(Reuters) - Weaker oil prices ate into Chevron Corp's first-quarter profits, while the second-largest U.S. oil company also took a hit from downtime at two of its three biggest U.S. refineries.

First-quarter net income fell 4.5 percent to $6.18 billion, or $3.18 per share, from $6.47 billion, or $3.27 per share, a year earlier. Analysts had expected $3.08 per share, according to the average on Thomson Reuters I/B/E/S, and shares of Chevron rose by 0.4 percent in premarket trading.

Fourth-quarter production was 2.65 million barrels of oil equivalent per day, up from 2.63 million a year earlier, though down from a particularly strong fourth quarter's 2.67 million.

Increasing output from oil wells has been a struggle for Chevron and larger rival Exxon Mobil Corp , which posted a drop in first-quarter production on Thursday.

Chevron's earnings from the production of oil and gas, or upstream earnings, fell by about a quarter in the United States to $1.13 billion, with operating expenses higher and the average sales price for liquids down to $94 per barrel from $102 a year before.

U.S. downstream earnings, from refining and marketing, declined more than 70 percent, with refinery crude oil input falling by 350,000 barrels per day to 576,000 bpd due to planned work at its largest refinery in Mississippi and an extended crude unit outage at its oldest refinery in Richmond, California.

Chevron is still trying to get Richmond working at normal levels after a fire last August damaged its crude unit, which is expected to start up this month.

Chevron shares rose 0.4 percent to $118.99 in premarket trading on Friday. The stock is up about 10 percent so far this year, compared with a 2 percent rise for Exxon.

(Reporting by Braden Reddall in San Francisco; Editing by Gerald E. McCormick)

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