Angola's deepwater frontier extends nearly 1,500 km in length by 250 km wide, in water depths ranging between 200 meters and 4,000 meters, an area that encompasses the present day upper, middle, and lower continental slope. Angola's offshore south Atlantic has relatively mild weather and calm waters offering easy working conditions for exploration-production activities.
The nation's exploration frontier is sub-divided into three provinces - North, Central, and Southern - that encompass the Congo, Kwanza, Benguela, and Namibe basins of Cretaceous-Tertiary age. The deepwater potential is proven in the North and is recognized in the other two provinces. Angola has adopted a four-fold strategy to identify prospectivity. This includes:
- An initial regional geological appraisal to identify the play potential
- Conducting non-exclusive seismic acquisitions of 2D and 3D for evaluation
- Using the geological-geophysical database to create a concession of 5,000 sq km in size to reduce exploration risk
- Awarding the concessions to oil companies that will operate in group association.
The agreements are defined under a production sharing agreement (PSA) to carry exploration and economic obligations to the concessionaire. This strategy has helped to recognize hydrocarbon-rich zones in a relatively rapid fashion.
Deepwater leases are drawn in 200 meters to 2,500 meters water depth. Plans are underway to draw lease boundaries in over 2,500 meters water depth. These are defined under deepwater (200-1,500 meters water depth), ultra-deepwater (1,500-2,500 meters water depth) and ultra-deepwater west (>2, 500 meters water depth).
Deepwater provinces
The deepwater and ultra-deepwater blocks are defined in the North province and include blocks 14 through 18, and 31 through 34. Here, the ultra-deepwater west blocks are yet to be defined. In the Central and Southern provinces, only the deepwater blocks, 19 to 22 and 23 to 30, are defined. In this region, the ultra-deepwater blocks are yet to be configured.
All the defined blocks in the North and Central provinces are under exploration-production obligation. In the Southern province, only blocks 24 and 25 are obligated. Overall, there are 21 existing deepwater frontier concessions in Angola and the region is further contemplated to be defined with new leases in ultra-deepwater west. Thousands of sq km offshore remain to be explored.
North province
The North province is the key commercial success due to its initial advances in exploration activities. In blocks 14 through 18, there are 35 exploration, 12 appraisal, and 12 development wells. This has resulted in 21 commercial discoveries such as Girassol, Dalia, Quito, and Hungo.
Amongst the prominent discoveries, Block 14 has four, Block 15 has six, Block 16 has three, Block 17 has nine, and Block 18 has two. In terms of recoverable reserves, Block 14 has 1 billion bbl, Block 15 has 2 billion bbl, Block 16 has 155 million bbl, Block 17 has 2.236 billion bbl, and Block 18 has 341 million bbl. The overall estimated oil-in-place from all combined discoveries is 17 billion bbl. The recoverable oil, at a 40% rate, will give 7 billion bbl. The ongoing exploration is intense and the rate of further discovery is optimistic.
Central south provinces
Exploration in the central province is at an early stage. Only two wells have been drilled in Block 20. The remaining blocks 19, 21, and 22 are yet to be explored. The region has a proven source rock having generated hydrocarbons. In the Southern province, Blocks 24 and 25 are gearing up for exploration with current 3D acquisitions. The remaining region is required to undergo further evaluation. Several blocks are opened for exploration lease bids.
Reservoir characteristics
The recognized play in the deepwater region is in Tertiary age Oligocene and Miocene channel system. The reservoir sandstone is trapped in four-way, fault-bound, or at salt-flank pinch-out. The average porosity is 30% and permeability is at 1-5 Darcy. The reservoirs are found between 700 meters to 1,500 meters below the mudline, with an N/G around 50%. The reservoir temperature range is 55-75°C. The API of the oil is 20-37° (heavy to light).
Biodegradation has occurred under 60°C reservoir temperature. As a result, certain reservoirs contain heavy crude that is expected to have price devaluation. The found crude is tied to marine sources. The oil migration is made feasible from the available fault conduits.
A model of the geological play suggests that the Tertiary age channels are trapped in salt withdrawal structures leading to four-way or fault-bound traps. In addition, there are salt-induced compressed structures and salt overhangs with subsalt trap dominating in ultra-deepwater. The Tertiary age channel complexes are predominantly recognized at basin lower slope.
The reservoir bearing channels are recognized from geophysical techniques such as differences in AVO (amplitude variation with offset) for oil-gas-water and flat-spot events to differentiate the fluid contacts. In addition, special acquisition parameters and pre-stack time-depth migration are routinely applied to chase the mentioned traps.
The deepwater potential in Angola is indicated to be quite large. Current exploration evaluation indicates the recoverable reserves would be in the North Province (15-20 billion bbl), Central Province (10-15 billion bbl), and the South Province (5-10 billion bbl). Currently, development-production evaluations are ongoing for the major discoveries. The concepts considered are either as stand alone or cluster development.
Kuito and Girassol
The discoveries Kuito in Block 14 and Girassol in Block 17 are at a relatively advanced stage of development as stand-alone fields. The field Kuito is in 350-450 meters water depth and is planned to go in production by the end of this year. The crude is heavy in quality and acidic. The planned development cost is estimated to be $ 2 billion to reach the production level of 200,000 b/d of oil. The production costs per bbl is estimated to be $5.
Girassol is located in 1,350-1,500 meters water depth and is planned to go in production in 2001. The crude is light in quality. The planned development cost is estimated to be $2.5 billion to reach the production level of 200,000 b/d of oil. The production cost is estimated to be $ 6/bbl.
Other developments
There are also many other stand-alone discoveries in very early planning stage to be considered for development. Cluster development planning is underway for the Block 15 discoveries. The fields are located within a radius of less than 13 km of each other, centered on the Hungo discovery. The operators have decided on a fast-track development plan with several development scenarios under consideration. The current preference is towards a surface wellhead entity such as a TLP (tension leg platform, DDCV (deep draft caisson vessel), or FPSO (floating production, storage, and offloading vessel).
The deepwater discoveries will have a bearing on production trends in Angola. A linear increase in daily production is predicted from the current level of 760,000 b/d to 2,500,000 b/d of oil by the year 2010. This is based on all the anticipated discoveries from the exploration efforts in deepwater and ultra-deepwater in the North, Central, and Southern provinces. However, it must be recognized that the controlling factors are in technology for development, such as crude price, markets, and investment options, that would dictate the policies of government to reach the production goal.
Assessment of Angola's deepwater potential is very optimistic, due to the region's geology, current and potential discoveries, and inflow of dollar investments for exploration and production. The availability of open acreage, favorable production sharing agreements (PSA) agreements, and compatibility of personnel in the country to work with has created a positive trend to find hydrocarbons relatively quickly. Estimates of potential reserves in the region are quite high, and is lucrative for deepwater development.
Author
Antonio Raposo is head of the Interpretation Department at Sonangol.
