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
Geologists talk about a ‘source rock’ for oil and gas reserves – this is the rock that contains the organic material that was converted into hydrocarbons which then move to other rocks where they accumulate in reservoirs. The giant Saudi oil fields are believed to have formed from shales such as the Qusaiba formed in the Silurian between 440 and 400 million years ago and the younger Hanifa. Shales are rocks made of fine clay minerals laid down in quiet, still seas. The seas where the shale was forming were teaming with microscopic life, especially algae. When these died, they sank to the bottom of the ocean along with the mud. The water immediately above the seafloor contained very little dissolved oxygen, so the organic remains of the algae didn’t decay. Instead, they accumulated to make up a considerable volume of the shale.
The ocean basin was gradually deepening, so more and more mud and organic ooze continued to accumulate. The pressure of the newer sediments piled pressure on the stuff below and as it got deeper it got warmer until the dead algae began to convert into hydrocarbons.
Over time, the oil and gas in the shale began to migrate upwards into younger reservoir rocks. In Saudi Arabia, there are huge thicknesses and extents of sandstones and carbonates containing pores and cracks where oil and gas can accumulate overlaid in turn by rocks that prevent them escaping. The reservoir for the gigantic Ghawar field is a limestone which is more than 1/3 empty space, leaving lots of space for oil.
There are even bigger fields in Venezuela, but the oil there is dominated by extremely sticky, high-sulfur crudes which are expensive to lift and transport and then need sophisticated refining to turn them into reasonably non-polluting fuels, so they have been much less preferable to Saudi crude. The best oils in the World for uses such as petrol are the very light, low-sulfur oils found in Libya and the North Sea which means they are sold at a premium over Middle Eastern oil.
TL;DR: a layer of rock that is really rich in dead plankton is getting cooked up and generating oil and gas thanks to the Arabian Peninsula slowly crushing into Asia and sinking to deeper depths (plate tectonics). The oil and gas flows out of this “source rock” and some of it gets stuck in spongy rocks with tiny holes that we can drill into. Other places (e.g., Libya or Egypt) also have similarly rich layers of rock that are getting cooked.
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To answer this question you need to understand the idea of a petroleum system. A petroleum system is a natural configuration of the geology that produces and concentrates oil and gas (petroleum) into economic deposits.
It’s hard to keep it ELI5, but you need 4 pieces for a functioning petroleum system:
1) Source rock. This is a rock with a high amount of organic carbon in it. “Organic carbon” refers to carbon from living things *other* than carbon in things like shells. So, bits of plankton (algae) or bits of plants. You generally need more than 1% of the rock to be made of this, higher values are better. Such a rock usually gets deposited as sediment on the surface of the Earth in lakes or oceans. You need plenty of productivity (lots of plankton or plants) and you need it to be preserved in the sediment, otherwise it gets broken down and recycled by life at the surface (think: decay — this needs to be avoided). Preservation is more likely in places with low amounts of oxygen available, so oceans or lakes that don’t have good circulation are best (think of somewhere like the Black Sea, a narrow ocean like the Red Sea, or a deep lake that does not have much life living on the bottom of it). On land, swamps and bogs can trap dead land plants. The dominant type of organic material determines whether mostly oil or mostly gas gets generated. Planktonic algae tends to produce oil, land plants tend to produce natural gas. Dinosaurs are not involved.
2) Maturation. Once deposited, the sediment needs to get buried to greater depths — great enough that it starts to heat up. You need a place where the Earth’s crust is sinking. At around 60°C it starts to “cook” oil and gas out of the organic material. The location this happens is known as the “petroleum kitchen”. This is usually at a depth of 1 to 4km, though it depends on how “hot” the local geology is. Hotter and it happens at a shallower depth. Once over about 225°C, it’s “overmature” and everything has been cooked out.
3) Migration. In conventional oil and gas deposits, the oil and gas is not found where the source rock is. It has been forced out of the source rock and flows from it into surrounding rocks. *Generally* it flows laterally and upward from higher pressures to lower pressures, which usually means to shallower depths, though downwards is possible depending on conditions. The migrating oil and gas can reach the surface, in which case you end up with something like the LaBrea tar pits in Los Angeles area. These are “leaky” petroleum systems that have all the right pieces except the next one.
