Crude oil and natural gas are formed in source rocks which contain a required quantity of organic matter. Shale rock is the best example of a source rock. In order for crude oil, biogenic or thermogenic gas to be generated, organic matter contained in the shale rock must be first processed by specific groups of bacteria or buried deep enough and heated so as to reach thermal maturity that is required for generation of thermogenic gas.
First of all, thermal maturity denotes diagenetic changes of organic matter that is contained in source rocks which generate hydrocarbons – from the immature phase through the main phase of oil and gas generation to the post-maturity phase.
The drivers of organic matter transformation are:
- temperature,
- time, and
- pressure
As a result of their action, mobile products (gases and liquid hydrocarbons) are released from the biomass at an increasing concentration of stable organic components. In subsequent maturing (diagenesis) phases the aforementioned components loose their functional groups with oxygen, sulphur and nitrogen, while being enriched in carbon. These processes change the optical properties of organic matter.
The degree of thermal maturity depends primarily on the top paleotemperature, as applied to the rocks in their geological history, and on the time of exposure to that temperature. Maturity increases with burial depth due to higher temperatures that prevail in deeper parts of the earth's crust. The increase is closely related to the geothermal gradient which is usually expressed as °C/km.
The effect of pressure on thermal maturity of organic matter is small, as pressure retards the chemical reactions that are involved in maturation processes, albeit it obviously affects the physical properties, such as porosity and structural layout of particles.
Diagenetic processes that have occurred in the Lower Paleozoic organic matter can be recreated using different methods, including:
- CAI (Conodont colour alteration index),
- characterization of volatile organic compounds,
- elemental analysis of carbon,
- Rock-Eval pyrolysis,
- chromatography,
- isotopic method, or
- smectite to illite transformation.
Nevertheless, white or UV light reflection microscopy is still one of the simplest and most accurate methods.
A sample examined under reflected UV light microscope reveals a decreasing intensity of fluorescent colours of primary macerals belonging to the liptinite group (e.g. alginite, bitumenite), up to the colour disappearance point. The primary material is transformed into secondary non-fluorescent components (e.g. stable bitumens). These diagenetic changes in organic matter normally occur at a depth of approximately 3000 m. In reflected white light, they are revealed by a higher reflectance (a brighter colour) of organic macerals, up to the point of bireflectance (anisotropy) of vitrinite macerals and vitrinite-like components (e.g. stable bitumens).
Different plant parts or other organic remains vary in terms of susceptibility to the action of physical or chemical agents. Lignin and cellulose are the most stable biomass constituents and initial components of the vitrinite group.
Reflectance of the vitrinite or, in its absence (as for example in the Lower Paleozoic sediments), of a vitrinite-like material (such as stable bitumens) is a measurable microscopic parameter that defines the degree of diagenetic alteration. Vitrinite maceral reflectance is increasing with thermal maturity. Reflectance values determine the different stages of diagenetic alteration and hydrocarbon generation phases.
Reflectance of autigenic vitrinite or of an optically vitrinite-like material is measured using oil immersion lenses and polished thin sections of the rock. Reflectance of these components increases linearly with increasing thermal maturity and the measured reflectance index value (expressed as % Ro) reflects a specific range of temperatures the sediments had been exposed to in their geological history.
Crude oil and natural gas are formed in source rocks which contain a required quantity of organic matter. Shale rock is the best example of a source rock. In order for crude oil, biogenic or thermogenic gas to be generated, organic matter contained in the shale rock must be first processed by specific groups of bacteria or buried deep enough and heated so as to reach thermal maturity that is required for generation of thermogenic gas.
authors: Izabella Grotek, Marcin Janas
