Chevron, ExxonMobil and Shell are facing a showdown with the Tax Office over tax-free profits of up to $3 billion a year they are making from the huge Gorgon project, even before they have shipped the first gas.

The key to understanding this ongoing battle over the $US54 billion ($73 billion) project lies in its long and colourful history. Its origins date back 14 years, when Chevron Corporation announced a takeover of rival Texaco.