A newly compiled list of the world's largest energy companies indicates that investors are buying into the growth potential of nationally run oil companies at the expense of independent integrated players who have long been the gems of the sector.

The PFC Energy 50 lists the world's 50 largest energy companies based on their market capitalization on their home exchange. It's tallied annually by the private energy consultancy bearing the same name, and it's used to analyze major trends in the energy space.

According to the latest version, PetroChina ( PTR), a major integrated nationally run oil company in China, grew 181% in 2007, overtaking Exxon Mobil ( XOM) to become the world's largest energy firm. PetroChina had previously held the third spot on the PFC 50.

Based on its float on the Shanghai exchange, PetroChina's market cap was $723 billion at the end of 2007, compared with Exxon's value of $512 billion.

Though Exxon fell to the second spot, it remains the major with the largest oil reserves. However, reserves aren't considered in the creation of the PFC 50 list.

The number of independent integrated oil companies in the list of the 10 supermajors fell from eight to five. Chevron ( CVX) dropped to the 10th spot from seventh in 2006. Italy's Eni ( E) fell to No. 11 from eighth. ConocoPhillips ( COP) slipped to 12th place from No. 9.

Sinopec ( SHI), another Chinese national concern, rose to the fifth spot on the list from 12th with an annual growth rate of 157%. Petrobras ( PBR), the national oil company of Brazil, rose to No. 6 from 11th with a 93% growth rate last year.

National oil companies saw their share prices grow at an average rate of 73% in 2007, compared with 20% for private majors and 42% for independent exploration and production firms.

The report comes at a time when energy analysts have been fretting that major independent integrateds like Exxon are losing out to national operators like PetroChina and Russia's Gazprom in the race to secure reserves around the globe. The results of the latest PFC 50 reaffirms the hypothesis that investors are putting their money behind this trend.

One reason for the advance of nationals over private integrateds is the soaring global demand for energy in all its forms. Growing domestic demand in traditional oil-exporting countries like Iran and Venezuela is encouraging these countries to take more control of their natural resources and restrict access for outsiders.

The sudden climb in crude oil prices from $50 a barrel in early 2007 to $100 a barrel in January 2008 is another factor. The tremendous inflows of cash that the energy business is bringing to exporters like Russia is encouraging such nations to tighten their grip on their holdings.

This newfound wealth is also allowing them to invest in advanced technology for finding and developing reserves, leaving them with less need to hire outside experts like Halliburton ( HAL) or Schlumberger ( SLB).

Geopolitical concerns are also weighing heavily on integrated names. Political differences between the U.S. and Venezuelan president Hugo Chavez likely led to Venezuela's 2007 expropriation of assets in the Orinoco River basin that were owned by Exxon and Conoco. And China's tremendous thirst for energy has encouraged Chinese nationals to enter politically sensitive areas, such as Sudan, where most integrateds either aren't allowed or aren't inclined to go.

BP's ( BP) recent decision to tap the Canadian tar sands, an expensive and environmentally unfriendly space, was considered by many analysts to be an effort to shore up reserves in a friendly part of the world after a long and tumultuous year for independent integrateds.

According to Charlie Maxwell, senior energy analyst at Weeden & Company, BP had long said that it wasn't interested in participating in the Canadian tar sands. However, if nationals continue to be favored over integrateds in major drilling projects around the globe, companies like BP will have to take whatever they can get.

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