Royal Dutch Shell PLC (RDSB) said Thursday that maintaining energy efficiency in its oil and gas production will prove a challenge in coming years.

"We expect that maintaining the energy efficiency levels of recent years will be more difficult in the future as existing fields age and production comes from more energy-intensive sources," Shell said in its 2012 sustainability report.

The Anglo-Dutch oil major said its energy efficiency record for oil and gas production, excluding oil sands and gas to liquids, worsened last year compared to the year before.

According to Shell's data, it used 0.78 gigajoules of energy per ton of production last year, up from 0.75 gigaoules the year before, a level of energy intensity last seen in 2008.

Energy efficiency at the company's chemical plants also worsened last year compared to 2011, although the performance of its refineries improved.

Despite the worsening record in terms of efficiency, the company said its greenhouse gas emissions fell to 72 million tons on a carbon dioxide equivalent basis from 74 million tons in 2011, largely due to reduced gas flaring in Nigeria and divestments in the company's downstream business.

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