Fossil Fuels: Origin and Formation

• Fossil fuels remain vital for industrialized nations
• They will continue to be critical contributors to world energy for decades

Origin of Fossil Fuels

• Fossil fuels originate as a detour on the right side of the global carbon cycle

Transformation of Organic Matter

• The transformation of organic matter to fossil fuels involves complicated reactions
• Studying these reactions requires knowledge of starting materials, reaction conditions (temperature, pressure, time, catalysis), and reaction mechanisms

Fossil Formation

• Fossils are remnants of once-living organisms preserved in Earth's crust
• Fossil fuels derive from the chemical components of living organisms
• Evidence includes biomarkers in petroleum and coal, such as 2,6,10,14-tetramethylpentadecane (pristane)
• Pristane is found in petroleum samples and living plant waxes
• Petroleum contains optically active compounds, indicating chirality from biochemical processes
• Coals provide visual evidence through coalified plant parts

Carbon Accumulation

• On average, 7 kg of carbon in organic matter results in 1 gram of carbon in fossil fuels
• In 2005, the world's annual fossil fuel consumption was equivalent to 7.5 gigatonnes of carbon, derived from 50 teratonnes of carbon in organic matter
• Environments favoring organism growth share abundant light, moisture, and warmth
• Precursors to fossil fuels accumulated in tropical or subtropical ecosystems like swamps, marshes, river deltas, and lagoons

Diagenesis

• Diagenesis transforms organic matter into kerogen
• The decay process for a simple monosaccharide is: C6H12O6 + 6O2 → 6CO2 + 6H2O
• Oxygen from the air is the key reactant in decay, facilitated by aerobic bacteria
• Aerobic bacteria exist in quantities of 10⁹ per gram of soil and 3 tonnes per hectare
• Effective aerobic decomposition requires oxygen concentrations above 1 mg/l
• Fossil fuel formation necessitates preservation against air or aerobic bacteria action
• Covering organic matter with stagnant water or sediments helps
• Oxygen can diffuse through water or sediment in the first meter of burial, maintaining decay processes
• Fossil fuel formation is a race between decay and burial rates of organic matter.
• Below 1 m, oxygen depletes, stopping aerobic decay
• New processes begin, facilitated by anaerobic bacteria that use sulfates or nitrates as energy sources
• Anaerobic conditions have oxygen concentrations typically less than 0.1 mg/l.
• Reactants for anaerobic reactions include cellulose, hemicellulose, starch, glycosides, lignin, proteins, fats, oils, waxes, steroids, resins, and hydrocarbons
• Organic matter below 1 m forms a rich chemical stew
• Molecules break apart by hydrolysis reactions
• Temperature a few meters into the Earth is close to ambient, so pyrolysis doesn't occur
• Pressure is also near-ambient
• Polysaccharides and glycosides hydrolyze easily under these conditions
• The peptide linkage is also susceptible to hydrolysis
• Enzymes in anaerobic bacteria greatly accelerate these processes, up to 10¹⁰ for polypeptide hydrolysis
• Anaerobic bacteria conduct oxidation processes via oxidative deamination of amino acids
• The product reacts with water to give ammonia and an α-keto acid

Hydrolytic Reactivity

• Hydrolytic reactivity and expected products from organic matter constituents:
  • Saccharides: high reactivity, sugars produced
  • Glycosides: high reactivity, sugars and phenols produced, CO2 + CH4 as secondary products
  • Peptides: high reactivity, amino acids produced, CO2 + NH3 + aldehydes as secondary products
  • Esters: moderate reactivity, fatty acids produced
  • Waxes: low reactivity, fatty acids and long-chain alcohols produced
  • Ethers: very low reactivity, phenols produced
  • Hydrocarbons: no reactivity, none produced

Anaerobic Bacteria

• Anaerobic bacteria attack monosaccharides: C6H12O6 → 3 CH4 + 3 CO2
• Anaerobic reaction involves no molecular oxygen
• Methane produced has the trivial name marsh gas or biogenic methane
• Fats and oils don't hydrolyze extensively, ester groups in waxes are virtually impossible to hydrolyze under mild conditions
• Methoxy and ether linkages in lignin do not react to any extent, resins and alkanes lack hydrolyzable functional groups
• Anaerobic reactions produce fulvic acids from monosaccharides, amino acids, phenols, and aldehydes
• Fulvic acids have molecular weights in the range 700–10,000 Da and dissolve in aqueous acid
• Further reactions lead to humic acids, high molecular weight solids (~10,000–300,000 Da) that dissolve in aqueous base but precipitate when acidified
• At about ten meters, anaerobic bacteria action ceases, consuming most metabolizable material, phenolic compounds function as bactericides

Kerogen Formation

• Actinomycetes produce antibiotic compounds like actinomycin and tetracycline
• The mixture includes humic acids, unreacted fats, oils, waxes, modified lignin, resins, and hydrocarbons
• These combine to form kerogen
• Kerogen structural arrangements resemble phenol-formaldehyde resins
• Kerogen is operationally defined as a brownish-black, high molecular weight, polymeric organic solid insoluble in aqueous base, non-oxidizing acids, and common organic solvents
• Three kerogen types are recognized, based on the dominant source of organic matter
• Kerogen formation continues to depths of about 1000 m, where temperatures might reach ≈50°C
• Kerogen represents the halfway point between organic matter and fossil fuels
• Kerogen formation is sometimes called the biochemical phase of fossil fuel formation

