Despite the ripple recently when the Brazilian government sought to win a bigger role in the country's pre-salt region, primarily in the deepwater Santos Basin, events continue to rock along in what has become one of the centers of the energy universe.

The latest event in the area is an announcement by Petrobras (NYSE: PBR) and the U.K.'s BG Group that the Guara field likely contains substantially more oil than had been thought. Following tests by the trio of partners -- Petrobras has a 45% interest, compared with BG's 30% and a 25% interest held by Spain's Repsol (NYSE: REP) -- it now appears that the field contains between 1.1 billion and 2 billion barrels of oil. Original assumptions had pegged Guara at about 500 to 650 million barrels of oil equivalent.

Guara is located in the BM-S-9 block, an area where Petrobras has drilledtwo previous wells and currently is in the process of drilling yet another. As a result, the amount of oil and gas trapped in the block could be higher than thus far has been assumed.

The only fly in this ointment (at least for international oil companies) is an indication that the nation's government, under President Luiz Inacio Lula da Silva, has aspirations of playing a bigger rolethrough Petrobras in the pre-salt. A proposed set of rulesthat he has set forth would grant to Petrobras sole operating rights in all pre-salt fields, and the company would have at least a 30% interest in all joint ventures in the basin. However, the rules might not get past the country's legislators.

Nevertheless, as a result, Royal Dutch Shell (NYSE: RDS-A)(NYSE: RDS-B) has already determined not to spend more exploration money in Brazil until the situation is resolved. And while the likes of ExxonMobil (NYSE: XOM), Anadarko (NYSE: APC), and Hess (NYSE: HES) remain activein the pre-salt, one can only wonder how long that activity will continue if faced with more stringent mandates.

At the same time, the positive news on the Guara field makes me even more convinced that it'll be difficult to top Petrobrasas an energy investment in the years ahead. With the company planning to spend nearly $175 billion on exploration and production projects during the next five years, and with its success in the pre-salt thus far, it seems that Fools would be wise to salt away at least a minimal position in the company's shares.

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This article was originally published as Don't Take Petrobras With a Grain of Pre-Salton Fool.com

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