What is a physical feature where most of the world's oil reserves can be found? it is in the middle east.?

i need to know that and a word for- agreements about how countries can use water in a region


the first question is 11 letters long and the 4th letter is an S. the 2nd question is 11 letters long and the 10th letter is a T.What is a physical feature where most of the world's oil reserves can be found? it is in the middle east.?
ghawar field