There may well be 10 trillion barrels left. But that's not the point. It's the cost of producing it. And not the dollar cost.

Good point.  The earth's atmosphere is 78% N, but try fertilizing your tomatoes with it.

I appreciate your underlying point, but in reality you can fertilize your tomatoes with atmospheric nitrogen.  Rotational planting or interplanting with nitrogen fixing plants will do the trick.  You can be sure that we'll be making a great deal more use of atmospheric nitrogen, than we ever will of the third and fourth billion barrel of oil down there, somewhere.  

And then there is the slightly more random trick of loading the atmosphere with greenhouse gases to stimulate more violent weather, bringing on lightning and rain storms.  My garden got a great fix of nitrogen this way last week.  Hmmm...maybe I should send a letter of thanks to the coal lobby.  On the other hand there was that damage to my roof...I wonder if I can sue for damages?  It's all so confusing.

I appreciate your underlying point, but in reality you can fertilize your tomatoes with atmospheric nitrogen.

Sure, I know that.  But it happens on The Great Spirit's terms.  You don't pour it out of a bottle.

Actually, you can pour some great spirits out of a bottle ;-)
your tomatoes get fertilized with it every time it rains