China's big three oil companies have all reported a decline in earnings in 2012, blaming government price controls and rising taxes.

But they still raked in an average of 660 million yuan ($106 million) a day in profit in the past year, showing the strong profitability of the largely government-regulated sector.

China Petroleum and Chemical Corp, CNOOC Ltd and PetroChina, the listed arm of China National Petroleum Corp, earned a total of 242.5 billion yuan, down more than 10 percent year-on-year. But the figure is still more than double that of Warren Buffett's Berkshire Hathaway.