The biggest publicly held oil company, ExxonMobil (NYSE: XOM), has followed ConocoPhillips (NYSE: COP) and Chevron (NYSE: CVX) in reporting out-of-sight earnings. But it seems all the members of Big Oil are changing at a rate unrecognized by even longtime energy investors.

For the quarter reported Monday, Exxon earned $9.25 billion, or $1.85 per share, compared with $6.05 billion, or $1.27 per share, in the fourth quarter of 2009. Analysts had a consensus expectation of $1.67 per share. Revenue increased 17% to $105.2 billion, versus $89.8 billion a year ago.

There were a number of factors behind the improved results, including increased production, which, on an oil-equivalent basis, was up 19% year-on-year. Partially as a result, the company's upstream earnings grew about 29%, to $7.5 billion. Liquids production was up nearly 6% in the quarter, while natural gas output was up 37%, based on the boost to U.S. unconventional gas output that resulted from the acquisition of XTO Energy, along with increased production in Qatar.

Downstream earnings jumped to $1.15 billion, compared with $189 million in the year-earlier quarter. At the same time, a substantially lower tax rate appeared to benefit the results. At just below 38%, the rate for the most recent quarter was nearly two percentage points below the year-ago rate and even further below analysts' expectations.

But of even more importance in looking at Exxon's quarter and full year was the company's transition through acquisitions to its new status as the largest natural gas producer in the U.S. In June, the company completed the $41 billion purchase of XTO, with its 45 trillion cubic feet of gas. Then, effective Oct. 1, Exxon agreed to pay $575 million for Petrohawk Energy's (NYSE: HK) natural gas wells in the Fayetteville shale, along with another $75 million for pipelines in the same play.

Just last week, Exxon released its latest version of "Outlook for Energy: A View to 2030," in which the company posited a shift toward natural gas. Given both environmental and geopolitical considerations (Can you say Egypt?), it's difficult to disagree with that belief, which obviously was the impetus behind the natural gas acquisitions.

There are other significant events occurring at Exxon. For instance, the company has announced an agreement with Russia's Rosneft to jointly develop oil and gas resources in the Black Sea. Rosneft is also involved in a new partnership with BP (NYSE: BP) for exploration in the Russian arctic. However, the British company is experiencing difficulty with its partners in its Russian joint venture TNK-BP over working with a Russian competitor. It is highly unlikely that Exxon will suffer a similar fate.

There are numerous other points to be made about Exxon, but I think you get the point: The biggest of the Big Oil companies is faring extremely well at a time when it is undergoing major change.    

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We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. Fool contributor David Lee Smith doesn't own shares in any of the companies named above. The Motley Fool has a disclosure policy.