Catagenesis

• Catagenesis transforms kerogen into fossil fuels
• Catagenesis occurs as a result of burial inside the Earth's crust
• Driven primarily by temperature, the natural heat in the Earth's crust.
• Catagenesis is a word developed in petroleum geology
• Heat derives mainly from the decomposition of radioactive materials like ⁴⁰K, ²³²Th, ²³⁵U, and ²³⁸U
• The geothermal gradient varies (typically 10–30 °C/km)
• Unusual events, such as magma intrusion, might create more heating in small areas
• Reaction conditions range from about 60 to several hundred degrees
• Pressures can be elevated due to the weight of overlying rocks or folding during mountain-building
• Reaction times can be thousands to millions of years
• Reactions represent the structural or compositional rearrangement of the components of kerogen and are driven by temperature.
• Radical chemistry dominates, resulting in hydrogen-rich products and carbon-rich products

Atomic H/C Ratio

• A decrease in the atomic H/C ratio of the starting material is accompanied by a decrease in the relative amounts of hydrogen-rich to carbon-rich products
• The possible ratios would be indicated by a decrease in the methane-to-graphite ratio upon complete catagenesis
• The ultimate end points of catagenesis are methane (most hydrogen-rich) and graphite (carbon itself)
• Catagenesis of kerogen normally proceeds only part way to graphite and methane
• The severity of reaction conditions determines the extent of transformation
• In natural systems, hydrogen-rich products increase H/C ratio by utilizing hydrogen pulled out of the carbon-rich products
• As catagenesis proceeds, hydrogen moves from low H/C to high H/C products
• The H-rich or C-rich products are determined by the original kerogen
• Kerogens with higher H/C and low O/C ratios characterize the elemental composition of lipid-rich Types I and II kerogens
• Deposits containing kerogen that transforms to petroleum and/or natural gas are called source rocks
• Source rock quality depends on the amount of organic carbon it contains (1-2% for a good source rock, >4% for excellent)
• Around 10% of the organic carbon dissolves in common organic solvents and this is called bitumen, the insoluble remainder is kerogen

Bond breakage

• Breaking C–C bonds accomplishes reduction in molecular size, thermal cracking is an important part of the process
• Petroleum contains thousands of individual compounds because thermal cracking reactions can result in many products
• The initial cracking of butane can proceed in two ways:
  • CH3CH2CH2CH3 → CH3CH2CH2• + •CH3
  • CH3CH2CH2CH3 → CH3CH2• + •CH2CH3
• C-C bond homolysis in butane gives rise to methyl, ethyl, and propyl
• Butane gives rise to five products: methane, ethane, ethylene, propane, and propylene
• Molecules in bitumen or kerogen are much larger than butane
• Triacontane's initial C–C bond cleavage can result in 29 different radicals
• Radicals undergo processes resulting in stable products
• The number of products from a single, large hydrocarbon can be large, since butane generates all possible alkanes and alkenes, 57 alkanes and 1-alkenes could form
• Further complication comes from that naturally occurring bitumen or kerogen does not consist of a single pure compound
• Natural materials contain branched-chain structures and cyclic structures
• Cracking in nature begins with a mixture of compounds containing roughly 16 to over 40 carbon atoms in linear, branched, and cyclic structures
• Petroleum, or the liquid product that forms from all of these reactions, occurs as a mixture of the hundreds of possible products
• Conversion of kerogen to bitumen begins at approximately 60°C
• One might expect any C–C bond in a given molecule to break, molecules will crack on the interior
• Temperature increase means that the rate of cracking also increases
• Oil formation increases linearly with time and exponentially with temperature
• The molecules become smaller, more amount of C–C bonds will be broken
• Gas hydrocarbons begin to appear around 110°C
• Formation of the liquid petroleum ceases around 170°C
• The ultimate gaseous product of this catagenesis would be methane

Thermal Cracking

• Thermal cracking drives systems to methane but not enough hydrogen is available for complete carbon conversion
• Internal transfer of hydrogen results in the formation of carbon-rich products
• Radical disproportionation and hydrogen abstraction reactions help drive these changes
• Progression from brown coals through anthracites marks a regular transition along the van Krevelen diagram
• Marked by an increase of carbon content
• Systems of coal rank indeed exist, with brown coal assigned the lowest rank and the anthracite the highest
• The deeper a coal is buried, the higher the temperatures it experiences, the longer it is exposed
• The source rock coal has probably been converted to a high rank
• The methane that forms along with the bituminous coals and anthracite has been a source of serious problems
• Slow seepage of methane from the coal into mine air can result in the concentration of methane eventually reaching the explosive limit, about 5-13% methane in air
• Accumulated methane, when released explosively, will blast anything left over
• An outburst will generate high-velocity, flying fragments of coal or rock
• Methane emission through coal seams can affect the greenhouse effect
• Methane from coal seams are a super fuel
• The transformations of kerogens can be summarized on a single van Krevelen diagram
• All kerogens show an initial region of CO2 and H2O formation via decarboxylation and dehydration
• Extensive gas formation accompanies severe thermal decomposition of the organic material
• Coal, oil, and gas could occur together
• Increasing temperature and time shifts materials to higher rank coal