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It reads:

Petroleum consists of crude oil and natural gas and contains a mixture of up to 300 hydrocarbons, including sulfur and nitrogen compounds. It is formed over thousands of years from the remains of fossilised marine organisms.

Is petroleum a single entity that is simultaneously a gas and a liquid oil compounds? How does that work? It is also a mixture of up to 300 hydrocarbons, so it is a liquid and gas in one? I don't understand? Do they mean that petroleum can be either gas OR it can be either liquid? What about the 'mixture of 300 hydrocarbons bit'. Does that mean you will never extract petroleum of a single hydrocarbon eg pure $\ce{C6H14}$ etc? I haven't revised this in a long time so I forgot..

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Petroleum is a mixture of many different compounds.
Some of those compounds are gas. Some are liquid. Some are solid.
Many of them are hydrocarbons, and there are many different hydrocarbons included in the mix (e.g. different numbers on the C and H).
Petroleum is generally refined into more specific products before use, at a refinery.
Another example to consider might be "mud" - that's a mixture of some liquids, maybe some dissolved gases, and a bunch of different solids, usually including clay, maybe a bit of gravel, and some organic materials formed from decaying plants and animals. If you take mud and compress it underground for thousands of years, then you'll have petroleum.

The second sentence seems clear enough.

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  • $\begingroup$ How can something be both a gas and a liquid? Or is petroleum used to refer to a collective group of hydrocarbons, some of which are either gases or liquids? $\endgroup$ – watsup22 Sep 8 '15 at 12:10
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    $\begingroup$ The latter. Have a soda with some ice - there in your cup you'll have solid, gas (which makes the soda fizzy), and liquid. Then have some ice water- you'll find the same substance in solid, liquid, and (a small amount) gas forms all in the same place at the same time (stable only at 0C). $\endgroup$ – WBT Sep 8 '15 at 15:44
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To understand what petroleum is you need to understand where it is usually found.

Deposits of crude oil are usually found deep in the earth in porous rocks under high pressure (the organic material that forms crude oil needs pressure and heat to convert it to oil). Often the rocks that store oil and not where the oil was originally formed, but, being daily mobile, it tends to migrate to the highest place in the rock formation that is still porous (and is often trapped by a higher layer of a non-porous rock).

The mix of components in oil varies a lot. There will usually be a range of hydrocarbons from methane to higher hydrocarbons with dozens of carbons or perhaps even hundreds. There will also be sulphur and nitrogen compounds, sometimes in large quantities. Sometimes the gaseous components will separate out into a layer in the deposit, but even then there will be a lot of gas dissolved in the liquid parts (oil is usually under a lot of pressure so there can be a lot of gas in solution).

When you drill into an oil formation one of the things that happens is that you release the pressure. As the pressure lowers much of the gas in the solution will be released even if there isn't a separate layer of gas in the deposit. The oil driller than has to handle the separation at the wellhead in a controlled way. But the major point is that what comes out of the ground is often a messy mixture of gas and liquid and your description is reasonably accurate there. This, however, would not normally be called petroleum which is usually reserved for certain particular refined mixtures.

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Petroleum is a mixture of many hydrocarbons and other things as you said. Due to the pressure and temperature, some light fractions will dissappear and your petroleum will be heavier. Let's think about the condensate. Condensate is mainly composed of C8 C11 fractions and if you put it in a open bottle in atmospheric conditions, light fractions will be gaseous and leave the bottle. The light color will be darker and the amount will be less. This means that your crude oil contains gas and liquid substances. The conditions are important. In reservoir, due to the high pressure and temperature, light fractions are soluable in it.

Hope it helps you to understand the situation.

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