Considering oil forms under compression of trees and the like, doesn’t that mean there must have been a lot of life and vegetation there a long time ago? Why did all of that dissappear and only leave mostly barren wasteland?
In: 4590
About 400 million years ago, the Persian Gulf area was under a large shallow-ish tropical sea. Oil is created not from trees (that’s coal) but from organic sediment washed down from rivers, and marine microorganisms like plankton and algae that love these warm climates (think about how coral reefs today are teeming with life).
Then huge amounts of limestone was also deposited on top of these organic rich sediments. Limestone also tends to form in these conditions, and limestone can create good reservoirs for the oil.
As the sea closed up because of plate tectonics, the layers of rocks were fractured, wrinkled and folded up. This created “compartments” in the rocks where oil & gas can get trapped. An oilfield needs 3 things: a source rock in thic case was mudstones and shales rich in organic matter, a reservoir rock where the oil sits which is the limestone here, and a cap rock that stops the oil escaping which is more shale and salts here. When the rocks get folded into an arch shape due to tectonics the oil from the source can float up and accumulate at the peak of the arch.
These arch shaped compartments in the middle east just so happen to be relatively shallow underground, massive in size, and made of relatively good rock with a lot of spaces to trap the oil like a sponge. This makes it cost effective to extract because you don’t need to drill using too much complicated equipment, and because of the type of rock the oil can flow relatively well so you don’t need too many wells to extract all of it. Something to bear in mind is that all petroleum, not just shale gas, is trapped inside rocks. Some rocks like sandstone and limestone just have more connected pores than others like shale, which makes it easier to extract from.
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