With increasing temperature the chemical bonds in these large molecules (kerogen) are broken and kerogen is transformed into smaller molecules which make up oil and gas. This requires that the temperature must be 80–150◦C over long geological time (typically 1–100 million years). The conversion of kerogen to oil and gas is thus a process which requires both higher temperatures than one finds at the surface of the earth and a long period of geological time. Only when temperatures of about 80–90◦C are reached, i.e. at 2–3 km depth, does the conversion of organic plant and animal matter to hydrocarbons very slowly begin to take place. About 100–150◦C is the ideal temperature range for this conversion of kerogen to oil, which is called maturation. This corresponds to a depth of 3–4 km with a normal geothermal gradient (about 30–40◦C/km).
In volcanic regions organic matter may mature at much lesser depths due to high geothermal gradients (high heat-flow areas). In large intracratonic sedimentary basins or along passive margins, however, the geothermal gradient may be only 20–25◦C/km and the minimum overburden required to initiate petroleum generation will be correspondingly greater (4–6 km). In general one can say that petroleum can not be generated near the surface except locally through the influence of hydrothermal and igneous activity. Shallow deposits of oil and gas which we find today were actually formed at great depths and either the overburden has been removed by erosion or the hydrocarbons have migrated upwards considerable distances. Large amounts of natural gas, chiefly methane (CH4), may be formed near the surface by biochemical processes.
Temperature increases with increasing overburden, causing the carbon-carbon bonds of the organic molecules in the kerogen to rupture. This results in smaller hydrocarbon molecules. When kerogen maturation reactions are completed, the kerogen's "organic" components, which may be derived from lipids, fatty acids and proteins, have been converted into hydrocarbons. As the temperature rises, more and more of the bonds are broken, both in the kerogen and in the hydrocarbon molecules which have already been formed. This "cracking" leads to the formation of lighter hydrocarbons from the long hydrocarbon chains and from the kerogen. The removal of gas, mainly CH4, leaves the residual kerogen relatively enriched in carbon. At the outset kerogen (Type I and II) has an H/C ratio of 1.3–1.7. Humic kerogen (TypeIII), which has high initial oxygen contents, gives off mostly CO2 gas and so its oxygen/carbon ratio gradually diminishes. This diagenetic alteration begins at 70–80◦C and as water and CH4 are removed the H/C ratio will fall to about 0.6 and the O/C ratio will become less than 0.1 at about 150–180◦C. In the North Sea basin, most of the oil is generated at temperatures around 130–140◦C, which equates with a depth of about 3.5 km. If temperatures higher than 170–180◦C persist for a few million years, all the longer hydrocarbon chains will already have been broken (cracked), leaving us only with gas mainly methane (dry gas). The kerogen composition will gradually be depleted in hydrogen and move towards pure carbon (graphite) (H/C→0).
What Factors Influence the Maturation of Kerogen?
The term "maturity" refers here to the degree of thermal transformation of kerogen into hydrocarbons and ultimately into gas and graphite. The conversion of kerogen into hydrocarbons is a chemical process which takes place with activation energies of around 50–60 kcal/mol. This energy is required to break chemical bonds in the kerogen which consists of very large molecules (polymers) so that smaller hydrocarbon molecules can be formed. It has been assumed that formation of oil is a first order reaction, the rate of which is an exponential function of time. Understanding the factors which influence the rate of this reaction is of great interest. Four factors are thought to contribute:
- Temperature
- Pressure
- Time
- Minerals or other substances which increase the rate of reaction (catalysts) or which inhibit reactions (inhibitors).
Temperature is clearly the most important factor, and hydrocarbons can be produced experimentally from kerogen by heating it (pyrolysis). This reaction is time-dependent and in laboratory experiments, where time is more limited than it is in nature, fairly high temperatures (350–550◦C) have to be used in pyrolysis. Pressure appears to play a minor role but increasing pressure should reduce the rate of the reaction because of the increase in volume involved in the formation of hydrocarbons. There is a relatively small volume increase when kerogen becomes oil, even though oil is lighter than kerogen. This is due to the residual (coke) which remains unaltered. When kerogen is converted directly into gas, or from oil which has been formed first, there is a marked volume increase. This should lead to slower reaction rates under high pressure in a closed system and retard generation of gas. Generation of petroleum, particularly gas, may contribute to the formation of overpressure but in a sedimentary basin the pressure will for the most part be controlled by the flow of water which is the dominant fluid phase. In the source rock however overpressure is likely to develop, causing hydrofracturing which helps to expel the generated petroleum.
