What is source rock?

In petroleum geology, the term source rock refers to rocks in which hydrocarbons have been generated or are capable of hydrocarbon generation. They are one of the necessary elements of a working petroleum system. They are organic-rich sediments that may have been deposited in a variety of environments: deep water, marine, lacustrine, and deltaic.

When the inorganic materials are in proper temperature and pressure, the organic material become a source rock, sometimes called kerogen. Oil shale can be considered as an organic-rich but immature source rock from which has little oil or none has been generated and expelled.
​Source rocks are classified from the kerogen type.

• ​ Type I – is hydrogen rich are formed from deposited algae the in the deep lakes. It trends to generate waxy crude oil.
•  Type II –are formed from pyto- and zootoplankton in marine environments. Mostly , It generated both oil and gas that had been buried in depth.
• Type III - are formed from plant that had been decomposed by fungi and bacteria. Most of them became to coal and coaly shale
•  Type IV – are formed in terrestrial environment with oxidize and recycled woody debris matter.

Algae

Plant- and Zoo plankton

Fungi

Woody bebris

How do we classify kerogen types?

​The classic Van Krevelen diagram is commonly used for classifying the organic matter in sediment. The ratio H/C (hydrogen : carbon) and O/C (Oxygen :Carbon) are plotted in diagram in vertical and horizontal axis respectively.
Rather than plot the elemental ratios it is common to plot indices determined by a pyrolysis technique referred to as Rock Eval. In the pyrolysis techniques two indices are determined: the Hydrogen Index (HI) which is milligrams of pyrolyzable hydrocarbons divided by TOC and the Oxygen Index (OI) which is milligrams of pyrolyzable organic carbon dioxide divided by TOC.
 
Cross-plots of both elemental H/C and O/C ratios or of HI and OI are utilized to discriminate four ‘fields’ which are referred to as Types I, II, III, and IV kerogen.

Classification of Kerogen Type (Oxygen and Hydrogen index )

Classification of Kerogen Type (by H/C and O/C ratio)

Author
Pooritat Toprasert

 

Maturation 

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Formation of a reservoir require a lot of time and the processes must be in the right order to create an environment that can trap hydrocarbons and to migrate the hydrocarbons from source rocks.

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Biological Theory of Oil and Gas Formation.
​The biological theory of oil and gas formation dates back to Georg Agricola in 1556 and later to Mikhail Lomonosov in the 18thcentury. According to this theory, organic material including algae, aquatic microbes, and other sea life, accumulated over time in oxygen-poor enviroment, eventually becoming covered with silt. Over time, as this organic-rich matrix was further buried, it was compressed and heated, eventually undergoing unspecified chemical reactions resulting in crude oil and methane (natural gas).
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The influence of temperature is important; if the temperature is too cold, the decomposition of the organic matter (called kerogen) will not occur, and if the temperature is too high, the liquid and complex hydrocarbons we call oil will breakdown to methane. The rather narrow range of acceptable temperature is called the oil window and it comprises a temperature range of about 60 to 120 degrees Celsius, corresponding to a depth of about 3 to 6 km.
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Oil window with depth and temperature

Measurement of maturation
            When source rocks were formed, these rocks passed through many processes to release oil and gas. Some rocks can release only heavy oil while some rocks can release both light oil and heavy oil. So how do we know the maturation of these rocks? 
           
Vitrinite reflectance (VR)
            Vitrinite reflectance is a measure of the percentage of incident light reflected from the surface of vitrinite particles in a sedimentary rock. It is referred to as %Ro. Results are often presented as a mean Ro value based on all vitrinite particles measured in an individual sample.
           At very first, we have to know what vitrinite is , how important it is and how to use it to classify maturation of the source rocks.
           Vitrinite is the common component of coal and kerogen. Vitrinite is formed from organic material such as plant's cell wall , woody tissue that are a polymer of lignin and cellulose. So vitrinite looks like shiny appearance because of having cell wall composition. 
         So vitrinite reflactance (VR) is a method for identifying the thermal history of sediments in basin. At the first time its utility was used for analyzing maturation of coal bed. Recently, its utility as a tool for the study of sedimentary organic matter metamorphism from kerogens to hydrocarbons has been increasingly exploited.
Because of its sensitivity to temperature that largely correspond to those of hydrocarbon generation ( 60 to 120°C) ,so this method is suitable calibration. 
           Vitrinite reflectance can be used as an indicator of maturity in hydrocarbon source rocks. Generally, the onset of oil generation is correlated with a reflectance of 0.5-0.6% and the termination of oil generation with reflectance of 0.85-1.1%. The onset of gas generation ('gas window') is typically associated with values of 1.0-1.3% and terminates around 3.0%. However these generation windows vary between source rocks with different kerogen types.

More infomation

References
www.homeaquaria.com
www.countrysideinfo.co.uk
www.hotnhitnews.com
www.fishbio.com
www.studyblue.com

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