Chevron estimates the size of an oil sheen drifting from the Brazilian coast represents about 18 barrels of oil, down from the 650 barrel-estimate last week.
The company started cementing operations to seal the appraisal well, which is near the Frade field in about 3,800 feet of water, to contain the spill.
Chevron referred to estimates from Brazilian counterparts, who put the size of the spill at 5,000-8,000 barrels of oil.
Brazilian authorities last week said they were frustrated with varying claims by the supermajor.
"Chevron takes full responsibility for this incident," Chevron Brazil country manager George Buck said in a statement. "We are committed to deploying resources until the sheen can no longer be detected."
Buck added that an investigation was under way to find out what happened at the Frade appraisal well. Findings, he said, would be shared with the Brazilian government.
"We believe no new oil is seeping from the reservoir," he said.
More than a dozen vessels were deployed to clear sheen from the water's surface. Chevron said it wasn't using chemical dispersants during the cleanup.