UPDATE 3-Nexen profit jumps but reserve revision weighs

(Adds executive, analyst comments, details; changes dateline from Toronto)

By Jeffrey Jones

CALGARY, Alberta, Feb 14 (Reuters) - Fourth-quarter profit at Nexen Inc (NXY.TO: Quote, Profile, Research) more than doubled on big jumps in oil output and prices, although a writedown of Gulf of Mexico reserves weighed on results, Canada's No. 4 independent oil explorer said on Thursday.

Nexen said it was disappointed by development drilling at the Aspen field in the U.S. Gulf, and that its accounting method had forced it to reduce its tally of proved reserves there by 13 million barrels of oil equivalent.

The company has not given up on the play, however, with expected returns on its capital of more than 30 percent.

It is getting a big boost from its 43 percent owned Buzzard oil field in the North Sea and months from starting up the C$6.1 billion ($6.1 billion) Long Lake oil sands project in northern Alberta, in which it has a 50 percent stake.

Nexen Chief Executive Charlie Fischer said the potential of the 200,000-barrel-a-day Buzzard field, which began producing last year, is expanding.

"The reservoir continues to be larger than what we had originally anticipated and drilling results are confirming that, and productivity is causing us to be confident about improved recovery factors," Fischer told analysts.

The production platform offers little opportunity for boosting output, but there many be opportunities for Nexen to take additional oil from Buzzard at its nearby Ettrick field, expected to start pumping crude later this year, he said.