Mali, dominated by the ever-encroaching Sahara, is heavily dependent on foreign aid and vulnerable to fluctuations in world prices for cotton, its main export. However, some experts think the country may be sitting on large volumes of hydrocarbons, waiting to turn this poverty stricken country into a net exporter of oil.
Unexplored country
Dr John Scott has extensive experience in the oil and gas sector, beginning in 1971. He founded Petroleum Geological Analysis Ltd in 1979 and provides exploration advice and appraisal to clients throughout the world. Between 1987 and 1991, he was Professor of Petroleum Geology and Director of the Key Centre for Resource Exploration at Curtin University of Technology, Western Australia. Photo: Jane WhaleyThe existence of hydrocarbons in Mali has been suspected since reconnaissance airborne surveys in the 1960’s. In the 70’s and 80’s several thousand line kilometres of 2D seismic were shot over the country, giving strong enough indications of potential resources for a few wells to be drilled. Poor results from these, coupled with a turbulent political history and lack of infrastructure, meant that the majors wiped their hands of the country, and no further serious investigations took place until 2004. But with rising prices and improved technologies, the industry is again looking for hydrocarbons in Mali, despite the remoteness of the country.
Australian Baraka Petroleum were the first to show renewed interest, taking five blocks in the north-western Taoudeni Basin in 2004. New airborne surveys were followed by seismic, while the first gravity and magnetic surveys of the southern Gao Graben since the 1970’s were recently undertaken. The majority of the country is now licensed, and over ten companies hold acreage.
Five sedimentary basins
The main target reservoirs in the Taoudeni Basin are Infracambrian carbonates, with massive fine-grained limey mudstones interspersed by horizontally laminated stromatolitic horizons. The primary reservoir potential of the carbonates is low, but fracture porosity is expected in the subsurface. Photo: John ScottMali has five sedimentary basins of potential interest, ranging in age from Infracambrian to Mesozoic-Cenozoic. Four of these are proven through drilling, with the fifth, the Nara Trough in the south-eastern corner of Mali, delineated through geophysical records. Most activity to date is in the Taoudeni Basin, partly due to its extension into Mauritania, a country which has itself undergone renewed interest in recent years.
As Jon North, CEO of Selier Petroleum, which has interest in a number of blocks in Mali, says, “The vast Taoudeni Basin, more than 1,000 km in diameter, has similar geology to the intracratonic basins of Algeria and Libya, but only 1 exploration well per 450,000 km2 of basin area (equivalent to more than 70 UK North Sea quadrants). Considering this, Mali must be regarded as one of the last frontiers for onshore oil and gas exploration worldwide.”
Over 600 MMbo in underexplored basin
The nomadic Touareg are the main inhabitants of the Saharan interior of North Africa, including Mali. They are famous for their distinctive blue head coverings and cloaks, traditionally covering the heads and faces of the men, while the women remain unveiled. Photo: Paul ArcherThe Taoudeni Basin is one of the largest Early Palaeozoic Basins in Africa, with sediment thicknesses of over 5,000m in places. A 2006 independent evaluation of its potential, undertaken by industry consultants RPS Energy, suggested that just the five blocks held by Baraka Petroleum, covering half the Malian Taoudeni Basin, could hold as much as 645 MMbo and over 9 Tcfg. Very large leads mapped from seismic data suggest that this barely explored, high risk basin is worthy of further investigation, with evidence of both Infracambrian gas and Palaeozoic oil plays.
The basin is essentially a depression centred on northern Mali, comprising two megasequences. The oldest is Late Proterozoic to Early Palaeozoic, with clastics, shallow marine carbonates and interbedded black shales. This shale source and fractured carbonate reservoir system forms the main Infracambrian exploration target. In 1982 the only well in Mali to reach this level, Yarba-1, in the south-eastern Taoudeni Basin, reported gas shows in Infracambrian carbonates, although recent reprocessing of old seismic suggests that the well was probably not drilled on a closed structure. Abolag-1, drilled in 1974 to the west in Mauritania, flowed 480,000 cfgpd from Precambrian fractured carbonates. Organic-rich Infracambrian black shales, with TOCs (Total Organic Content) up to 20%, crop out along the northern margin of the Basin.
This Infracambrian petroleum system is the main interest for Mali Petroleum, which holds acreage in excess of 35,000 km2 in the southern part of the Taoudeni Basin. It has mapped promising anticlinal structures, over 50 km long, which could house large hydrocarbon volumes. These are predicted to be primarily gas, with some condensate.
