Strength in refining and chemicals led to higher-than-expected fourth-quarter earnings for Exxon Mobil and Chevron, the two largest U.S. oil companies.

A flood of oil produced from U.S. shale formations has pushed refining margins higher for many companies with plants in the United States, while chemical companies are benefiting from the low price of natural gas, a key feedstock.

Brian Youngberg, energy company analyst with Edward Jones in St. Louis, said bigger-than-expected refining earnings "were a common theme for this quarter" for Exxon and Chevron.

Exxon's overall profit was $9.95 billion, or $2.20 per share, compared with $9.4 billion, or $1.97 per share, in the same period a year earlier, while its refining arm's earnings quadrupled to $1.77 billion.

"As we look at just our U.S. Gulf coast refining circuit, we have more than tripled the processing of advantaged North American crude over the last couple of years," David Rosenthal, Exxon's investor relations executive, told analysts.

Chevron's net income grew to $7.2 billion, or $3.70 per share, from $5.1 billion, or $2.58 per share, a year earlier - though the latest profit included a $1.4 billion one-time gain.

Chevron's refining operations made a profit of $925 million, compared with a loss of $61 million a year before.

Increasing output from the wellhead, on the other hand, has been a struggle in the past year for big oil companies.

Exxon's oil and gas output fell 5% to 4.29 million barrels oil equivalent per day (bpd), while Chevron managed an increase to 2.67 million bpd after a year of underperformance.

Ernie Cecilia, chief investment officer at Bryn Mawr Trust and a Chevron shareholder, noted PhillipsConoco was struggling while its spun-off refining arm, Phillips 66, did well in what remains a highly cyclical business.

"The refining side can be a two-edged sword, being a tailwind as well as a headwind," Cecilia said.