A mistake in calculating the oil pressure caused a spill off the coast of Rio de Janeiro state, Chevron said Saturday, amid uncertainties over the extent of the slick.

"The pressure of the deposits was underestimated," Chevron Brazil president George Buck told local media.

Based on the incorrect calculation, the company used a type of material that lacked the sufficient weight needed to contain the oil, which then leaked and reached the surface, Folha de Sao Paulo newspaper explained.

Buck insisted the leak was fully contained on November 13, but noted that the incident was still being investigated.

According to Chevron, the slick reached a volume of 882 barrels of oil at its worst, on Monday, and dropped to 18 barrels on Friday.

The well, which Chevron said began leaking on November 8, is near the Frade field located some 370 kilometers (230 miles) northeast of Rio de Janeiro, in an area that is a migratory route for whales and dolphins.

The National Petroleum Agency said Friday that the oil slick "continues to move away from shore" and estimated that it was now 18 kilometers (11 miles) long and 11.8 square kilometers (4.5 square miles), down from 163 square kilometers (63 square miles) on Tuesday.

It estimated that between 200 and 330 barrels of oil have seeped into the sea since November 8. The energy ministry, for its part, said 220 to 230 barrels of oil were seeping into the ocean daily.

Those estimates were contested by Greenpeace, which said satellite pictures showed a spill "10 times bigger," and likely reached closer to 3,700 barrels a day.