Interestingly enough, there was a map published about 25 years ago, showing all of the continents and how much of the surface area had been explored and to what degree it was explored. The US was almost all back showing a high level of exploration. The entire middle East was white, showing virtually no exploration. Oil was so plentiful there and the fields so large that all one had to do was drill a hole and the oil would start flowing.
Now that some of those huge fields are starting to run down, just a moderate amount of exploration would uncover substantially more reserves.