Source Rocks:
A Petroleum source rock is defined as any rock that has the capability to generate and expel enough HC to form an accumulation of oil or gas. A potential source rock is one that is too immature to generate petroleum but will form significant quantities of HC when heated in the laboratory or during deep burial.
An effective source rock is one that has already formed and expelled petroleum to reservoirs.
The term source rock is applied irrespective of whether the organic matter is mature or immature. This concept is commonly referred to as the “maturity of source rock”.
Three basic parameters are required to make a source rock:
- Amount of organic matter
- Type of organic matter
- Thermal maturity of organic matter
Amount of organic matter:
The quantity of organic matter is usually expressed as total organic content (TOC).
The lower limit of organic carbon for petroleum source rocks has been established in an empirical manner.
For shale-type sediments, the lower limit is 0.5% wt. TOC, while for carbonate rocks, it is 0.3% wt.
However, these minimum values should be considered only as necessary background values rather than as positive indications for source rocks.
Types of Organic Matter:
The quality of petroleum generated and expelled increases as the atomic H to C ratio (H/C) of organic matter increases. Hence is the importance of quality of Organic matter which have different types with different H/C ratio.
Organic matter, in general, may be classified into two major groups:
i) Sapropelic kerogen:
The term Sapropelic refers to the decomposition products of fatty, lipid organic material such as spores and planktonical algae deposited in subaquous mud (marine and lacustrine) under oxygen-restricted water.
Sapropelic organic matter such as fats, oil, resins and waxes show high H/C ratio in the range of 1.3 to 2.0. It is capable of generating both oil and gas.
ii) Humic kerogen:
The term Humic refers to organic matter derived from terrestrial plant cell and wall material which is composed mainly of lignin and cellulose. Humic kerogen is produced from the lignin of higher woody plants, which grow on land.
Humic kerogen has an H/C ratio of < 0.84. It is capable of generating a lot of gas but little oil. In addition to the classification into Sapropelic and Humic types, there are other specific types of classification like those by coal petrographer, by petroleum geologist, etc.
Thermal Maturity of Organic Matter:
Determination techniques:
- Carbonization of polynomorph – Pollen coloration
- Vitrinite reflectance
- Rock Eval Pyrolysis
- Elemental analysis: van Krevelen diagram
