* Long Lake output may not meet forecasts
* Wells have encountered "lean zones" in reservoir
* Partner Opti meeting commitments
* Nexen shares drop as much as 3.5 pct
* Opti declines to discuss finances (Adds performance of Opti bonds; updates shares. In U.S. dollars unless noted.)
By Scott Haggett and Jeffrey Jones
CALGARY, Alberta, Feb 10 (Reuters) - Nexen Inc NXY.TO, the operator of the Long Lake oil sands project, warned that output may fall short of forecasts because of operational problems that analysts say may take heavy spending to fix.
Nexen shares dropped more 3 percent on Thursday when it said January output fell at the underperforming oil sands project in the Canadian province of Alberta.
It said production at some wells isn't rising no matter how much steam is pumped into the reservoir to liquefy the tarry bitumen that's mixed with sand, a process that brings it to the surface. Nexen blamed "lean zones"in the reservoir.
"We have not seen bitumen production increase during December and January," Marvin Romanow, Nexen's chief executive, said on a conference call on Thursday.
On Wednesday Nexen said its fourth-quarter output rose 10 percent from the third quarter to 28,100 barrels per day. In January it fell to 27,000 bpd.
Production at the C$6.1 billion ($6.16 billion) steam-driven project and upgrader began more than two years ago, but output has yet to reach half of the 72,000-barrel-per-day capacity.
That has depressed Nexen's shares and forced its partner Opti Canada Inc OPC.TO to search for a buyer as its debts weigh on its stock. Opti has 35 percent of the project.
Nexen also had issues with some pumps and excess water flowing to the surface in December and January.
But analysts said the most crucial issue remains the quality of the project's reservoir, which requires more expensive steam to be pumped into the ground than rival projects.
"We believe the issue at Long Lake remains reservoir quality," George Toriola, an analyst with UBS Canada, wrote in a research note. "In our opinion, this means that well productivity would be lower than expected, while requiring a higher (steam to oil ratio) than initially envisaged. This implies significant future capital requirements to monetize this asset."
Romanow said he expects the wells that have encountered the lean zones to produce as much bitumen as expected eventually.
But he said the issue may mean Long Lake output will not match the promised range of between 38,000 and 45,000 barrels per day of bitumen this year, well below the 72,000 bpd it is designed to produce.
Romanow added Opti Canada has been paying its share of Long Lake's costs even though its shares have plunged as investors fret that the company is carrying to much debt.
Opti has hired Lazard Freres & Co LLC as an adviser to pursue a debt restructuring as efforts to find a buyer or some other deal as its efforts have so far come up dry.
The company acknowledged on Thursday that its time to keep looking for a deal is limited.
Opti's shares have been battered in recent months as rating agencies downgraded its large debt load over concern it may not be able to meet its financial obligations through the year due to Long Lake production levels lagging expectations.
At least one analyst cut his price target on the shares to zero while Opti's bonds <0#USOPCRRPS> have also plunged.
Standard & Poor's said in a note on Thursday that Opti's second-lien bonds dropped more that 5 points in heavy trading.
Opti's two-year second-lien note due in December 2014 and carrying a 7.875 percent coupon were trading for $46.25 on Thursday, while the 8.25 percent note due in December 2014 was at $46.75.
In a conference call to discuss its fourth-quarter results, Opti Chief Executive Chris Slubicki declined to answer questions about the company's finances, steering all discourse to Long Lake's operational issues.
"Our objective to successfully carry out a strategic transaction remains unchanged," he said in his opening remarks. "Working through our plant and reservoir issues is imperative to succeeding and finding a strategic transaction that will address our balance sheet and recognize value for the company."
In the fourth quarter, Opti had a net loss of C$26 million, or 9 Canadian cents a share, compared with a year-earlier loss of C$212 million, or 75 Canadian cents. That included an C$81 million unrealized foreign exchange gain in the quarter.
Nexen shares fell 55 Canadian cents to C$23.30 by early afternoon the Toronto Stock Exchange after earlier touching C$23.02.
Opti, whose shares fallen by more than half so far this year, rose half a Canadian cent to 30 Canadian cents.
$1=$0.99 Canadian Editing by Frank McGurty