HOUSTON (Bloomberg) — Marathon Oil Corp., the fourth- largest US energy company, said appraisal drilling on its Droshky discovery in the Gulf of Mexico indicates a recoverable deposit equivalent to 80 million to 90 million barrels of crude.Production from the prospect could begin as early as 2010, depending on the availability of rigs and other equipment, Houston-based Marathon said yesterday in a statement. The site is in 2,900 feet of water, 137 miles south-southwest of Venice, Louisiana. The well was drilled to a depth of 21,190 feet, the company said.

Major oil producers struggling to increase output and reserves from mature fields in North America and Europe are looking to deepwater projects in the Gulf of Mexico and off the coast of West Africa. If Droshky proves to hold 90 million barrels of oil, it would be equivalent to more than 7 percent of Marathon's reserves as of the end of 2006.

"Ultimately, what's proved up and put in as proved reserves remains to be seen," Marathon spokesman Paul Weeditz said in a telephone interview. "We're certainly not at a point to estimate what a proved reserve would be."

Marathon's Alvheim/Vilje project in the North Sea, expected to play an important role in the company's growth, is estimated to hold resources equivalent to up to 250 million barrels of oil. In July, Marathon estimated recoverable resources from its Piceance acquisition in Colorado at 900 billion cubic feet of natural gas, or the equivalent of 150 million barrels of oil.

Marathon reported the discovery in April, after its initial well encountering 250 feet of oil-bearing rock. Two subsequent wells encountered 600 feet and 300 feet of so-called net oil pay, the company said. Marathon owns 100 percent of the deposit.

The project will be linked by pipeline to the Troika underwater development system, then to Royal Dutch Shell Plc's Bullwinkle platform 14 miles away, where oil is processed, Marathon said.

Shares of Marathon rose $3.21, or 2.7 percent, to $122.10 in New York Stock Exchange composite trading. The stock has climbed 32 percent this year.

Exxon Mobil Corp., Chevron Corp. and ConocoPhillips are the three largest US energy companies.