In 2003, 182 billion barrels of oil reserves worth about US $4.5 trillion were discovered worldwide (Johnson et al., 2004). Moreover, between 1950 and 2002 the total volume of reserves discovered has run to more than 1500 billion barrels for oil and 7.5 trillion cubic feet for gas (Bentley, 2002). More than half of these resources have already been produced and have driven the global economy for the last 50 years.

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