What deternines whether a given area has oil or NG?

Title mostly says it all. But I want to make clear that I’m not asking how you can determine by looking at the geography whether or not there is oil and/or NG in the area.

Rather the question is why is there oil and/or NG in some areas and not in others; what caused this uneven distribution?

Oil and gas are mostly found in ancient sedimentary basins, whether marine or sometimes freshwater. Under heat a pressure the organic matter in the sediments eventually is transformed into oil and gas.

The major difference is that you can put oil on a train car or truck and haul it around the country without specific infrastructure for it, whereas natural gas is dependent on the pipelines to haul it around the country and the delivery lines to get it to your house.

Whether your house has one or the other mainly depends on how old your house is. Usually if there was natural gas infrastructure where they were building a house, they would have used natural gas because it’s generally cleaner, has historically been cheaper, and is less of a hassle. Of course, when the natural gas infrastructure arrived in your region is dependent on a lot of things, mostly on how close you are to a geologic setting that produces natural gas.

I believe a lot of places like the Northeastern US that were almost all oil heat really started getting natural gas lines in the wake of the oil crunches in the 70’s and early 80’s. Part of why natural gas has been cheaper historically is because oil fluctuates on the global market whereas shipping natural gas overseas is difficult, so the price is more regional.

It depends on the source material , and the temperature and pressures the source rock is cooked at .

Organic material is laid down under anoxic conditions. It gets buried and compressed with sediments and eventually forms into a shale. The organic material, as it is buried and heated turns to kerogen. This shale is often referred to as the source rock , as it is where the hydrocarbons come from.

The Kerogen is heated down in the earth, and eventually forms oil or gas. Generally speaking the higher the temperature , the more likely gas is produced. That said , the organic material laid down can have an impact, algae, plankton etc often form a kerogen that forms oil, plant matter tends to form gas.

The oil and gas then tends to migrate up through the rock, and eventually gets to surface and evaporates off into the atmosphere. If the migrating oil and gas hits a seal, with a porous formation under the seal, we end up with an oil or gas (or often both) reservoir. The gas and oils can all migrate at different rates through the rock to get to the reservoir as well.

So really what determines the reservoir is

  1. the type of organic material laid down,
  2. the temperature it was heated up to , and to a certain extent, the rate at which it was heated.

On one further note, the shale oil drilling you hear about is basically going straight for the source rock.

So that is the - why oil or gas or both

The question to there being any hydrocarbon is basically,

  1. was there any organic material being deposited in a sedimentary basin under anoxic conditions several million years ago.
  2. And if so, was there sufficient burial and heating to form kerogen and then oil and gas,
  3. Did subsequent sediment deposits also form a porus reservoir rock
  4. did a seal form over that reservoir.

OK, thanks. But why would such things be more likely to happen in one place (e.g. the Middle East) than another (e.g. India)?

The major oil/NG deposits tend to be in locations that were river deltas many eons ago. Organic matter was swept out with the water flow, deposited in the sediment at the mouth of the river, and as Colibri mentions, eventually geological processes produced oil from the residue.

So you tend to find oil in places like the Mesopotamian delta (in a big way); Venezuela’s oil is at the outflow of the Orinoco. Of course, a number of oil fields may relate to watersheds deltas that have somewhat disappeared - eons ago, the whole east side of the rocky mountains used to front onto a shallow sea; (in the days before grass, erosion was a bigger problem) so the oil resources from Mexico and Texas to Alberta were the drainage deltas of rivers running off the Rockies into that shallow sea. Similarly, the oil fields of central Asia relate to time when there was much more run-off there.

I’m guessing the big river(s) of India are a much more recent phenomenon; and the Indian subcontinent was still free-floating and hadn’t crashed into Asia (forming the Himalayas) when the oil deposits were being formed.

Yup. You need a primordial source or organic matter, it needs to be sealed in, and it needs to get cooked just right. Overcooked oil is called coal. It won’t flow up a pipe.

But what exploration geologists are most interesting in looking for are traps. You need to trap the oil or gas under some form of impermeable seal. If there is no such trap, the oil or gas will have long since vanished. Traps can be a range of materials: appropriate sedimentary layers, volcanic lows, salt. But you are looking for an intact seal that traps the oil and gas under some form of simple dome. You drill into the top of the dome.

So to have oil/gas you need to have had a conditions for the creation of lots of organic matter, a situation where it gets laid down in the strata - such as under active sedimentation. It gets cooked over time properly, and it remains trapped under rocks that stop it leaking out.

Not necessarily deltas. Major deposits tend to be found in old oceanic basins, as I said. The oil fields in the Middle East are mainly due to the closure of the ancient Tethys Sea as Africa collided with Eurasia.

But the vast majority of Middle Eastern oil deposits are some distance away from the Mesopotamian deltas. Saudi Arabia and the UAE, for example. Iraq has lots of oil, of course, but Israel, Jordan and Syria are relatively oil-poor.

Oh, oops, I very much misread the OP!

Modern deltas are not necessarily a good guide to the location of oil and gas, unless they have been in the same location for a very long time.

Most US oil and gas fields don’t correspond to deltas, but to old ancient basins, particularly the shallow seas that once occupied the center of the continentin the Mesozoic.

Middle Eastern deposits generally correspond to the part of the Tethys Sea that now underlies the Persian Gulf and surrounding areas.

Colibri has it right. A lot of the geology/geography is remarkably different than it was 100M-plus years ago.

So the question is - is the oil a residue of land matter swept into the sea (i.e. deltas, marshes, coastal deposits) or is there a substantial sea life component?

In larger basins, most of it is going to be sea life, especially plankton.

Basically the geologic makeup of most areas has been mapped, or at least the geologists have an idea of what went on there geologically (say… 500 million years ago)

So they know that there was an ocean basin there in the Cambrian period, which means that it would have the basic raw materials for oil formation.

Then they start looking for the right types of rock formations- permeable rocks capped by impermeable ones, folds, etc… basically identifying reservoirs.

Once they find likely areas, they start doing seismic testing to get an idea of what’s actually under there, and if that’s favorable, then they drill test wells, and scale up if they’re succesful.

The thing is… over geologic time, the Earth moves a LOT and things change a bunch. At one point in the distant past, the mountains in the Scottish Highlands, the Tell-Atlas mountains in Morocco and the Appalachian mountains in the US were all once part of the same mountain range in Pangaea. And… at about the same time, the areas that are now England, the Netherlands and nearby were actually part of a huge desert, instead of being as wet as they are today.

That makes sense. Thanks.

This is a cool story; a fossilized forest was found in an Illinois coal mine.



The fact you’re a geology guy makes that an especially fun goof.

Neeeveeeer miiiiiind!!1!


At Epcot Center in Orlando, they explain how fossil fuels come from dinosaurs. They don’t go into sufficient detail to answer the OP, but it is only natural to expect coal from land dinosaurs (such as brontosaurus), natural gas from flying dinosaurs, and oil from swimming dinosaurs.