Gazprom is one of the world's largest companies. By market value it's now worth over two-hundred billion dollars. That puts it in the top ten of global firms, just behind energy giants like Royal Dutch Shell but ahead of the huge US retailer Wal-Mart. And with good reason - Gazprom, in effect the successor to the Soviet Union's giant gas ministry, controls a fifth of the world's reserves of gas plus many of the pipelines that supply it to Europe.