Petroleum Sources: Kerogen Types and Their Significance

Sources of Petroleum

Petroleum originates from a small fraction of the organic matter deposited in sedimentary environments.

“Source Rock”

The typical “source rock” contains 99% mineral matter and 1% organic matter. The organic matter is composed of 90% kerogen and 10% bitumens.

This organic matter is usually a combination of marine- and terrestrially-derived organic matter (plant) and zooplankton (animal), which constitutes more than 95% of the life in the oceans.

Terrestrial organisms are mainly wind-blown spores and pollen, along with some woody debris from rivers and swamps.

All living matter is composed of four main constituents: lipids, proteins, carbohydrates, and lignins.

Kerogen Types

1. Type I Kerogen

This type of kerogen has a high initial hydrogen to carbon atomic ratio [(H/C) of 1.5 or more], and a low oxygen to carbon atomic ratio [(O/C) of less than 0.1]. It has a hydrogen index greater than [300] and an oxygen index less than [50].

Its primary source is from algal sediments, such as lacustrine deposits. Type I kerogen is also called alginite kerogen, containing high concentrations of alkanes and fatty acids. It is the best source for oil-prone maturation, but unfortunately, it is very rare.

2. Type II Kerogen

Type II kerogen has a relatively high [H/C ratio (1.0 to 1.4)], and a low [O/C ratio (0.09 to 1.5)], [hydrogen index between 200 and 300], and an [oxygen index between 50 and 100].

It consists of abundant moderate-length aliphatic chains and naphthenic rings.

Type II kerogen is also called exinite and is usually associated with marine sediments, where autochthonous organic matter (bacteria, phytoplankton, and zooplankton) have been deposited in a reducing environment. It is a good oil or gas-prone kerogen. It is more common than alginite.

3. Type III Kerogen

Type III kerogen has a relatively low [H/C ratio (usually <
1.0)], and [low O/C ratio (0.2 to 0.3)], [hydrogen index below 300 and an oxygen index above 100].

The main source of this type of kerogen is continental plants found in thick detrital sedimentation along continental margins. This type of kerogen is also called vitrinite. It is less favorable for oil generation but will provide a source rock for gas.

Humic material is thought to be derived from plant matter, while sapropelic material originates from algae or plankton.