The conversion of organic matter to petroleum

1. Suez University Faculty of Petroleum & Mining Engineering The Conversion of Organic Matter to Petroleum Student Belal Farouk El- saied Ibrahim Class / III Section / Engineering Geology and Geophysics The Reference / Pet. Geology ( F.K.North ) Presented to Prof. Dr. / Shouhdi E. Shalaby

2. Organic Matter When an organism (plant or animal) dies, it is normally oxidized Under exceptional conditions: organic matter is buried and preserved in sediments The composition of the organic matter strongly influences whether the organic matter can produce coal , oil or gas .

3. Basic components of organic matter in sediments • PROTEINS • CARBOHYDRATES • LIPIDS (Fats) • LIGNIN All of these + Time + Temperature + Pressure = KEROGEN

4. Biomolecules in Living Organisms Lipids , mostly fats, oils and waxes, have the greatest potential to be hydrocarbon sources. They are combinations of the fatty acids of the general formula CnH2nO2 with glycerol, C3H5(OH)3. An important example is glyceride C17H35COOCH3 formed from the stearic acid. Proteins are giant molecules that make up the solid constituents of animal tissues and plant cells. They are rich in carbon but contain substantial amounts of N, S and O. Carbohydrates are based on sugars Cn (H2O)n and their polymers ( cellulose, starch, chitin). They are common in plant tissue. Lignin is a polymer consisting of numerous aromatic rings. It is a major constituent in land plants and converts to coal through desoxygenation .

5. We have examined the type of raw material needed and how it must accumulate in the natural environment. The next link in the process is to examine what happens to this organic matter (OM) when buried and subjected to increased temperature and pressure. One thing to remember is that not all of the organic carbon (OC) in sedimentary rocks is converted into petroleum hydrocarbons. A portion of the Total Organic Carbon (TOC) consists of Kerogen . We will look at the transformation of OM first to kerogen , then to petroleum hydrocarbons. Environment of the Transformation

6. The only elements essential to the transformation of organic matter (OM) into petroleum are hydrogen and carbon. Thus the nitrogen and oxygen contained in the OM must somehow be removed while at the same time preserving the hydrogen-rich organic residue. The formation of petroleum at this point must occur in an oxygen-deficient environment, not be subjected to prolonged exposure to the atmosphere or to aerated surface or subsurface waters containing acids or bases, come into contact with elemental sulfur, vulcanicity , or other igneous activity, and have a short transportation time from the time of death to that of burial. All of these conditions must be met in order to avoid decomposition of the OM. All of this implies that as dead organic matter falls to the sea floor (organic rain), the hydrocarbon constituents needed for creating the end product will be preserved only if the water column through which they are falling is anoxic - lacking living organisms, fall is rapid - the particle size must not entirely be microscopic, bottom dwelling predators are lacking, and there is a rapid sedimentation rate - rapid deposition buries the OM below the reach of mud-feeding scavengers.

7. Once the organic material is buried within the sea floor, transformation begins. It is a slow process that occurs to the OM. The general process can be illustrated by the following formulas: OM + Transformation = Kerogen + Bitumen (by product) Kerogen + Bitumen + more Transformation = Petroleum There are three phases in the transformation of OM into hydrocarbons : Diagenesis , Catagenesis , and Metagenesis ( Tissot , 1997). Diagenesis occurs in the shallow subsurface and begins during initial deposition and burial. It takes place at depths from shallow to perhaps as deep as 1,000 meters and at temperatures ranging from near normal to less than 60oC. Biogenic decay aided by bacteria (such asThiobacillus ) and non-biogenic reactions are the principal processes at work producing primarily CH4(Methane), CO2 (Carbon Dioxide), H2O (Water), kerogen , a precursor to the creation of the petroleum, and bitumen.

8. Temperature plays an important role in the process. Ambient temperatures increase with depth of burial which decreases the role of bacteria in the biogenic reactions because they die out. However, much of the initial methane production begins to decline because it is the bacteria that produces the methane as a by-product during diagenesis . Simultaneous to the death of the bacteria however, the increased temperatures accelerate organic reactions.