The main cause for overpressuring is, however, not only the increase in fluid volume but the transformation of solid into fluids. When solid kerogen is transformed into fluid oil or gas the ratio between the solid phase and the fluid phase is changed, as expressed by the porosity and the void ratio. Temperature is however the most important factor controlling petroleum generation. It has long been suspected that minerals, particularly clay minerals, might affect the rate of hydrocarbon generation. A number of laboratory experiments have been carried out in which kerogen is mixed with various minerals but the results have not been conclusive.
The conversion of organic matter begins at 70– 80◦C, given long geological time. Between 60 and 90◦C the transformation of kerogen proceeds very slowly, and it is only in ancient, organic rich sediments that significant amounts are formed. Most of the maturation process occurs between 100 and 150◦C. Here the degree of kerogen transformation is also a function of time. This means that rocks which have been subjected to 100◦C for 50 million years are more mature than rocks which have been exposed to this temperature for 10 million years. As the organic-rich sediment (source rock) is buried in a sedimentary basin, it will normally be subjected to increasing temperature as a function of increasing burial depth. If we know the stratigraphy of the overlying sediment sequence and the geothermal gradient and the subsidence curve, we can calculate the temperature as a function of time.
At low degrees of maturity we find more of the alkenes (olefins) and cykloalkenes (naphtenes), which have high H/C ratios, while with greater maturity there is an increase in the proportion of aromates and polyaromates (low H/C ratio). Oil thus acquires increasing gas content with increasing maturity. During this transformation of organic matter, water and oxygen-rich compounds are liberated first, then compounds which are rich in hydrogen. This conversion results in enrichment of carbon and the colour of the residual kerogen changes from light yellow to orange, brown and finally black. These gradations can best be registered by measuring light absorption of fossil pollen and spores (palynomorphs). It is also possible to analyse colour changes in other kinds of fossils, for example conodonts. For application in exploration a rapid semi quantitative method has been developed where by these colour changes are estimated from smooth spores examined under transmitted light and compared with a standard colour scale. This parameter is called the Thermal alteration index (TAI) and will give a rough idea of the thermal maturity of the sediments and their temperature history.
Another way of assessing paleo-temperatures at which alteration took place in sedimentary rocks is to record the degree of carbonisation of other plant remains which are usually present. Vitrinite, which originally was fragments of woody tissue, is a common component of coal but is also found in smaller amounts in marine source rocks. This material is analysed by measuring the amount of light it reflects. It becomes shinier and reflects light better as the degree of carbonisation increases. By measuring the reflectivity of vitrinite particles under a reflected light microscope an exact value is obtained for this maturity parameter, expressed by the reflectivity coefficient R0 (% vitrinite reflectance). If R0 is less than 0.5% in a shale it can not have generated much oil and is classed as immature. Shales with R0 = 0.9–1.0 have been exposed to temperatures corresponding to maximum oil generation. R0 =1.3 represents the upper limit for oil generation, above which the shale will only produce condensate (light oils) or gas.
For certain source rocks the ratio between extractable alkenes (paraffins) with an even number of carbon atoms per molecule and those with an odd number may also be an expression of maturity. In plant material and in marine algae, one finds a higher abundance of alkenes with an odd number of carbon atoms than in transformed organic matter like waxes and fatty acids. The decrease in this predominance of odd over even in source rocks with increasing maturity is due to the dilution of the original biologically derived n-alkane mixture with a newly generated mixture which has a regular carbon number distribution. This odd/even ratio is normally expressed by means of an index called the Carbon Preference Index (CPI):
CPI = n-alkenes (odd)/ n-alkenes (even)
It is based on analyses of alkenes with carbon number between 25 and 33 (C25 and C33) in oils. However, organisms also start off with different CPI index values, and land plants have high ratios between odd and even carbon numbers. Bacteria have a predominance of even carbon numbers. The maturation process will cause a shift in the carbon number distribution towards smaller molecules, particularly in the range C13–C18. Oil that comes from carbonate source rocks often has a low CPI index, while oil derived from plants has a high index value. With increasing temperature the CPI index goes towards 1, that is to say, equal even and odd carbon numbers. The isotopic ratio also changes because the bonding between hydrogen and 13C, and between hydrogen and 12C, are not equally stable. Light gases such as methane, are enriched in the light isotope 12C, and the hydrocarbons that remain will therefore have an increasing 13C/12C ratio with increasing temperature. There is isotopic fractionation of carbon, and when kerosene is releasing petroleum, this phase is somewhat enriched in 11C corresponding to the precursor kerosene. Gases, particularly methane, normally have lighter carbon isotopes than kerosene and oil. When methane is formed from larger hydrocarbon molecules by thermal cracking, the 12C–12C bond is less stable than the 13C–12C bond and the product becomes enriched in 12C (low δ 13C).
