This includes the resurgence of U.S. liquid production in recent years (5.5 million barrels of oil per day and trending upward), as well as conventional gas production's six-fold increase over the last two decades (to approximately 32 billion standard cubic feet of gas per day in 2010, nearly equaling U.S. liquid production).

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The increase in cash cost per ton was due largely to fixed costs being spread over lower than expected production, as well as higher sales related costs related to higher average sales prices.

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However, it says while average emissions per oil barrel are declining, since the demand for oil production has increased, overall emissions have increased as well.

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