Tanks containing the fuel were expected to rupture because of the water pressure in the depths of the Atlantic. The oil will be at the mercy of ocean currents as it rises to the surface.
The 40-storey platform, which had about 9,500 barrels (1.5 million litres) of oil on board, went down on Tuesday - five days after powerful blasts tore through it, killing 10 workers.
But the BBC's Brazil correspondent, Tom Gibb, says the clean-up operation will be difficult given the rig's location - 125km (80 miles) offshore, where the sea is more than a kilometre deep. This will also make the investigation into the accident difficult.
Salvage experts battled high winds and rough seas for five days, but abandoned their efforts after the platform "shifted suddenly" early on Tuesday morning.
The rig went down off the coast of Macae, 190 km (120 miles) northeast of Rio de Janeiro, taking with it the bodies of nine oil workers believed to be trapped inside.
Naturalists fear that that at least one critically endangered species of seabird and other threatened species could be affected by a significant oil spill.
Tanks containing the fuel were expected to rupture
because of the water pressure in the depths of the Atlantic. The oil will
be at the mercy of ocean currents as it rises to the surface.
Petrobras says this will help disperse the slick, inflicting minimal environmental damage.
Company officials say the quantity of oil involved is a fraction of other disasters. But environmentalists are not convinced, especially as Petrobras has in the past played down the amount of oil dumped in other spills.
Petrobras says its fiscal 2001 earnings could be hurt by as much as $450m because of the disaster on the platform.
The company's shares have lost 7.6% of their value since the explosions on the platform last Thursday.
In addition to the costs of lost output, Petrobras will face higher insurance costs from now on.
The sunken rig was insured for $500m and the accident caused panic in the insurance market.
The company could also face a fine of more than $70m for the oil and diesel that are already leaking from the sunken structure, apart from compensation payments to the families of dead crew members and other related costs.
However, experts say the company, which posted a net profit
of about $5bn last year, is unlikely to be badly damaged
by the rig's loss.
If there is a significant spill from the rig or one of the wellheads below, the conservation organisation BirdLife International warns of the danger to a threatened species of local seabird, the spectacled petrel.
The bird is thought only to have a population of 2,500 to 10,000 and is found in the waters off Brazil at this time of year.
Other birds thought to be at risk include the near threatened Atlantic yellow-nosed albatross, as well as migrant birds such as shearwaters.