Exxon has 19,400 acres in the Marcellus shale

HOUSTON (Reuters) - Exxon Mobil Corp (XOM.N), the world's largest publicly traded company, has leases on 19,400 acres in the Marcellus Shale, a formation that is said to hold vast amounts of natural gas.

In September, Exxon bid $85.2 million for 18 blocks in the Marcellus, a large shale formation that runs through parts of New York, Pennsylvania, Ohio and West Virginia.

Exxon, based in Irving, Texas, was the high bidder on six Marcellus blocks, paying a total of $22.4 million for acreage in Tioga and Lycoming counties in Pennsylvania, company spokesman Patrick McGinn, said.

Until recent technological advances, the natural gas trapped in shale was considered too expensive or difficult to retrieve.

But now, processes like hydraulic fracturing are used. In a "frac job," propellants like sand grains and liquids are pumped into a well under pressure high enough to crack the shale and allow the release of oil and gas.

McGinn would not comment on Exxon's plans in the Marcellus.

European oil majors StatoilHydro (STL.OL) and BP Plc (BP.L) have struck billion-dollar shale asset deals in North America.

Elsewhere in North America, Exxon Mobil has acquired 152,000 net acres since 2007 in the Horn River Basin in northeast British Columbia. Exploration drilling activity to evaluate the Horn River shale gas play began in late 2008, the company said in its annual report.

(Reporting by Anna Driver in Houston)