of light oil charge from unknown source rocks together with excellent reservoir quality in Cretaceous clastics open up new exploration possibilities in the Majunga basin.
At least six plays remain to be tested in this frontier area. Similarities can be drawn with the oil and gas producing province in Southeast Pakistan.
Political changes in Madagascar the last four years have led to an open door policy for foreign investment. Favorable terms are on offer for investment in the petroleum sector, and high potential exists for development on this island continent.
REFERENCES
- Besairie, H., Geologie de Madagascar: I, Les Terrains Sedimentaires, Annal. Geol. Mada. No. 35, 1972.
- Unpublished Omnis reports.
- Plummer, P., Evidence of actively migrating hydrocarbons found off Seychelles, OGJ, Aug. 2, 1993.
- Dolan, P., Pakistan: a history of petroleum exploration and future potential, in Classic Petroleum Provinces, Brooks, J., ed., Geol. Soc. London Special Pub. 50, 1990, pp. 503-524.
- Smith, M.A., Schwab, K.W., and Bissada, K.K., organic facies analysis of Cretaceous petroleum source rocks, southern Indus basin, Pakistan, Vol. 76, 1992, pp. 1,126-27.
Coffin, M,F., and Rabinowitz, Evolution of the conjugate East African-Madagascar margins and the western Somali basin, GSA Special Paper 776, 1988.
Hindermeyer et al., Pre-reconnaissance du bassin de Majunga, Rev. Inst. Fr. Petrole XIV No. 1, 1959.
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