Geol Supplementary Notes 463-RWR-6,7 GEOL RWR-6 ORGANIC COMPOSITION OF PETROLEUM COMPOSITION OF LIVING ORGANISMS

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1 GEOL RWR-6 ORGANIC COMPOSITION OF PETROLEUM Recommended sections to read in the textbook: Chapter 4 COMPOSITION OF LIVING ORGANISMS The main components of living organisms: Carbohydrates Sugar chains Rapidly break down; unstable Include cellulose, starch chitin Lignin Walls of higher plants Very resistant to decay Contains aromatic rings Tannin Contains aromatic rings Proteins Amino-acid polymers Contain most N-compounds in organic matter Rapidly decomposed Include enzymes, hemoglobin, structural components of shells, corals, sponges, etc. Lipids Animal fats and vegetable oils Insoluble in water Include spores, fruit, muscle, waxes, etc. Some contain paraffin chains 1

2 Plant and animal pigments Essential oils Resins Average Chemical Composition of Natural Organic Substances in weight % (from Hunt, 1996: Table 4.1) Substance C H S N O Carbohydrates Lignins Proteins Lipids Petroleum If we compare the C / N + S + O atomic ratios for each group: Carbohydrates: ~ 1 : 1 Proteins: ~ 3 : 1 Lipids: ~ 10 : 1 Diagenetic degradation of equal amounts of each group should produce more hydro-carbons from lipids (H-rich) than the other two groups. Composition of living matter in weight, dry, ash-free basis (from Table 4.2 in Hunt, 1996) Plants Proteins Carbohydrates Lipids Lignin Spruce wood Oak leaves Pine needles Phytoplankton Diatoms Lycropodium spores Animals Zooplankton Copepods Oysters Higher invertebrates Plants: mainly carbohydrates and lignin; Animals: protein and carbohydrates. LIPIDS AND LIGNIN ARE MOST RESISTANT TO DECAY. 2

3 DIAGENESIS of organic matter leads from BIOPOLYMERS synthesized by organisms through humin to KEROGEN, a GEOPOLYMER, by partial destruction and rearrangement of the main organic building blocks: Evolution of organic matter to kerogen Proteins Carbohydrates Microbial degradation Amino-acids Sugars Polymerization Condensation Re-enter the biological cycle Used by microbes for energy CO 2, H 2 O Humic- and flulvic acids Humin Lipids Hydrocarbons Preserved with little alteration Increasing polymerization, condensation, insolubilization Geochemical fossils KEROGEN Between modern organisms and recent sediments, the main changes are: A large decrease in carbohydrates An increase in lignin-humus, and N-compounds Early changes caused by chemical and microbial reactions hydrolyze some OM to sugars and simple molecules that polymerize to form lignin-humus and nitrogenous compounds that are the precursors of KEROGEN. 3

4 DIAGENESIS OF ORGANIC MATTER organic matter WATER COLUMN SEDIMENT DIAGENESIS Minerals Interstitial (pore) water Organic matter Benthic microbes C (few hundred to thousand m) Residence time in upper metre: CATAGENESIS During early diagenesis, the composition of the organic matter changes, and its potential to produce petroleum is partly determined. Microbial degradation begins in the water column: Biomass: up to 500 g/m 3 Less than 10% of the organic matter settles to the bottom. Much of that arrives in fecal pellets. Much of it is degraded in the water column, initially by oxidation. Microbiological degradation of organic matter is due to the activities of bacteria, fungi, and protozoa, and other microbes. In the oxic zone, directly below the sediment/water interface, the main reaction is oxidation of organic matter (microbial induced): 4

5 CH 2 O + O 2 = CO 2 + H 2 O Oxygen can penetrate to depths of 20 cm in permeable sands, but water circulation and diffusion in fine sediments are slow, limiting possibilities for oxygen renewal. Aerobic breakdown is thus more effective in coarse-grained sediments than in fine-grained. The environment in the sediments becomes reducing as oxygen becomes depleted. The ph may rise, and carbonate concretions and nodules may form. The products of oxidation of organic matter are H 2 O, CO 2, SO 4, and NH 3. The depth of the oxic zone can be < 1 cm to several m. ANOXIC ZONE In the anoxic zone, anaerobic microbes use enzymes to decompose proteins, carbohydrates and lipids into simpler molecules, often by fermentation and reduction. Three zones are typically present: Nitrate Reduction After oxygen is depleted, NO 3 - is used as an energy source: When nitrate is exhausted: Sulphate Reduction - 6CH 2 O + 4NO 3 = 6CO 2 + 6H 2 O + 2N or 5CH 2 O + 4NO 3 = 2N 2 + 4HCO 3 + CO 2 + 3H 2 O 2CH 2 O + SO 4 2- = H 2 S + 2HCO 3 - SO 4 2- = S + 2O 2 (mainly by Desulfovibrio bacteria) If iron is available in the sediment, H 2 S may combine with Fe to form pyrite. This process explains why pyrite is so common in black, organic-rich shales (and coal, which also accumulates under reducing conditions). If no Fe is available, the H 2 S may migrate into oxic zone (to become SO 4 2- ) or may combine with organic molecules. This can result in an S-rich crude oil. 5