Links to prolific North African basins
Two of the four wells drilled in the Taoudeni Basin reached the target Infracambrian carbonates and both had evidence of hydrocarbons. Image: Sphere PetroleumThe Infracambrian is overlain by Palaeozoic sediments, an erosional remnant of a major basin stretching from north-west Africa to Arabia, which links the Taoudeni Basin prospectively to the prolific Palaeozoic basins of Libya and Algeria. Potential reservoirs include Ordovician and Devonian sandstones, although initial geophysical evidence suggests that the Palaeozoic system is not as extensive as the Infracambrian. The only other well in the Malian part of the Taoudeni Basin, Atouila-1, had minor gas shows in the Siluro-Devonian.
The Taoudeni Basin shows interesting geological similarities to the productive North African basins, including the southern Algerian Reggane and Illizi Basins. These shared Early to Late Palaeozoic stratigraphy with the Taoudeni, covering the depositional interval of source, reservoir and seal rocks.
In the eastern part of Mali the Palaeozoic Tamesna Basin and overlying Mesozoic Iullemedden Basin, extend into neighbouring Niger and Algeria. Not much is known about these basins, as little seismic has been recorded and only two wells, In Tamat-1 in the Tamesna and Tahabanat-1 in the Iullemedden, have been drilled, both back in 1983. The second of these recorded minor bitumen shows in the Lower Cretaceous and Upper Jurassic. With only these two wells on 80,000 km2 of basin area, and similarities to the Cretaceous rift basins of Chad and the Palaeozoic sag basins of Libya and Algeria, as well as the probable presence of good Silurian source rocks, this remote area is worth further investigation.
Half graben complex
About 65% of Mali is desert or semi desert, and less than 2% of the land is cultivated, although agriculture is the main occupation of the majority of the population. Photo: Paul ArcherMali Petroleum also has a 37,500 km2 block in the Gao half-graben complex in south-east Mali. This is part of the Central African Rift System, which formed during the Atlantic opening in the Cretaceous, extending from Nigeria and Mali in the west to Sudan and Kenya in the east and containing commercial oil in Chad and the Sudan. Only one deep well has been drilled within the Gao Graben, and one in its southern extension in Niger, both of which were dry. However, minor shows of both oil and gas were recorded in a shallow water well in the Gao Graben, indicating the presence of hydrocarbons in the area.
Dr. John Scott, Technical Director of Mali Petroleum, believes that reprocessing old seismic data has shown the potential of this area. “Ansongo-1, in the Malian sector, found predominantly sandstones and siltstones, without thick shale layers, but we think that in a classic continental half-graben complex such as this, considerable facies change can be expected down dip. We expect older lacustrine sequences which may contain potential source rocks in the deeper parts of the graben complex. Similar systems are known in grabens in the Central African Rift System and elsewhere.”
In the south-west of Mali there is a totally unexplored basin, the Nara Trough, which is recognised solely from gravity and magnetic data. It is presumed to be of Mesozoic origin but may be Infracambrian, and aeromagnetic data suggests that it may contain a sedimentary section up to 14,000 m thick.
Geological and practical challenges
Unlike many of its neighbours, Mali has a progressive, democratic government and a reputation for being fair and open. Keen to encourage foreign investment, in 2004 it introduced a new petroleum law aimed at attracting new entrants. This has been remarkably successful, and only a few blocks remain unlicensed.
Whether Mali ultimately enters the club of hydrocarbon producing nations will depend on the result of the ongoing analyses of the various basins. As well as questions about the viability of proposed reservoirs and the existence of sealing rocks, the level of maturity of the potential source rocks is unknown. The Taoudeni Basin, for example, is extensively intruded by Jurassic dolerites, which could have locally destroyed reservoir potential. In addition, the sills have proved a problem in the interpretation of old seismic data where they can cut bedding planes, giving rise to false anticlines. Detailed investigations to determine the extent and influence of these intrusives are required. Some answers may soon be delivered, as Mali Petroleum hope to drill in the Taoudeni Basin in 2009.
Practical issues in exploration revolve around the remoteness of the country and its lack of infrastructure. Mali is landlocked, and the eastern basins of Geo and Iullemedden are about 1,000 km from the Gulf of Guinea coast. Roads are very poor and the nearest hydrocarbon pipelines are many miles away, although there are suggestions that a spur to the proposed Nigeria to Algeria gas reticulation line could be built if sufficient resources were discovered in the Malian sector of the Taoudeni.
Gauntlet thrown down
Mali remains a basically unexplored country, offering excellent frontier basin opportunities to companies prepared to take the challenge. It remains to be seen how many of those who take up the gauntlet will be successful, but it will be interesting to follow their fortunes.
