* Adj EPS C$1.21 vs est C$1.27
* Cash flow slightly lower at C$2.77/shr
* Oil production rises 23 pct to 406,266 bpd (Adds details, background)
CALGARY, Alberta, May 6 (Reuters) - First-quarter profit at Canadian Natural Resources Ltd (CNQ.TO), the country's biggest independent oil explorer, more than doubled on higher oil production and stronger prices, the company said on Thursday.
Canadian Natural, which operates the new Horizon oil sands project and produces oil and gas in Canada, the North Sea and elsewhere, benefited from an 86 percent rise in oil prices, to $78.37 a barrel and strong heavy oil prices.
But natural gas prices remained weak and a strengthening Canadian dollar also cut into returns.
"Production in North America and in our international operations are doing as or better than expected," Allan Markin, Canadian Natural's chairman, said in a statement.
"Ramp-up at Horizon continues, with production at the high end of expectations as our team continues to work towards production at levels near targeted capacity of 110,000 barrels per day."
Net income rose to C$866 million ($825 million), or C$1.60 a share, from C$305 million, or 56 Canadian cents, in the fourth quarter of 2009.
Adjusted profit, which removes most one-time items, dropped 9.5 percent to C$658 million, or C$1.21 a share, from C$727 million, or C$1.34, in the year-prior quarter.
The adjusted result lagged the average analyst forecast for the measure of C$1.27 per share.
The company's cash flow, a measure of its ability to fund new projects, fell less than 1 percent to C$1.51 billion, or C$2.77 per share, from C$1.52 billion, or C$2.80.
The company's oil production rose 23 percent to 406,266 barrels per day, including 86,995 bpd from the Horizon project, up from just 3,384 bpd in the first quarter of last year, when the plant was starting up.
Natural gas output fell 10 percent to 1.23 billion cubic feet a day.
The company expects average oil production of 405,000 to 450,000 bpd this year. Gas output for 2010 is forecast at between 1.2 bcf and 1.27 bcf per day.
Canadian Natural's revenue rose 63 percent to C$3.58 billion.
The company's shares fell C$1.13 to C$72.70 on Thursday on the Toronto Stock Exchange. The earnings were released after markets closed.
$1=$1.05 Canadian Reporting by Scott Haggett; editing by Rob Wilson