UPDATE 1-Williams adds Piceance acres in $258 mln deal

* May hold 795 Bcfe of reserves

* Shares up 1.7 pct (Adds details, share price)

HOUSTON Aug 10 (Reuters) - Williams Cos Inc (WMB.N), a U.S. pipeline operator and natural gas company, on Monday said it agreed to pay $258 million for natural gas assets in the Piceance Basin that may hold 795 billion cubic feet equivalent of net reserves.

Williams, based in Tulsa, Oklahoma, already holds about 190,000 acres in the Piceance Basin in western Colorado. The new acreage, located east of the company's existing assets, holds 150 Bcfe of proved reserves.

The purchase, expected to close near the end of the third quarter, covers 21,800 net acres and includes 28 wells currently producing 24 million cubic feet equivalent per day.

Williams plans to fund the $258 million deal, along with $15 million in projected 2009 development costs and $50 million of the 2010 development costs, with cash on hand, it said.

Shares of Williams rose 28 cents, or 1.65 percent, to $17.21 in morning trading on the New York Stock Exchange. (Reporting by Anna Driver, editing by Gerald E. McCormick)