By Yereth Rosen

ANCHORAGE, Alaska, Jan 7 (Reuters) - Three months after bidding $13.7 million for the rights to search for oil and gas on more than 820,000 acres of federal land on Alaska's North Slope, Petro-Hunt LLC has relinquished its leases because falling oil prices made exploration there uneconomical, the U.S. Bureau of land Management said Wednesday.

BLM announced it has formally awarded 10-year leases to four other high bidders in the Sept. 24 lease sale for the National Petroleum Reserve-Alaska - Anadarko Petroleum Corp. (APC.N: Quote, Profile, Research), ConocoPhillips (COP.N: Quote, Profile, Research), Petro-Canada PCA.T and Talisman Energy Inc. TLM.T unit FEX L.P.

But the 72 tracts for which Dallas-based Petro-Hunt was the high bidder will revert back to government ownership.

"Petro-Hunt decided not to buy the leases that they bid on in our September sale," said Ted Murphy, deputy director for the BLM's Alaska state office. The company notified the BLM of its plans on Dec. 20, Murphy said.

In relinquishing its bids, Petro-Hunt forfeits a deposit of $2.7 million, the BLM said.

Prices for Alaska North Slope crude plunged from a record $144 a barrel in July to less than $30 a barrel in late December, tracking a slump in global commodity markets. "A $100 swing in six months is significant," Murphy said.

Petro-Hunt's high bids accounted for nearly half of the $30.9 million in high bids submitted at the September lease sale. It was the fifth in a series of lease sales in the area held by the BLM that started with a sale held by the Clinton administration in 1999.

The National Petroleum Reserve-Alaska sprawls over 23 million acres of tundra, lakes and rivers in the western part of the North Slope.