Petroleum Intelligence Weekly's annual ranking of the world's 50 largest oil companies, a perennial benchmark survey recognized industry-wide, is the leading source of comparative performance assessments on all the world's oil companies. The Rankings are based on six operational criteria that allow the comparison of private sector and state-owned oil companies and is the precursor to the more comprehensive Energy Intelligence Top 100: Ranking The World's Oil Companies.

Saudi Aramco and Exxon Mobil remain entrenched at the top, while mergers, acquisitions and state consolidation continue to make their mark. With high oil prices continuing to fuel resource nationalism among the traditional 100% state-owned national oil companies, the most impressive gains came from hybrid, partly state-owned firms from emerging economies including Russia, China and India. Half of the 20 firms advancing in this year's Top 50 are state-controlled, evidence of their continued and increasing dominance.

Here are five key findings from the PIW Top 50:

• Saudi Aramco, perennial holder of the Rankings' top spot, continues to solidify its supremacy with significant investment in both upstream and downstream oil and gas operations.
• Russian gas giant Gazprom, aided by its acquisition of Sibneft, continues to forge ahead, gaining nine places to reach number 15 on its march toward the top ten. Similarly, new assets helped Rosneft advance five spots to number 26.
• Occidental returns to the Top 50 with help from its purchase of Vintage Petroleum, while Canadian Natural Resources makes its Top 50 debut after spending the previous four years waiting in the wings.
• PetroChina moved deeper into the top ten to reach 7th place. In the process, it overtook both Chevron and Total, which tied for 8th.
• The US again dominates this year's rankings with ten firms in the Top 50, followed by Russia with six and the UK and Canada with three each.

The PIW Top 50 can only give you a glimpse of the performance of the world's key energy companies. You can find out exactly how your company, client or competitor ranks against industry peers with The Energy Intelligence Top 100: Ranking The World's Oil Companies.

This annual special report from Energy Intelligence Research provides a detailed picture of how the leading energy companies -- both publicly traded and state-owned firms -- have performed. With coverage of more than 160 operational and performance criteria for the 100 largest oil companies in the world, it allows you to assess operating strengths and weaknesses, set goals based on peer performance and identify your best-in-class competitors.

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PIW's Top 50: How The Firms Stack Up

Methodology of PIW Ranks The World's Top Oil Companies

PIW's ranking of the world's 50 largest oil companies is based on operational data from over 130 firms. The focus on operations allows meaningful comparisons of all types of companies -- including state-owned firms -- and thus differs from more financially oriented corporate rankings. PIW's unique system uses as criteria oil reserves and production, natural gas reserves and output, refinery capacity, and product sales volumes.

Firms are compared in six different operational areas in the table below, with companies assigned a separate rank within each category. The six individual ranks are then added together to determine the cumulative, overall position, giving each of the six criteria an equal weighting.

The rankings above are based on the 2005 operational results for the companies as they existed at the end of that year or as they reported them. Estimates are used mainly for state-owned oil companies that do not release regular or complete annual reports in a timely fashion. Some numbers reflect estimates when complete corporate data are not available. PIW's system of ranking tends to favor national oil companies with large oil and gas reserves, and to favor integrated concerns over firms that specialize in one industry sector.

To round out the picture, basic financial data are provided for the companies. But these data are not used to create the PIW top 50 rankings, since they are based on widely differing accounting practices.

Data in the tables below are primarily from company sources and annual reports. In some cases, secondary sources or PIW estimates have been used to fill in the gaps. Data are usually shown as reported by the companies but are sometimes adjusted for interests or shares held by others. Data for the Russian and Caspian firms are drawn from Energy Intelligence Research's 2006, Almanac of Russian & Caspian Petroleum.