Hydrocarbon Source rock types

There are four types of Source rock (SR). The source rock are more oil prone as the Hydrogen/Oxygen ratio increases. 

Type I deposition environment marine. eg lacustrine

Derived from algae present in fresh water lake and lagoon.

Type II mixture of marine and terrestrial depositional environment.

Type III terrestrial environment.

Occurs abundantly in coals.

Type IV terrestrial depositional environment. 

High in carbon BUT low in Hydrogen. Often termed as "dead carbon".

Type II are the most common SR found. 

The depth and temperature where the source rock(SR) buried also plays an important role on hydrocarbon generation. If the SR depth is <2km, at 30 deg C/km the SR is in diagenesis process whereby only small quantities of HC generated.

The optimum SR depthto generate oil should be from 2-4km whereby it is at Katagenesis process.

As the SR depth are more than 5km, it has enter Metagenesis and Metamorfic processes. The SR will generates gas.