Figure 1. Area of study (illustration: Petrobras)

Part 1: Report from Peter Howard Wertheim and Dayse Abrantes, in Rio de Janeiro, Brazil, based on papers and interviews with Professor Cleveland M. Jones, DSc., and Professor Hernani Aquini Fernandes Chaves, DSc., geologists. For Scandinavian Oil-Gas Magazine (Oslo, Norway).

Both geologists, Jones and Chaves, are familiar with the Brazilian marginal basins of Campos and Santos and have long known about the existence of the evaporate sequence and the pre-salt rocks below. Thus, they produced forward-looking analyses which are rather appropriate at a time when Petrobras is in the midst of oil and gas production and administrative turmoil.

The Professors recognised the possible hydrocarbon accumulation potential, but it was only when seismic technology finally allowed better resolution of features below thick salt layers, that pre-salt exploration blocks were included in the second Brazilian bidding round conducted by ANP (Agência Nacional do Petróleo, Gás Natural e Biocombustíveis, the Brazilian oil regulatory agency), in 2000.

First major discoveries were confirmed by Petrobras, in 2006. Since then, discoveries in the region have ranked among the largest in the world in the last ten years, including Lula (formerly Tupi), Iara, Libra and others. This led to various estimates, from within and outside the government and Petrobras, which mentioned extremely large total possible accumulations, ranging from 50 to over 300 billion barrels of recoverable oil in the “Picanha Azul” region, a legal boundary of approximately 150 thousand square kilometres (approximately 800 km by 200 km).

Since none of these estimates were accompanied by technical data or a description of the methodology used, in 2010 the Professors decided to carry out the first assessment of the yet-to-find (YTF) oil potential in the Brazilian pre-salt region, utilising an assessment methodology that had already been used by the authors to successfully assess other areas, such as the shallow waters of the Campos Basin. (see Figure 01)

An exploration modelling tool (GeoX® software from Schlumberger) and a stochastic simulation method (Monte Carlo) were utilised for estimating the potential accumulations discovered and remaining to be discovered in that region. Information is entered via probabilistically defined variables, and results are also defined probabilistically.

Typical confidence levels used to define entry variables and to describe results are P90 and P10 (exceeding probabilities of 90% and 10%, respectively), where P90 means 90% chance that the result is equal or greater than the given value, and P10 means 10% chance that the result is equal or greater than the given value.

The “Assessment of yet-to-find-oil in the Pre-Salt area of Brazil” was presented during the 14th International Congress of the Brazilian Geophysical Society, in Rio de Janeiro.

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Part 2: Current Issues Affecting Petrobras

Part 3: Petrobras Step by Step Assessment of Pre-Salt Plays

Peter Howard Wertheim and Dayse Abrantes

Part 4: Bright Future Trends

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Peter Howard Wertheim and Dayse Abrantes can be reached at peterhw@frionline.com.br