RIO DE JANEIRO, July 8 (Reuters) - Brazil's most promising offshore oil province, the Santos Basin, has 400 times the number of oil leaks as its older and more prolific neighbor, the Campos Basin, a Brazilian university study says, raising concern about the environmental sustainability of the country's new oil wealth.
The Santos Basin, south of Rio de Janeiro likely contains more than 30 billion barrels of recoverable oil, enough to supply all U.S. oil needs for more than four years.
Most of the oil comes from the basin's so-called "subsalt" reservoirs, which hold oil trapped beneath up to 7 km (4.3 miles) of ocean and seabed by an ancient layer of salt.
Those fields, announced in 2007, are the focus of state-led oil company Petroleo Brasileiro SA, or Petrobras' $221 billion five-year investment plan and are seen as a key to financing the health and education policies of Brazilian President Dilma Rousseff.
The oil in Santos, though, is some of the most difficult discovered or produced anywhere. New unpublished research from the State University of Rio de Janeiro (UERJ) given exclusively to Reuters asks if the high leak rate may be a result of Santos' technological difficulties.
"The more audacious the exploration, the greater the risk of dangerous accidents and the harder it is to control them," said David Man Wai Zee, professor of oceanography at UERJ who conducted the study.
The study does not specify which companies working in the Santos Basin were responsible for spills or which of the leaks were specifically from subsalt wells. About 75 percent of the Santos Basin's output in May came from deepwater subsalt wells.
The analysis, done in conjunction with Brazil's environmental protection agency IBAMA, found that in the Santos Basin between 2008 and 2012 one liter of oil was leaked into the ocean for every 33,300 litres of oil produced.
It is usual for areas with more exploration to have a greater number of oil leaks, but UERJ says the number in Santos is unusually high.
The Brazilian national average for the period was one litre of leaked oil for every 349,600 litres of production, or more than 10 times less.
In the Campos Basin, responsible for nearly 80 percent of Brazil's oil output, the leak rate was a tiny fraction of the national average at one liter per 13.6 million liters of oil.
IBAMA data includes any oil leaks damaging to the environment such as crude petroleum, drilling fluids, fuels and oil contaminated water from the entire petroleum industry.
The Santos leakage factor was also higher than in the United States where one litre of oil was leaked for every 50,000 liters of oil between 1999 and 2009, according to data from the American Petroleum Institute (API). API data includes onshore and offshore production as well as spills related to imports.
Petrobras, which produces about 85 percent of Brazil's oil and the bulk of subsalt oil, said it rejected the assertion that Santos had more oil leaks than other formations.
"The affirmation that the company has more leaks from the subsalt region than other exploration areas is absolutely unfounded," Petrobras said.
"In 2014 for example, there has not been a single leak of oil in the Santos Basin, where most of the subsalt activity is taking place," the company added. (Writing Stephen Eisenhammer; Editing by Jeb Blount and Cynthia Osterman)