4) Trap. Somewhere along the migration paths there is a porous (small holes) and permeable (connections between the holes) rock that the oil and gas flows into. This is known as a reservoir rock. It is not a gigantic cave, it’s more like the rock equivalent of a sponge with tiny holes. It looks like a fairly ordinary rock, but it has space for liquids in it. The amount can vary, but 10% to 30% space is common. Sandstones or limestones are common rock types (think of the spaces between the sand grains), but not all such rocks have porosity and permeability. It depends on the details of how the sediment got cemented into rock. Sometimes there’s little or no space left as the rock cements. The reservoir rock is up against a rock that is the opposite: not porous or permeable. This is known as a seal and is a barrier to the continued flow of the oil and gas so that it gets stuck and concentrated in the reservoir. This combination (reservoir + seal) is known as a trap.
This is a “conventional” oil and gas situation where oil and gas flows easily out of the reservoir when you drill into it. There are also “unconventional” ones. These can be situations where there isn’t necessarily much migration or even a conventional trap. If you cook up the source rock so that there is oil and gas in it, and then drill into the source rock and fracture it hydraulically you can make the permeability needed to make it flow even if the rock is naturally a pretty bad reservoir. Or if it is a coal seam, you can extract the water and get natural gas to flow out of it (think of the methane exposed during coal mining — you pump that gas out of the rock). Or if it’s really thick tar, you can inject steam to heat it up and get it to flow, or mine it (“oil sands”). As conventional deposits are depleted (think: easy to get flowing), unconventional deposits are increasingly being developed (difficult and expensive). This is one of the reasons for increasing oil price.
Finally we can get to the Middle East question. Why is it there so much oil there?
Short answer: because that area has *really good* and widespread source rocks that happen to be the oil-prone type (i.e. from plankton), and the plate tectonics of the area (the way the Earth’s crust is moving) has created places that are sinking and heating up those source rocks (maturation), and deforming the rocks to make a wide variety of traps. It’s also timed about right.
The Persian Gulf is particularly productive because the northern edge of the Arabian plate is like a gently-dipping slab that’s being fed into hotter temperatures, progressively cooking oil and gas out, and then that stuff flows laterally up the layers of rock laterally, getting stuck along the way in huge traps with (luckily) good reservoir rocks and good seals. The seals are particularly interesting in some areas because they include anhydrite (dehyrated gypsum) that are exceptionally good.
Some of it is just the quirks of geological history and luck, but the key factor is the source rock that is foundational to the whole process. In the Middle East there are multiple ones that are prolific. The most exceptional one is in organic-rich marine limestones of the Jurassic-age Tuwaiq Mountain and Hanila Formations, which have an average of about 3% organic carbon and sometimes much more. They were deposited in a shallow ocean, not on land with plants. It was at the edge of Pangaea (see below). Whole paper about it here if you’re into geochemistry (though, obviously, not ELI5, you can still look at the maps): [https://www.researchgate.net/publication/279386725_Geochemistry_of_the_Upper_Jurassic_Tuwaiq_Mountain_and_Hanifa_Formation_Petroleum_Source_Rocks_of_Eastern_Saudi_Arabia](https://www.researchgate.net/publication/279386725_Geochemistry_of_the_Upper_Jurassic_Tuwaiq_Mountain_and_Hanifa_Formation_Petroleum_Source_Rocks_of_Eastern_Saudi_Arabia)
[Edit: Forgot there are also Silurian source rocks that are prolific too: the Qusaiba Shale Formation — having multiple source rocks is even better]
Why did the ancient conditions disappear? Because the Earth’s climate has changed and the entire crust of the Earth moves around, closing up oceans and tearing apart continents. Back in the Jurassic the Arabian Peninsula was at the edges of an ocean that mostly doesn’t exist anymore (the Tethys — the Mediterranean is sort of a remnant) and the Red Sea hadn’t yet formed. It looked something like this: [https://www.researchgate.net/figure/A-Upper-Jurassic-paleogeographic-configuration-demonstrating-the-paleo-occurrence-of_fig1_347075763](https://www.researchgate.net/figure/A-Upper-Jurassic-paleogeographic-configuration-demonstrating-the-paleo-occurrence-of_fig1_347075763) It was a shallow tropical ocean with poor circulation, which allowed the source rock to form. Fast-forward about 150 million years or so, and it’s being nicely cooked up as the Arabian Peninsula slowly collides with Asia.
My FIL worked for Aramco in Saudi Arabia. I visited him there and asked why there are no derricks pumping in the oil fields like I see in California. He laughed and said Saudi is like sticking a straw in the ground and the pressure just pushes it out. No need to pump much. That’s why it’s cheaper. Much cheaper.
Other countries have an abundance of oil. Like the 🇺🇸 and Venezuela. Our gas now should be $2.00 gal.
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