6 After sulphate is depleted: Methanogenesis (fermentation) Methanogens produce CH 4 from the residue of the overlying zones: e.g., CH 3 COOH (cellulose) = CH 4 + CO 2 (acetate fermentation) CO 2 + 8H + = CH 4 + 2H 2 O (CO 2 reduction) These processes give rise to swamp gas (CH 4 ), as well as CO 2, H 2, H 2 S, NH 3 and P 2 O 5. Biogenic gas may form commercial accumulations (e.g., Western Siberia) or be a hazard (fire, blowouts) during shallow drilling. Anaerobic degradation of OM is much less efficient that aerobic (oxic) degradation. It produces a more reduced (H-rich) residue, rich in lipids. The early microbial reactions: remove much of the N, S, O, and P lead to an early concentration of C and H in the residue Microbial activity is most intense in continental shelf settings, decreasing with increasing water depth. It is lowest in deep oceanic floors and below stratified (anoxic) waters. Residual organic products following anaerobic diagenesis continue to transform into kerogen by partial destruction and rebuilding of the organic building blocks. The many processes include polycondensation, which may take place during degradation. This involves rebuilding (condensation) of organic residues into large organic molecules. Humin is a partially soluble product of this process. Further condensation (insolubilization) eliminates much of the remaining nitrogen and converts the humin into insoluble kerogen. These processes occur at depths of 10s to 100s of metres over periods of ~ 1 million years. Many inorganic reactions also take place during this phase, such as decarboxylation, which converts fatty acids to paraffins, and H-disproportionation, which depletes on enriches molecules in hydrogen. 6

7 GEOL RWR-7 KEROGEN AND CATAGENESIS Recommended sections to read in the textbook: Chapters 5 (esp. pages ) and 10 (esp. pages ) Kerogen Kerogen, which is macro-molecular complex with a polymer-like structure, is the fraction of sedimentary organic matter that does not dissolve in organic solvents. It forms in the upper few hundred metres of the sediment column from organic precursors that have been modified during diagenesis. It may include organic particles with a recognizable morphology, including algal and fungal spores, cuticles and remains of woody tissue. Kerogen resists oxidizing acids, and can be recovered from sedimentary rocks by dissolving them in HCl or HF, or by using heavy liquids of different density: kerogen is lighter than minerals. Kerogen can then be studied using a range of optical and spectroscopic methods. Although difficult to analyse, heating in an inert atmosphere (pyrolysis) will break it up into small fractions that can be analysed by mass spectrometry and gas chromatography. Kerogen is usually classified according to type (Tissot and Welte, 1984, Petroleum Formation and Occurrence, Springer), based mainly on its H/C ratio and O/C ratio. Several variations on the original classification have been published, but three main types are generally recognized: TYPE I (Sapropelic) High initial H/C ratio ( ) Low initial O/C ratio (<0.1) Derived from microbial breakdown of spores, planktonic algae, animal organic matter Enriched in lipids (e.g. fatty acids, oils, alcohols, waxes) Produces mainly oil with thermal maturation Typical kerogen in oil shales (freshwater and marine), but rare compared with Type II TYPE II Intermediate initial H/C ratio ( ) Fairly low initial O/C ratio ( ) Can be S-rich Commonly derived from phytoplankton, zooplankton and other marine organisms deposited under reducing conditions; also minor plant material Most common and richest source rocks for oil 7

8 TYPE III (Humic) Low initial H/C ratio (<1) High O/C (>0.2) Derived from organic matter from land plants such as lignin, tannins and cellulose Generates abundant CO 2 and methane (CH 4 ). Coal has a similar composition and structure CONVERSION OF KEROGEN TO OIL AND GAS Conversion of kerogen to petroleum needs temperatures of at least C (equivalent to 1 2 km of burial) and a long period of geological time. The optimum temperature range for maturation is C, equivalent to burial depth of about 3 4 km for a typical geothermal gradient (i.e C). Temperature increases with increasing burial, and causes carbon-carbon bonds in the organic molecules of the kerogen to break. This is termed cracking. With rising temperature, more C-C bonds are broken, both in the kerogen and in the hydrocarbon molecules that had formed previously. Cracking leads to formation of lighter hydrocarbons from long hydrocarbon chains in the kerogen. Van Krevelen Diagram CO 2, CH 4, H 2 O OIL GAS Cg: catagenesis Mg: metagenesis H/C Ratio Cg Diagenesis MAIN ZONE OF OIL FORMATION Immature zone 0.5 Residual OM wet Zone of gas formation dry O/C ratio 8

9 The removal of CH 4 and other light hydrocarbons leaves the residual kerogen relatively enriched in carbon, because initially Types I and II kerogen have a H/C ratio of Type III kerogen, which has a high initial oxygen content, produces mainly CO 2 gas, thus its O/C ratio gradually falls. These relationships can be shown on a Van Krevelen diagram (previous page), which plots the changes in the H/C and O/C ratios during the course of hydrocarbon evolution. The alteration that begins at C continues until the H/C ratio is about 0.6 and the O/C ratio is less than 0.1 at about 150 C. The peak of oil production (catagenesis) is reached at about C. At higher temperatures, the longer hydrocarbon chains will have already cracked, leaving only gas, mainly methane (dry gas). During this transformation, the kerogen composition will gradually move toward pure carbon (H/C = 0). On completion of oil maturation: aliphatic structural components, derived from lipids, fatty acids and proteins, etc., will have been converted into hydrocarbons there will be a reduction in molecular weight of the hydrocarbons but aromatization (sharing H+ to make larger molecules) may have occurred the number of n-paraffins will have increased geochemical fossils will have been liberated 50 C GAS Diagenesis Biogenic gas: CH 4, CO 2, H 2 S, etc. Heavy oil and gas 120 C 150 C OIL Catagenesis (OIL WINDOW) Metagenesis Medium and light oil and gas Condensate and wet gas Dry gas (methane) Residuum (graphite) The oil window 9

Lecture 19: Soil Organic Matter

Lecture 19: Soil Organic Matter Effects of OM Properties on Soil Property Dark color Excellent water retention Binds to clay minerals Metal chelation Low water solubility ph buffering High CEC Nutrient