GEO ExPro

An Underexplored Salt Province

A thirty year independence struggle with Ethiopia means that the natural resources of Eritrea remain largely untapped, but there is ample evidence that this now peaceful country is highly prospective for oil and gas.
This article appeared in Vol. 7, No. 3 - 2010

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Alec Robinson is a petroleum geologist who has spent nearly 40 years in the oil industry, many of them with Amoco. He has lived and worked in many parts of the world, including Colombia, Peru, Oman, Norway and Argentina. Now based in the UK, he has been President and CEO of Centric Energy since 2006. Photo: Jane Whaley Most of the hydrocarbon potential of Eritrea lies in its 16,600 km2 Red Sea acreage. Image: NASA The Dahlak Islands in the Red Sea, where oil seeps encouraged exploration in the 1920’s. They have been famous for pearl fishing since Roman times Photo: Boris Kessler Like many of us, when Eritrea was first mentioned to Alec Robinson, he reached for the atlas. “It’s not the first place that springs to mind when thinking of African hydrocarbon exploration,” the CEO of Centric Energy admits. “But that’s the point. It’s one of the few relatively unexplored places left in the world – and it has great potential. The Eritrean Red Sea has all the classic features of pre- and post-rift sedimentation, including syn-rift evaporites, as well as a known oil-rich source, and sediment feeder channels leading to prospects in less than 50m of water. There are also oil seeps on the islands of the Dahlak archipelago, and along the coast.”
 
“In addition to all the positive geological indicators,” Alec adds, “it is now one of the safest and most peaceful places to work in Africa. The government has made great efforts to encourage oil companies to the country.
 
The geology of Eritrea is dominated by metamorphosed Precambrian rocks, which were deformed by the pan-African orogeny, and then further altered by the intrusion of Lower Palaeozoic granites. As a result, the country abounds in minerals such as gold, copper, and zinc, but most of the areas prospective for hydrocarbons are found offshore.

“... one of the safest and most peaceful places in Africa”

Massive gas blow-out

The Eritrean Red Sea offers potential both above and below the salt Image: Centric Energy Eritrea lies at the southern end of the Red Sea, which was formed in Oligo-Miocene times (36 – 20 million years ago) as the African land mass broke away from the Arabian shield. Pre-rift arching and break-up of the continental crust led to the formation of the Red Sea, with block faulting on the flanks and strong subsidence in the central part, both very important for the formation of hydrocarbon traps. As rifting continued, fluviatile and lacustrine sedimentation was followed by shallow marine clastics and, with circulation in the Red Sea diminishing, the deposition of evaporites. Rifting through the late Miocene was accompanied by horst-graben faulting and fault block rotation, until by the Early Pliocene, about 5 Ma ago, small pull-apart basins along the axis of the Red Sea and Gulf of Aden were developing oceanic crust. These eventually coalesced to form the passive margins found today lying on the edge of the deep water trough of the central Red Sea, which drops rapidly to over 2,000m.

Alec and his colleagues at Centric believe that these sediments could hold large quantities of hydrocarbons. “Although exploration started back in 1921, only eleven wells have ever been drilled in Eritrean waters, with a further 12 shallow holes back in the 1940's on offshore islands. Eight of the offshore wells had good oil or gas shows, and the widespread occurrence of seeps is further evidence of a working petroleum system.”

“In fact one well, C1, drilled in 1969 by Mobil, suffered a massive gas blow-out, and continued flowing for 55 days before finally stopping naturally. From the limited and varying quality data available, a number of prospects and leads have been identified in both the pre- and sub-salt formations of the Eritrean Red Sea,” Alec says.

Pre-and post-salt plays

Parts of Eritrea are surprisingly green and fertile Photo: Michael Hopley/Sunridge Gold As with other classic rifts, there are two distinct play types, separated by the syn-rift evaporites. “A number of promising source horizons have been identified, both above and below the salt,” Alec explains. “There are Late Jurassic organic-rich shales and marls, as well as several promising source horizons in the syn-rift, with TOCs ranging from 3% – 8%. The most recent wells, drilled by Anadarko in 1998, also found potential source shales in the post-rift Desset Formation.

