(Adds CFO comment, details on taxes, export sales and volumes)
By David Alire Garcia
MEXICO CITY, Feb 27 (Reuters) - Mexican state-owned oil company Pemex said on Thursday its losses for the fourth quarter more than doubled from a year earlier after it lowered the value of some assets, including its Burgos gas field.
Lower crude oil sales and volumes also contributed to the weak quarter, the company said.
The Mexican oil giant, whose 75-year monopoly ended last year with the passage of sweeping energy reforms, said it lost 76.508 billion pesos ($5.84 billion) during the October-to-December period, compared with 28.761 billion pesos a year earlier.
It reported fourth-quarter sales of 409.5 billion pesos, down 2.6 percent compared to revenues during the year-earlier period.
The reduced sales revenue was dragged down by lower market prices for Maya crude, Mexico's main export blend, as well as a drop in domestic gasoline sales.
Export sales were down 4.6 percent during the quarter, while export volumes slid 5.4 percent, the company added.
Pemex said its operating profit fell nearly a third as it lowered the value of certain assets by 25.6 billion pesos, primarily its Burgos gas field in northern Mexico, near the border with Texas.
The lower value of company assets such as the Burgos field is due to ongoing exploration activity that provides updated information.
"What happened is the expected production from (Burgos) was greatly reduced, and as a result you have to cut the value of the asset," said Mario Beauregard, Pemex's chief financial officer, in a meeting with reporters at the company's Mexico City headquarters.
While Pemex's tax burden eased by nearly 4 percent during the quarter, taxes paid by the company during 2013 reached 124 percent of pre-tax profits for the year, up from a 99 percent effective tax rate in 2012.
Mexico's federal government relies on Pemex revenues to cover about a third of total public spending.
Mexico is the world's No. 10 crude oil producer and the third-biggest exporter to the United States, but has to import nearly half of its gasoline due to a lack of domestic refining capacity.
The company said that average crude production reached 2.523 million barrels per day (bpd) for the quarter, down 1.6 percent compared to the year-earlier period.
Crude oil production has fallen by a quarter from a peak of 3.4 million bpd in 2004, while crude exports have fallen by a third over the period.
In December, President Enrique Peña Nieto signed into law an energy overhaul that aims to breathe new life into the ailing company by allowing it to enter into joint ventures with international oil majors as well as providing it with more budget and management autonomy.
The overhaul also ended Pemex's decades-long monopoly on production, refining and retail activities, and promises to boost output by luring major new streams of foreign investment into the sector via new contracting options. ($1 = 13.09 pesos at end December 2013) (Reporting by David Alire Garcia; editing by Sophie Hares and G Crosse)