NEW YORK (CNNMoney.com) -- Record oil prices in the third quarter won't necessarily translate into record profits at the nation's biggest oil companies.

Crude prices hit record highs in the quarter but then sank about 20 percent from their highs. Still, prices were up about 12 percent for the quarter, on average, from a year earlier.

Corrosion in pipelines, pictured above, forced BP to shut its big Alaska oil field in August. The closure is impacting profits at not only BP, but partners Exxon and Conoco as well.

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But much of that increase was offset by higher production costs, sharply lower natural gas prices and a series of charges - leading to mixed results for the country's biggest producers.

ConocoPhillips (Charts) was the first of the domestic majors to report Wednesday. The company's overall earnings were slightly higher - $3.88 billion versus $3.80 billion the same time last year - and Conoco shares rose about 2 percent.

But revenues were slightly lower - $48.4 billion this year versus $48.7 billion last year.

Earnings per share were also lower, although that was expected.

Conoco bought Burlington Resources last March, part of which it had to pay for by issuing more shares.

The company cited a disruption in oil production in Alaska as one reason for the lower revenue.

Conoco, along with Exxon, was a partner in BP's Alaska oil field in Prudhoe Bay. BP was forced to shut in 400,000 barrels of production at the site in August for several weeks due to maintenance problems.

ExxonMobil (Charts), the world's biggest corporation by revenue and the record holder for the most profitable quarter in history - it earned $10.7 billion in last year's fourth quarter - is set to report Thursday.

Analysts are expecting the company to post higher profits but fall well short of setting any new milestones.

Analysts expect Exxon earned $9.7 billion in the third quarter, or $1.59 a share, up about 20 percent from a year earlier, according to First Call.

Higher oil prices and production are expected to have boosted results in the quarter.

And the per share earnings are still expected to be near record levels thanks to a share buyback program. Exxon has plowed $11 billion into repurchases just this year, according to Oppenheimer oil analyst Fadel Gheit.

Yet Exxon, like its competitors, will probably take a hit from natural gas prices, which have fallen 28 percent from a year earlier as production returned to the Gulf of Mexico after the devastating hurricanes of 2005.

Gheit said natural gas typically makes up one third of the revenue at the biggest oil companies.

Also weighing on results: a profit tax.

Although oil companies have so far been able to avoid such a tax in the U.S., they've not been so lucky in Britain.

The Brits instituted a "supplemental tax" on oil and gas production in the North Sea, payment due this quarter.

The tax was one reason, along with production problems in Alaska, BP (Charts) cited for a 3.6 percent fall in net profit when the British oil producer reported Tuesday. Conoco also cited the tax in its earnings report.

With oil prices still near $60 a barrel, three times higher than they were in 2002, one analyst said investors will be looking at production levels at all the companies.

"We like to see a continuation of volume growth year over year," said Bruce Lanni, an oil analyst at A.G. Edwards. "That will be one of the key measures."

Chevron (Charts), due to report Friday, is expected to post earnings of $4.4 billion, or $2.03 a share. If it meets estimates, it will be a record profit for Chevron.

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