China's energy consumption will peak by 2035, and its oil demand growth will slow to 2% per year by then from about 4.3% currently, state-owned flagship company China National Petroleum Corp. (CNPC) said in a rare long-term energy outlook.

The research paper said China's total energy consumption will reach its ceiling at 3.75 billion tonnes of oil equivalent by around 2035, compared with estimates of about 3.04 billion tonnes of oil equivalent for 2016. That marks a roughly 23.3% increase in overall national energy consumption over the next two decades.

The world's second-largest economy set its first cap on energy consumption during this year's annual parliament meetings as demand slowed and the country sought to increase its energy efficiency.

In March, the state planner said China will consume no more than 5 billion tonnes of standard coal equivalent (3.49 billion tonnes of oil equivalent) by 2020.

CNPC also forecast that natural gas will play a bigger role in China's energy consumption mix over the next three decades, echoing the government's ambitions to increase the use of gas in power generation and to reduce its reliance on coal.

Natural gas consumption will triple to 510 billion cubic meters (Bcm) by 2030, and will rise to 710 Bcm by 2050, CNPC said.

By contrast, oil consumption growth will ease to 2% annually, peaking at 670 million tonnes per year by 2027, or about 13.5 million barrels per day, according to the research paper.

"Natural gas will be the only fossil fuel resource that keeps growing by 2030," said Du Wei, the vice director of CNPC's economic and technology research institute.

CNPC said investment in the oil industry slowed during the first five months of this year, while losses related to exploration and refining increased by 2 billion yuan (US$299.22 million) in May.

CNPC added that oil output will grow 0.5% annually to 230 million tonnes per year by 2030, compared with 1.5% growth in 2015.

Coal will remain the largest source of energy, though its share in the energy mix will decline to 37% of the total by 2050 from 66% in 2014, CNPC said. (US$1 = 6.6840 Chinese yuan renminbi)