9. k=A*e [-E/RT] Arrhenius equation R =Gas constant (0.008314 KJ/mol K) T=absolute temperature E=activation energy A=frequency factor The dependence of chemical reaction rates upon temperature is commonly expressed by:

10. The Catagenesis (meaning thermodynamic , nonbiogenic process) phase becomes dominant in the deeper subsurface as burial (1,000 - 6,000 m), heating (60 - 175 o C), and deposition continues. The transformation of kerogen into petroleum is brought about by a rate controlled, thermocatalytic process where the dominant agents are temperature and pressure

11. The temperatures are of non-biological origin; heat is derived from the burial process and the geothermal gradient that exists within the earth's crust. The catalysts are various surfactant materials in clays and sulfur. Above 200 o  C, the catagenesis process is destructive and all hydrocarbons are converted to methane and graphite. And at 300 o  C, hydrocarbon molecules become unstable. Thus thermal energy (temperature) is a critical factor, but it is not the only factor The time factor is also critical because it provides stable conditions over long periods of time that allows the kerogen sufficient cooking time - exposure time of kerogen to catagenesis . Thus the Catagenesis phase involves the maturation of the kerogen ; petroleum is the first to be released from the kerogen followed by gas, CO 2  and H 2 O.

12. The Third phase is referred to as Metagensis . It occurs at very high temperatures and pressures which border on low grade metamorphism. The last hydrocarbons released from the kerogen is generally only methane. The H:C ratio declines until the residue remaining is comprised mostly of C (carbon) in the form of graphite.

13. Preservation of Organic Matter The biomolecules described before are reduced forms of carbon and hydrogen . Their preservation potential depends crucially on anoxic conditions, i.e. the absence of oxygen that could oxidize them. Stratified basins that prevent vertical circulation and thus the transport of oxygen to greater depths provide excellent conditions for this. An example is the Black Sea, which is salinity-stratified , but many lakes are also anoxic in their deeper waters because of thermal stratification or abundance of nutrients and lack of circulation.

14. Preservation of Organic Matter Access to air (oxygen) rapidly - at geological scales - oxidizes organic matter and converts it into CO2 and H2O. The total carbon content in the Earth ’ s crust is 9 ·1019 kg (the hydroand biosphere contain less than 10-5 of this). Over 80% of this is in carbonates . Organic carbon amounts to 1.2 ·1019 kg and is distributed approximately as follows: Dispersed in sedimentary rocks (~) 97.0 % Petroleum in non-reservoir rocks 2.0 % Coal and peat 0.13 % Petroleum in reservoirs 0.01 % This illustrates the low efficiency of the preservation process.

15. Total Organic Carbon (TOC ) If a rock contains significant amounts of organic carbon, it is a possible source rock for petroleum or gas. The TOC content is a measure of the source rock potential and is measured with total pyrolysis. The table below shows how TOC (in weight percent) relates to the source rock quality . TOC Quality 0.0-0.5 poor 0.5-1.0 fair 1.0-2.0 good 2.0-4.0 very good >4.0 excellent

16. TOC Types TOC in sedimentary rocks can be divided into two types: • Bitumen , the fraction that is soluble in organic solvents such as chloroform • Kerogen , ( κεροσ = wax) the insoluble, nonextractable residue that forms in the transformation from OM Kerogen is an intermediate product formed during diagenesis and is the principal source of hydrocarbon generation . It is a complex mixture of high-weight organic molecules with the general composition of (C12H12ON0.16)x

17. Conversion of OM to HC The principal condition is that this conversion take place in an essentially oxygen-free environment from the very beginning of the process . Anaerobic bacteria may help extract sulfur to form H2S and N , in addition to the earlier formation of CO2 and H2O. This explains the low sulfate content of many formation waters. On burial, kerogen is first formed. This is then gradually cracked to form smaller HC , with formation of CO2 and H2O. At higher temperatures, methane is formed and HCs from C13 to C30. Consequently, the carbon content of kerogen increases with increasing temperatures . Simultaneously, fluid products high in hydrogen are formed and oxygen is eliminated.

