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.