Possible Clues:

• Shaft found under a nodding donkey
• Keep it properly lubricated - that's the drill
• Money comes gushing out of this, so keep it running smoothly
• 'There Will Be Blood' site
• Said to be whole basis of Texan prosperity?
• Gusher, perhaps
• Borehole for petroleum
• Cap site
• Source of 'black gold'
• Whence petroleum is extracted