Transpetro, a subsidiary of Brazil\'s state-run energy giant Petrobras, said Thursday it had detected an oil leak off the coast of Rio Grande do Sul state but did not know how much had spilled. \"An oil leak was detected at a single bay in the Osorio Terminal during offloading from a vessel,\" it said in a brief statement. Petrobras\'s transportation unit said it was still not possible to estimate how much oil had leaked but said personnel and equipment were deployed to contain and remove the spill off the coast of the southern state. In November, another oil spill was detected in a well operated by US energy company Chevron near the Frade field, 370 kilometers (231 miles) northeast of the Rio de Janeiro coast. The spill was eventually contained and Chevron was subject to several environmental fines. The case was a wake-up call for Brazil as it prepares to tap huge underwater oil fields, which the national oil agency ANP says hold reserves that could surpass 100 billion barrels of high-quality recoverable crude. These fields are off Brazil\'s southeast Atlantic coast beneath kilometers of ocean, bedrock, and hot salt-beds. In 2010, Petrobras was the world\'s third biggest oil producer in terms of market value at $228 billion. By late last month it had fallen to 5th place, with its value down to $156 billion, according to the daily newspaper O Globo.