system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is there so much oil in the Middle East?

Why is that geographic region so rich in the resource? Where did it all come from? (Geohistorically)
gpt
Crude oil is formed from the decayed remnants of aquatic plants and animals that lived hundreds of millions of years ago. Large portions of the modern day Middle East were once submerged under a large, now non-existent sea called the Tethys Sea. 

This sea eventually disappeared, sealing large deposits of organic matter under a salty crust. Over the eons, this crust was in turn covered by layer upon layer of sediment. As the sediment was compressed under the increasing weight of the layers above, it hardened into limestone. About 15 million years ago, the shifting of tectonic plates of the region formed large, underground fissures. As the organic matter migrated through the layers of limestone, much of it seeped into these fissures. 

These deposits of organic matter became crude oil.


**TL:DR:** The middle east was a huge ocean before that contained a lot of organic material, then it dried out and all the organic material died out and eventually became oil.