After some initial missteps, in late 2010 a massive reserve of oil was discovered deep underneath the ocean floor about 140 miles off the coast of Rio di Janeiro, Brazil, in the Santos Basin.

The Libra prospect, buried underneath almost 7,000 feet of water, and another 16,000 feet of sand, rock, and salt layer, is expected to contain at least 8 billion barrels of reserves, and could very well be Brazil’s largest discovery ever, ahead of both the Franco and Tupi oil fields that are currently at the top of that list.

With yet another massive deep water discovery, one would expect the usual suspects lining up in droves for the upcoming October 21 state-run auction of licensing contracts for the Libra prospect. While the turnout has been surprisingly low, with only 11 companies registered so far, leading privately-owned and state-run firms will indeed be in attendance.

Conspicuous, however, is the absence of two major US companies, Chevron (CVX) and ExxonMobil (XOM) . And even though about 40 companies were originally projected to be in attendance at the auction, there are some indications that there are specific reasons for the no-show on the part of the world’s two largest oil firms.

There is considerable reason to believe that both companies are overlooking the project for the same reason that Brazilian President Dilma Rousseff canceled her October 23 visit to the US, if for very different motivations- namely revelations of the vast National Security Agency spying regime, which, according to an ongoing release of classified documents, targeted Brazilian politicians, including Rousseff herself, as well as the country’s major business concerns, including the oil giant Petrobras.

While other majors such as Royal Dutch Shell ($RDS.A) and Total SA (TOT) , along with major state-owned producers from China, India, and Malaysia look to be positioning themselves to tap the Libra field, Exxon and Chevron might be avoiding the prospect for fear of a massive upheaval resulting from unfavorable Brazilian public sentiment.

Brazilian protesters are already warmed-up, having fought riot cops and tear gas to prevent an increase in public-transit fares and generally express their displeasure with economic inequality in the country, and by all indications are outraged at the revelations about the NSA’s activities that have been released to the public by the UK’s Guardian newspaper. On September 9, for instance, Brazil’s largest oil union demanded that the US majors be precluded from the bidding process.

It is not clear how much money both companies stand to lose. Chevron is already reviled in Brazil as a result of the 2011 offshore spill in the country for which the company was held responsible. The NSA spying revelations, in which Brazil was revealed to be a main target, have only exacerbated the situation.

Exxon has said only that it has “decided not to participate in the Libra bid,” and it has been suggested that “supermajors” in general do not like such projects if they are not themselves the operators, but this claim is debatable given the willingness of Total and Royal Dutch Shell to get involved.

A more plausible explanation for the general low turnout for the bidding would be that such a deep-water project, backed by an insufficient amount of ecological data, would constitute an experimental and risky venture for any company regardless the size. Still, this explanation does not necessarily contradict the thesis that Chevron and Exxon have been told, or have decided on their own, to keep their distance.

The larger context could very well turn out to be that the NSA’s distressing behavior is starting to have a negative effect on the ability of major US businesses to make money abroad. The fact that oil companies, and majors in particular, could be the first victims is not surprising, as many outside the US, especially in oil-rich countries where American intelligence agencies have historically played a covert and pernicious role in local politics, such as Iran and Venezuela, have for years become accustomed to seeing them as a tool of US foreign policy objectives.

[Image: a Petrobras drilling platform, courtesy of Wikimedia Commons]

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