Organic matter is transformed into petroleum through a process of maturation over time. As Kerogen, the organic material in source rocks, is buried deeper underground under anaerobic conditions, it matures into various hydrocarbon molecules through the action of heat and microbes. This process converts Kerogen first into bitumen and eventually produces crude oil, wet gas, condensate, and thermogenic natural gas as the organic matter is subjected to greater depths and higher temperatures. Source rocks containing the organic matter that forms petroleum can be of either terrestrial or marine origin.

1. Organic matter transformation
by
Judith Kobusingye

2. Origin of petroleum
Petroleum is derived from the remains of living things which
contains a material called Kerogen.
Before dead organic matter becomes petroleum with time, the
Kerogen matures into an assortment of hydrocarbon molecules
of all sizes and weights.
The light ones being gas and the heavy ones being oil
.

3. Preservation of organic matter

4. Mechanism of organic matter transformation

5. Origin continues
Petroleum source rocks are of terrestrial and marine origin
 Terrestrial source rocks are deposited in lakes, delta and river basins having
woody plant matter, algae etc. Marine source rocks contain dead planktons, algae,
organic remains etc.
 In both the settings, the mixture is buried under conditions of no oxygen. The
Kerogen are classified as type I,II & III as per their origin and are capable of
producing oil or gas or both

6. Conversion of Organic Matter

7. Kerogen transformation
 Under the anaerobic conditions, the Kerogen is transformed into a flammable substance called bitumen
by the action of heat and anaerobic microbes in the sediment and natural catalysts. Most of the bitumen is
eventually cooked into tarry asphalt releasing hydrocarbon molecules (as well as water and carbon
dioxide) out of the source rock as it heats.
 Thermal alteration of Kerogen forms crude oil by increasing the carbon contents.
 At shallow depths (< 3,000 ft), bacteria actions on organic materials form Biogenic Gas(natural gas).
 At great depths (high temperature & overburden), Thermogenic Gas is formed.
 Later stages of thermogenesis will form wet gas and condensate.

8. Hydrocarbon maturation