It took a long time until the first giant gas discovery was made in Mozambique in 2009. Since then, the Rovuma Basin in the north of the offshore sector has been a centre of activity with many wells drilled and discoveries made.
In August, Bloomberg reported that an LNG tanker was soon to arrive at Eni’s new Floating Liquified Natural Gas Terminal Coral Sul (South), indicating that it will not be long before the first load of LNG will be on its way to its buyer. Combined with Eni’s intention to build a second terminal in Mozambique that will be operational in less than 4 years, and TotalEnergy’s plans to finalise the Rovuma onshore LNG plant, Mozambique will soon be an important player in the international gas market.
Although Mozambique has been a producer of onshore hydrocarbons for years, the eyes of the global exploration community only turned to this east African country when Anadarko made a gas discovery in the Rovuma Basin in the north of the country in December 2009 when it completed the well Windjammer-1 in Area 1. Before that, there had been little interest in the East African margin as most of the attention was focused on the western side of the continent.
However, after more than a decade after the discovery of gas in the Rovuma Basin, the majority of Mozambique’s offshore remains poorly explored. With the Sixth Licensing round coming to a close for bids in November, and blocks being on offer across major parts of the offshore sector, will there be an appetite to also explore the waters further south?
Map showing existing licences across Mozambique as well the areas on offer in the current Sixth Licensing Round (pale green). The gas fields (red) currently under development are all situated in the northern offshore of Mozambique.
The success in the Rovuma Basin followed on from the award of seven onshore and offshore blocks during the Second Licensing Round in 2006, resulting in the completion of 30 exploration and 32 appraisal wells from 2010 to 2015.
Major gas resources have since been proven, with the Instituto Nacional de Petróleo (INP) estimating that the Rovuma basin holds 125 Tcf in Areas 1 and 4 only. This amounts to around 3,500 Bcm or almost 22 billion barrels of oil equivalent.
Areas 1 and 4 are the only currently licensed areas in the Rovuma Basin, with TotalEnergies operating Area 1 in the west and a joint venture between Eni, ExxonMobil and CPNC operating Area 4 in the east.
With the Coral field in the south of Area 4 containing around 16 Tcf of recoverable gas, it is easy to see that the first FLNG terminal is only just the beginning of a major phase of development for this northern area of Mozambique’s offshore sector. TotalEnergies estimates that it has so far discovered 65 Tcf of recoverable gas in Area 1, so almost four times as much as in Coral.
But, even though around 50 offshore exploration wells have now been completed in Mozambique in total, large parts of the offshore sector remain to be tested. Only one well was drilled to date in the Angoche Basin, and only a limited number of attempts were made in the Zambezi and Beira High areas. Legacy 2D and 3D seismic data sets demonstrate high potential in these areas though, which must be the driving force behind Geoex MCG recently announcing the acquisition of a total of 8,550 km3 of broadband seismic mainly in the Angoche Basin in 2023.
The Sixth Licensing Round, which is still open for bids, also focuses on the Angoche Basin. In addition, several blocks are on offer to the north and south of Areas 1 and 4 in the Rovuma Basin as well. The closing date for submissions is the 11th of November this year.
Mozambique’s offshore geology
Focusing on the history of Mozambique’s offshore sedimentary infill, we do not need to go back in time very far. During the Middle Jurassic (165 Ma), Africa, Madagascar, India and Antarctica still formed part of the Gondwana continent. Therefore, sediments dating back to the Middle Jurassic and older are relatively scarce and have a limited extent, even though the onset of a tensional regime and the formation of half grabens had already started as far back as the Triassic.
Around 160 million years ago, during the Late Jurassic, the first rifting episode took place, with initially the formation of the Somali Basin in the north and a little bit later the Mozambique Basin further south. Rifting had a strong dextral strike-slip component; the Mozambique and Somali basins were separated by the Davie Fracture transform fault.
The Rovuma Basin formed the southernmost part of the Somali Basin whilst the Angoche and Zambezi Delta basins form part of the Mozambique Basin.
Until the Late Cretaceous, the Somali Basin and the Mozambique basins formed separate basins. The sedimentary environments of both basins have been similar since.
The Cenozoic succession in both the Rovuma and Mozambique basins is characterised by a deltaic succession with proximal fluvio-deltaic facies in the west to deep water facies in the east. Two sedimentary cycles have been distinguished in the Cenozoic succession, separated by an unconformity; a Paleocene- Eocene and an Oligocene to Pliocene interval. The detachment plane of folds and thrusts that formed in the Oligocene and younger succession in the offshore Rovuma Basin are thought to be Paleocene or Eocene mudstones.
The seismic line shown here, which is from the Rovuma Basin, nicely shows a thrusted interval in the deltaic succession.
East-West seismic line from the Rovuma Basin, nicely showing a thrusted succession in what is believed to be the Cenozoic delta succession. Source: Instituto Nacional de Petróleo (INP).
Rovuma Basin
In the Rovuma Basin, the Paleocene to Oligocene discoveries are characterised by stacked high-porosity deep-water channel and fan sandstones. The most commonly observed trapping mechanism is stratigraphic, but structural traps have also been found in the deep-water thrust belts of Palma and Mocimba.
In the Rovuma Basin, there is additional prospectivity in the areas flanking the main delta depocentre and the offshore extension of the Mozambique Plain further east, with proven high-quality reservoirs in the Cretaceous and throughout the Tertiary section. The Cretaceous is only penetrated by a limited number of wells, leaving this succession largely unexplored.
Rovuma gas is believed to be sourced from yet undrilled, gas-mature Lower Cretaceous marine shales. However, the presence of oil in Cretaceous sands in the Ironclad-1 well and coastal seeps in the southern Rovuma suggests that a potential oil-prone petroleum system also exists, with Lower Cretaceous, and Upper and Lower Jurassic successions as source rock candidates.
It is interesting to see that it is the Ironclad well that is the first well of a string of three wells south of Area 1. The two wells further to the south (Perola Negra-1 and Buzio-1) were both dry. The wells both fully tested the Cenozoic succession, thereby casting some doubt as to whether the trend of gas discoveries in Area 1 can be further extended towards the south.
Angoche basin
In the Angoche Basin, where just one well has been drilled, the sedimentary succession of up to 7,000 m is a first indication that all requirements of a working petroleum system can be met. Similar to the Rovuma Basin, oil seeps have also been observed on the Angoche coast.
Current seismic data also suggests the presence of large-scale slope fan sandstones, presumably within the Cenomanian/Turonian and Paleogene successions. Stratigraphic trapping is also thought to be the main trapping mechanism in this basin, with additional structural closures draped over syn-rift highs.
The Zambezi delta. Source: NASA.
Zambezi delta basin
The Zambezi Delta Basin is slightly different from the basins further north in the sense that below the 4 to 5 km of Upper Cretaceous to recent deltaic sediments of the delta, an up to 9 km thick succession of Upper Jurassic to Lower Cretaceous sediments can be found over a failed Middle Jurassic rift basin. In addition, the Beira High forms a prominent block of continental crust against which the onlap.
The Beira High has yet to be tested by the drill bit, whilst the Zambezi Basin wells drilled to date are believed to have been drilled off-structure or having missed the reservoir at the Upper Cretaceous target level. At the same time, in this area, seismic data also suggest the presence of deep-water channel and fan systems in the Upper Cretaceous succession. These may also drape the Beira High. Potential source rocks in the Jurassic and Lower Cretaceous are thought to be in the oil window away from the Zambezi Delta Basin depocenter.
HENK KOMBRINK
An important source of information for this article was the INP and GeoPartners Sixth Round Legacy Data website, which can be found here.
