State-owned oil companies are a rising force in the global energy sector as private rivals such as BP and Total seek aMore than half of the world’s top energy firms, 28 out of 50, are fully state-owned, and these national oil companies control almost 80 percent of crude reserves, according to industry publication Petroleum Economist

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Heading the biggest companies, as measured by reserves, are state firms from Iran, Saudi Arabia and Venezuela. ExxonMobil, the first entirely-private international oil company, or IOC, in the top 50, comes in at 11th position.

International oil companies such as British energy giant BP, which is the 15th-biggest company in terms of oil reserves, are relying on closer links with national oil companies, or NOCs, to gain access to huge oil reserves in Gulf countries and Russia.

BP chief executive Bob Dudley recently described Russia as a favorable environment for foreign investment, despite his group’s recently collapsed deal with Russian state oil giant Rosneft over exploration in the Arctic.

However, Dudley also acknowledged that “like trees, partnerships (between NOCs and IOCs) take time to bear fruits.”

The chairman of state-owned Kuwait Oil Company, Sami al-Rushaid, recently acknowledged that NOC-IOC tie-ups, were a delicate balancing act. But he also stressed that such partnerships were essential if NOCs and IOCs were to keep down their costs.

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It’s increasingly difficult and costly to develop oil and gas production,” said al-Rushaid, whose company has struck an agreement with Royal Dutch Shell to enhance technology used to secure oil from reservoirs.

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Our ageing reservoirs need use of customized enhanced oil recovery technologies to extend their like and keep production levels,” al-Rushaid recently said in London. “Partnerships are critical to sustain technological innovation. It’s vital to have a joint effort on technologies. We are proud of what we do now with Shell on the reservoirs technologies.”

However, relations between NOCs and IOCs are not always so smooth. The head of Venezuela’s state oil giant Petroleos de Venezuela said last month that the “imperialist” United States could “go to hell” after it slapped sanctions on the company over alleged ties to Iran.

NOCs are increasingly looking to extend beyond their own borders in a bid to compete with their IOC rivals. China National Petroleum Corp., or CNPC, last week announced that it had begun operations in Iraq’s Al-Ahdab oil field.

The project marks “the realization of the Chinese oil industry’s goal to develop the high-end oil market in the Middle East,” CNPC said.

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NOCs… are increasingly active in cross-border partnerships or other strategic deals,” said UBS analyst Philip Wolfe.

Boris Zilbermints, a senior executive at Gazprom, said his company sees itself as an “international NOC.” Speaking recently in London, he said: “We want to explore new areas. For us, the main issue with international majors is that everyone wants to go to Russia, no-one is willing to give us access to their own [energy resources].”

IOCs are also eyeing opportunities in Libya and the Middle East. The head of strategy at France’s Total, Olivier Cleret, believes that the ongoing unrest should not hamper efforts to forge ties with NOCs in these regions. “It may even be the opposite because the uprisings present new needs,” Cleret said.