18. Dehydrogenization and Carbonization The dehydrogenation and carbonization of organic source n be illustrated with the H:C ratio during the formation of coals : Source material H:C ratio Wood 1.5 Peat 1.3 Lignite 1.0 Bit. coal 0.8 Anthracite 0.3-0.0 Average, in weight %

19. Deoxygenization and Carbonization The deoxygenation and carbonization of the source material is illustrated with the formation of petroleum : Source material O:C ratio Organisms 0.35-0.6 Pyrobitumen ( kerogen ) 0.1-0.2 Petroleum (average) 0.004 Average, in weight %

20. Source Rock Quality The primary factor determining source rock quality is the level of TOC. Additionally, the quality of the source rock is better for higher H:C ratios before thermal maturation. As thermal maturation proceeds and HCs are formed, the kerogen will continuously deteriorate as a source for HC formation.

21. Lipid-rich kerogen ( phyto - and zooplankton ) Sapropelic kerogen ( algae) Humic kerogen (land plants) “ Van Krevelen diagram ” TAI , VR: Maturation indicators

22. Transformations with Depth LOM = level of organic metamorphism; BTU = British Thermal Unit; VM = volatile matter Source: North, F.K . ( 1985) Petroleum Geology, Allen & Unwin

23. Source: North, F.K. (1985) Petroleum Geology , Allen & Unwin Temperature is the single most important factor in thermal maturation. Rate of Maturation

24. Source: North, F.K. (1985) Petroleum Geology, Allen & Unwin Time is the second most important factor in thermal maturation Rate of Maturation ctd .

25. Purposes of Maturation Indicators • To recognize and evaluate potential source rocks for oil and gas by measuring their contents in organic carbon and their thermal maturities • To correlate oil types with probable source beds through their geochemical characteristics and the optical properties of kerogen in the source beds • To determine the time of hydrocarbon generation, migration and accumulation • To estimate the volumes of hydrocarbons generated and thus to assess possible reserves and losses of hydrocarbons in the system .

26. Lopatin ’ s TTI Index V. Lopatin (1971) recognized the dependence of thermal maturation from temperature and time . He developed a method where in the temperatures are weighted with the residence time the rock spent at this temperature. Periods of erosion and uplift are also taken into account. This so-called time-temperature index TTI is still in use, although in variations. The plot on the right shows a simple depiction of it. Rock of age A enters the oil-generating window at time y, while the older rock B has been at that time already in the gas-generating window and will stay there until the present. Source: North, F.K. (1985) Petroleum Geology , Allen & Unwin

27. Other Maturation Indicators Several approaches to quantify the degree of maturation have been proposed aside from the TTI. Most of them are sensitive to temperature and time. • Vitrinite Reflectance (Ro) measures the reflectance of vitrinite (see Kerogen maturation diagram) in oil, expressed as a percentage. It correlates with fixed carbon and ranges between 0.5 and 1.3 for the oil window. Laborious but widely used. • Thermal Alteration Index (TAI) measures the color of finely dispersed organic matter on a scale from 1 (pale yellow) to 5 (black). This index has a poor sensitivity within the oil window (TAI around 2.5 to 3.0) and is not generally used. • Level of Organic Maturation (LOM) is based on coal ranks and is adjusted to give a linear scale.

28. Correlation of TTI, Ro, and TAI

29. The Oil and Gas Windows Source: North, F.K. (1985) Petroleum Geology, Allen & Unwin The Oil and Gas Windows A similar slide as before. It shows clearly at what temperatures oil generation peaks. Gas generation diminishes above ~180°C

30. Oil Source Rock Criteria The criteria for a sedimentary rock to be an effective oil source can be quantitatively described. They are as follows: • The TOC should be 0.4% or more • Elemental C should be between 75% and 90% (in weight) • The ratio of bitumen to TOC should exceed 0.05 • The kerogen type should be I or II (from lipids) • Vitrinite reflectance should be between 0.6 and 1.3%

31. Source: Hunt, J.M. (1995) Petroleum Geochemistry and Geology , 2nd edition. W.H. Freeman & Co This diagram shows the development of biomolecules into petroleum and, with further maturation, into gas (left branch at bottom) which causes the residues to become increasingly more carbon-rich (right branch at bottom) Summary: Origin and Maturation