The United States is located on a relatively mature oil field.  
Therefore the yield per oil well is quite low.  Even the oil
fields of alaska are not all that rich.   As a result, that
resource, which was tapped in the early 70's had only about
a 30 year lifetime.  In contrast, Saudi Arabia sits on a very
young, rich deposit of oil.  Their production, which is about
equal to that of the US, is based on only 860 wells!  Hence,
each well is worth several billion dollars.