Most of the exploration to date has centred on the post-salt plays, and initially Centric Energy will be concentrating on these, although Alec believes that the syn- and pre-rift plays are also very prospective. “To date, all the seismic shot has been 2D, which has not effectively imaged the pre-salt. Modern 3D seismic is required in order to clearly delineate the sub salt horizons,” Alec says.

Post-rift reservoirs include Neogene lenticular sandstones and reefal limestones, with hydrocarbons trapped in structures formed by salt tectonics, including turtle backs, rollover anticlines, fault detachments and drapes over salt diapirs. “We believe there are also interesting stratigraphic traps, like deepwater ponded turbidites and channel sands, which have not been investigated yet by the drill bit.”

“The syn-rift sediments, just below the salt, are also promising – in fact, the major blow-out in well C1 occurred just as the drilling reached this point,” Alec continues. “While the Late Miocene formation is predominantly evaporitic in the areas drilled so far, it includes marginal fluvial, aeolian and beach sandstones, which may form thicker horizons as they extend into the basin. And, of course, with all that salt, along with proven shale intervals, seal is not expected to be a problem.”

Further potential is offered by the pre-rift, pre-salt horizons, which are completely unexplored, although they can be visually checked at outcrop along the coast. Plays are expected to include rotated fault blocks and horsts, particularly in the less eroded downdip areas, with potential reservoirs including Mesozoic fluvio-deltaic to shallow marine clastic sequences and dolomitised limestones, and Paleozoic fluvio-glacial sandstones.

“A number of promising source horizons have been identified”

Why so little exploration?

Centric Energy is finalizing the terms of a Production Sharing Contract for an offshore block and has been given exclusivity while finding a partner. At the moment, it is the only oil company active in Eritrea, although an Eritrean-Chinese company, Defba Oil, holds two large areas near the Sudan border. Which begs the question: why have previous companies exploring in Eritrea either not done more exploration or not followed up the minor discoveries?

“Obviously, after 30 years of war, the security situation was off-putting and left companies with concerns about operating in the country,” Alec replies. “But there were a number of other factors. Several of the wells, particularly those drilled in the 70’s, found gas, which at a time was not a sought-after commodity. That is no longer an issue, as Eritrea has a clear need for additional energy to support development - at the moment there is only one 85 MW power station for the whole country - and there would be a definite local market for any gas we find. In places, the presence of volcanics has resulted in abnormally high geothermal temperatures, which deterred some explorers who did not realise that the high geothermal gradients are localised.”

“Of course, seismic technology has only recently reached a point where we can properly image the salt and see the horizons beneath it, opening up the potential of the pre-rift. And it takes a little nerve to explore in a frontier area like this, which may be easier for a smaller organisation like us.”

Centric Energy is still a very small company, having started out in 2006 as West African Energy, focussing on West Africa with blocks in a similar rift basin in Mali (see GEO ExPro 2008, no 4, pp26 – 30). Wanting to move further afield, it changed its name to Centric Energy and has recently signed a production sharing contract in Kenya.
 

Oil, gold – and pizza!

Eritrea is trying hard to overcome the violent historical perspective most people hold of the country. The infrastructure is being rebuilt and efforts have been made to develop the economy, including putting in place attractive oil and gas licensing terms.

Sixteen foreign minerals exploration companies are now working there, reflecting the confidence of foreign investors, and the country’s first gold mine, Bisha, is due to start producing later this year, bringing much-needed revenue to the country.

“It’s a great place to visit,” says Alec Robinson. “Asmara, the capital, is in the hills, so it has a very pleasant, temperate climate, and is very safe to walk around. The people are delightful, and don’t give you any hassle. Since it was once part of an Italian colony, the architecture in the town is often in an interesting sort of Italian-colonial art deco style. And they make great coffee and pizza!”

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