BP shares , which had dropped more than 6 percent after engineers detected seepage on the floor of the Gulf, recovered slightly in late New York trade to close down 3.6 percent, at $35.75 a share.
BP spokesman Mark Proegler told Reuters: "Scientists have concluded that the seep was naturally occurring."
Investors had feared that seepage could signal that the April 20 blowout that preceded the leak damaged the wellbore, which could allow oil and gas to leak out the sides and possibly breach the seabed.
Officials are monitoring the pressure in the well to gauge whether it is structurally sound. An intact well would help when a relief well intercepts and tries to plug the leak, but damage could complicate that effort.
White House energy adviser Carol Browner told CBS's "The Early Show" the seepage was found less than two miles (three km) from the well.
The worst oil spill in U.S. history has caused an economic and environmental disaster in five states along the Gulf Coast, hurt President Barack Obama's approval ratings and complicated traditionally close ties with Britain.
Also Monday, BP announced that it had spent $3.95 billion so far on efforts to tackle the well and cleanup the millions of barrels of spilled oil.
The explosion that led to the spill and a three-month long effort to plug the leak and clean up the oil has impacted finances of BP, which has begun canvassing shareholders about a restructuring that could include a breakup of its businesses, the Sunday Times reported.
BP's talks to sell half its stake in Alaska's Prudhoe Bay oil field to Apache Corp, which had stalled over the weekend, were back on, CNBC reported.