|MadSci Network: Chemistry|
The reason you haven't been able to get a good answer to this question is that there isn't an easy answer. Crude oil does not have a simple chemical formula.
Don't despair entirely, though - there are some ways to classify the chemical compounds present in petroleum, and this will allow us to make some general statements about crude oil's properties. We'll get to that in a minute - first, let's talk a little bit about crude oil.
Petroleum is one of the most important resources available to us. Using the oil and natural gas that come out of reservoirs underground, we can produce fuels to power our cars, trucks, aircraft, and such, we can heat our homes, and we can produce electricity. Crude oil also provides the raw materials that go into making plastics, which are very important in the modern world.
The reason that petroleum lends itself to such a variety of uses is that it has a very complex chemical structure. Crude oil is a blend of many different compounds - sometimes thousands of them - and the actual compounds present and their proportions vary widely from one crude oil to another. Thus, the lightweight, sweet crude oil produced in the western Gulf of Mexico is made of different stuff and has different properties and capabilities than the heavier, more sour petroleum produced in Alberta. However, all crude oil is made of the same types of compounds. It is almost entirely made up of hydrocarbons, which are molecules which contain only hydrogen and carbon. The simplest hydrocarbon is methane (CH4), but crude oil is generally made from hydrocarbons with more than 5 carbon atoms. While the actual compounds present in crude oil defy listing, we can classify them into groups and describe the oil by the proportions of these groups present.
The most common system for doing this type of classification is the PONA system - PONA is an acronym for Paraffins, Olefins, Naphthenes, and Aromatics. In this context, paraffins are straight-chain or branched hydrocarbons in which there are no double or triple bonds between carbon atoms. Olefins are similar to paraffins, but they contain at least one multiple bond in their chemical structure. Naphthenes are saturated hydrocarbons, just like paraffins, but they incorporate a ring of carbon atoms into their chemical structure. Aromatics contain a benzene ring in their structure. Oil refineries are factories for separating these different types of compounds from one another and for converting compounds from one type into compounds of another type.
I hope this helps answer your question. If you need more information, try the links below. Good luck!
American Petroleum Institute: Facts About Oil
American Petroleum Institute: FAQs
How Stuff Works: How Oil Refining Works - Crude Oil
Try the links in the MadSci Library for more information on Chemistry.