This is the story of the exploration that yielded one of the most extraordinary gas resources in the deep waters of East Africa. In 2006, Mozambique launched a tender for a series of exploration blocks in the conventional deep and ultra-deep waters of the Rovuma Basin, on the border between Tanzania and Mozambique. There was very little geophysical data available for the first licencing round. However, on that data, was built a geological model. It followed the decision to submit an offer for two blocks, obtaining an operating licence for Block Four, named Area 4, which is what this story is about.
The first seismic surveys
Eni spent the following year negotiating the contract and proceeded with the geophysical exploration programme using the 2D (bidimensional) seismic acquisition. Analysis of the data led to the conclusion that the first chosen model was the path to pursue.
However, another area reveals an alternative prospect, and it was decided to go ahead with an initial three-dimensional survey of this new area in order to improve the indirect image of its subsoil. The block’s surface area was huge, some 17,000 square kilometres, of which over 4400 square kilometres were acquired. A “play” formed by gigantic clastic sediments (sands) emerged that had been generated by the deposition of sediments rich in suspended material and denser than the water in which they moved. Typically, these deposits are found at the bottom of oceans but are difficult to retrieve in surface geological outcrops even for specialists.
Imagine a large undersea river flowing under the seabed and carrying large quantities of suspended sediment. When these flows slow down, turbidites are deposited on the seabed. These phenomena are triggered by natural causes: earthquakes, heavy rainfall, underwater landslides. Each sediment bed is of considerable: tens, sometimes hundreds of square kilometres. The 3D seismic survey showed depositional systems, with a series of seismic anomalies interpreted as a possible evidence of hydrocarbons presence, known by the acronym DHI (Direct Hydrocarbon Indicators). The geologists decided to carry out two more 3D seismic surveys. By the end of 2009 and the beginning of 2010, they had a fairly clear indication of where to drill, but the data showed potential deposits so large that there were doubts about whether the interpretation of the data from the surveys was correct.
Come 2010, and while the drilling campaign was being prepared from the block adjacent to Area 4, word came that a considerable amount of gas had been found in a well that had just been drilled. In October 2011, drilling began on our first exploratory well in Area 4. The location of the exploration well is usually decided on the basis of a number of considerations, but geology is not an exact science; geologists drill where they think they will find gas mineralisation of such significance that the discovery is of economically viable, but this is not the only factor that comes into play. The location of wells is determined where one thinks the turbiditic systems have better rock quality but also depending on the condition of the seabed. The situation in Area 4 in Mozambique was quite complex: there were several deep underwater canyons cutting through the seabed.
Drilling conditions at a depth of 2000 metres are challenging as there are usually very powerful seasonal submarine currents, with a speed of five or six knots that make it difficult even for powerful vessels such as the SAIPEM 10000 to withstand. The submarine wellhead must be positioned in a safe place and there must be no stability issues. The first well was named Mamba South 1 and was drilled by SAIPEM 10000 - a dynamic positioning ship - that had come from Australia across the Indian Ocean. The discovery was of a much greater size than expected. The predictions had been right, but the thickness and quality of those reservoir rocks had been underestimated. The team relied on the best models derived from discoveries in Angola: out of 100 metres of total reservoir rock, it’s estimated that30-40 are good. In Mamba, the full thickness of sands is deliverable! An astounding success!
The Mamba field was located at a depth of about 3400 metres and was more than 300 metres thick. Its geological age was estimated to be around 30 million years. The field was larger than the whole city of Maputo.
Following the first major discovery, which revealed the out-of-scale size of the field, geologists continued non-stop with the drilling campaign. They already had four to five exploration prospects ready. Each well added a significant quantity of gas: all wells proved to be larger than expected. A total of eleven wells were drilled successfully, an outstanding achievement for the history of an exploration campaign. In 2012, we discovered Coral, a high-quality Eocene-age field with enormous exploration potential. It covered an area of 300 square kilometres and lay at a depth of about 4500 metres. In 2013, the Agulha field followed, allowing the mining potential of older geological sequences to be explored. Exploration operations lasted 725 days. In addition to the eleven exploratory wells, appraisal wells were drilled and production tests were carried out. The exploration campaign ended with 15 wells and a 100% success rate. Over 85 trillion cubic feet of natural gas (Tcf) had been discovered, equivalent to 2,400 billion cubic metres. These were gigantic figures.
Eni’s geologists’ curiosity went further. They drilled, found gas, and were fascinated by the underground world they had not seen anywhere else. A seismic survey revealed unusual phenomena: the ripples created by the waves of the sea on the sand appeared on the surface of these bodies. Geologists tried to understand what these geometrical shapes could represent, and how to fit them into their depositional model. During the drilling operations, the drilling chisel delved into the history of these rocks and seas, grasping useful hints for understanding the mystery surrounding the deep waters of the Rovuma Basin. The rocks where the gas was trapped were so thick as to suggest the existence of mighty river systems, capable of carrying exceptional amounts of water and sediment. An example of this in Africa is the Congo River. In East Africa, instead, rivers are poor today, but their geological history reveals a different truth: 30-40 million years ago, there were powerful river systems also on the East African coast. The team of geologists, led by Luca Bertelli (Head of Exploration from 2011 to 2022), built a model which would become famous because it explains the deposition of these sands: how and where, under the pressure of strong deep-sea bottom currents, fine particles settle in all the systems in Rovuma in Mozambique.
Success as a result of Eni’s expertise
The discovery of the Mamba, Coral and Agulha fields in the Rovuma Basin offshore Mozambique confirms the effectiveness of our exploration strategy.
In this story, success was a combination of three factors: a good deal of risk-taking; the building of sound predictive geological models; technology, through the acquisition of three-dimensional seismic data, its rapid processing and interpretation. These elements allowed Eni to pioneer exploration activities in the Rovuma Basin. Courage lies at the root of great discoveries in exploration, and this has always been the company's hallmark. Then there are geology-based ideas. In Mozambique they were right, and yet we were not aware of the extent of the discovery. Technology allowed us to come to supergiant discoveries and to refine the model. Data must be constantly updated and revised. Frontier exploration is like this: it is a mixture of intuition, concepts and technology.
A large part of the discoveries in Area 4 also extended into Area 1 besides Coral. There were also other smaller discoveries. Unitisation treaties, which are used to estimate the percentage of resources in fields straddling two areas for which Eni has obtained exploration licenses, are the basis on which we decide how we will subdivide the amount of gas we have discovered, once it has entered production. These are complex processes in which everyone tries to make the most of their share, especially when a field is big. In Coral’s case, however, the field was entirely within Area 4. We therefore decided to start production with the Coral South project and to do it with a floating plant, as this was the best solution from a technical point of view, the way leading to the first offshore gas in Mozambique. Coral South is a landmark project for the industry and firmly places Mozambique onto the global LNG stage: first LNG cargo departed from Coral Sul FLNG on 13th November 2022.
Extracts from a conversation with Luca Bertelli (Head of Exploration at Eni from 2011 to 2022) published in the photobook "L'Energia di Coral".
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