Crude Oil (2): How did petroleum form?

So here is the question today: how did petroleum form?

Here are a few pictures of different types of liquid crude oil:

This is heavy crude or very dense and thick crude oil could be from Saudi or other Middle Eastern area

Here is a range of different fractions taken from the fractional distillation of crude oil

Here is a lighter crude, note how much runnier it looks: could possibly be from the North Sea fields.

The question is how do these different types of crude oil form in the first place? 

All crude oil consists of molecules of hydrocarbons i.e. compounds formed from hydrogen and carbon only.  

There are no molecules containing oxygen in the oil. 

Where could the carbon and hydrogen have come from for these molecules to exist in crude oil? 

Carbon is the molecule in all life forms on Earth so crude oil, we assume, started life in the from of living sea organisms: marine animals and plankton. 

But how did these living organisms turn into oil?

The argument goes that when these organisms died they rotted down in the absence of oxygen.  

Since oil is found in layers of sedimentary rock the argument goes these dead organisms were covered with layers of sediment that sealed them into layers of soft rock.  

Given a long time span, changes occurred that turned organisms into oil. 

Heat from below the earth and the pressure of the millions of tonnes of sediment above also played a part. 

If impervious rock forms above the soft sedimentary rock it traps the oil molecules and they remain there until drilling hits the sweet spot and the oil, under immense pressure, shoots out the ground in what's called a blow out!!

 Here is a video of the effects of a gas blow out in the Gulf of Mexico.

Pages on the "Mole" and "Using the Mole" in chemical calculations are here