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Tanzania Ultra-Deepwater Exploration*

Lino Teixeira¹, Veronica Martinez¹, and Salvador Chrispim¹

Search and Discovery Article #10189 (2009)

Posted April 20, 2009

*Adapted from extended abstract prepared for AAPG International Conference and Exhibition, Cape Town, South Africa, October 26-29, 2008.

¹ Petrobras, International Exploration Department , Rio de Janeiro, RJ, Brazil ([email protected])

Abstract

Petrobras’ ultra-deepwater technology is internationally recognized for its innovative approach. In 2001 Petrobras received OTC’s award for the development of the ultra-deepwater Roncador Field in Campos Basin, Brazil, and recently, it discovered Tupi oil Field in Santos Basin, the 2nd largest oil field discovered in the last 20 years worldwide. Petrobras has the proficiency of producing at water depths up to 2000m and now is going even farther, improving the knowledge and expertise that should lead to a capability to produce at 3000m water depths (Figure 1).

Eastern Africa is becoming a challenging but encouraging area for petroleum and gas exploration. The Eastern Africa margin, inclusive of six countries (Mozambique, Tanzania, Kenya, Somalia, Madagascar, and Seychelles), was subjected to different tectonic regimes. Its structural framework evolution started with Eastern Gondwana’s break-up, during the Juro-Triassic, and it is still active at the present time. Extensional and compressional periods controlled by strike-slip movement of the David Fracture Zone, created many sedimentary basins and multiple petroleum systems.

Tanzania is considered an exploratory frontier area with only 35 wells drilled, mainly onshore and shallow water (including the islands). The presence of oil slicks suggest the existence of active petroleum systems, which encourage ultra-deepwater exploration, even though of no well has been drilled in such an environment. Hence, Tanzania becomes an attracting exploration challenge for Petrobras deepwater expertise in the near future.

The tectonic framework (Figure 2) was outlined through a methodology based on regional integration of geology and geophysical data, particularly seismic reflection and potential field (mainly gravity and magnetic anomalies).

Between 2004 and 2007, Petrobras acquired three offshore exploratory blocks located in deepwater basins, Tanzania, each of them with an approximated area of 9000 km² (Figure 3), and water depth that ranges from 500 to 3000m (Martinez and Teixeira, 2008). The region between Alpha Fault and Davie Fracture Zone (DFZ), in Block 5, is in ultra-deepwater (1600-3000m), where the main leads and prospects were mapped (Figure 4).

The structure of the basin is dominated by prominent N-S linear trend corresponding to Gammathe Alpha Fault and Davie Fracture Zone (DFZ). Both of these are supposed to be dextral strike-slip faults, which were developed in response to the southerly movement of Madagascar in relation to Africa. Locally there is an associated series of NNW-striking faults related to both extensional and compressional regimes caused by Neogene inversion movements along the fracture zone. Both inversion-related anticlines and faulted closures are associated with the displacement of the major fault lineation that provides a number of potential traps for hydrocarbons.

There are two main seismic surveys covering the area; one survey includes 1040 km of 2D SPEC data acquired by WesternGeco in 1999 and 5398 km of 2D data acquired by Petrobras in 2006 (Figure 5). The interpretation of this data in Block 5 resulted in many exploratory prospects and leads with multiple objectives in Miocene, Oligocene, Eocene, and Cretaceous (Figure 6).

As a result of the work done so far, it can be concluded that the G&G studies reveal the presence of the basic elements of a petroleum system, as well as at least one adequate prospect supporting a reasonable chance of an economic hydrocarbon accumulation existing in Block 5.