How Dead Plankton Become Crude Oil

It starts with a rain of dead algae — and a lack of oxygen

The story begins in the sunlit top 100 m of the ocean, where phytoplankton fix inorganic carbon into biomass following the Redfield ratio of roughly 106 C : 16 N : 1 P. Photosynthesis writes the net reaction as:

• 106 CO₂ + 16 NO₃⁻ + HPO₄²⁻ + 122 H₂O + 18 H⁺ → C₁₀₆H₂₆₃O₁₁₀N₁₆P + 138 O₂

When these cells die, most of that organic matter is recycled on the way down — respired back to CO₂ by bacteria and zooplankton in the biological pump. Roughly 1% of surface primary production reaches the seafloor intact, and only about 0.1% of primary production ultimately escapes remineralisation to be buried. The single most important control on whether carbon survives is the absence of oxygen. In a normal oxygenated basin, aerobic bacteria strip organic matter down completely (ΔG ≈ −479 kJ per mol CH₂O respired). Survival requires an anoxic bottom water — a stagnant, stratified basin like the modern Black Sea, or an ocean-wide anoxic event. Under anoxia, the electron-acceptor ladder shifts to weaker oxidants — NO₃⁻, then Mn⁴⁺/Fe³⁺, then SO₄²⁻ (sulfate reduction, the dominant marine anaerobic pathway), and finally methanogenesis. Each step yields less energy, decomposes organics more slowly, and lets more carbon slip into the sediment.

Diagenesis: building kerogen, the insoluble parent of all oil

Once buried in fine-grained mud, the organic debris enters diagenesis — the low-temperature (< ~50 °C), shallow reworking that happens in the first few hundred metres over the first few million years. Sulfate-reducing bacteria run reactions like:

• 2 CH₂O + SO₄²⁻ → H₂S + 2 HCO₃⁻ (sulfate reduction; the H₂S then reacts with Fe²⁺ to precipitate pyrite, FeS₂)
• CH₃COO⁻ + H⁺ → CH₄ + CO₂ (acetoclastic methanogenesis, producing shallow biogenic gas)

The labile molecules — sugars, most proteins — are consumed. What resists is the lipid fraction: algal cell-wall biopolymers, long-chain fatty acids, sterols, and pigments. Through condensation, defunctionalisation (loss of –OH, –COOH, –NH₂ groups as CO₂, H₂O, NH₃), and polymerisation, these fragments cross-link into kerogen — a huge, insoluble, three-dimensional macromolecule that is the true precursor of petroleum. Kerogen makes up over 90% of the organic carbon in sedimentary rocks and is the largest organic-carbon reservoir on the planet (~10¹⁶ tonnes C). Geochemists sort it by hydrogen richness on a van Krevelen diagram (H/C vs O/C atomic ratios):

• Type I (H/C > 1.5): lacustrine algae — oil-prone, the richest source.
• Type II (H/C ≈ 1.2–1.5): marine plankton — oil- and gas-prone; most petroleum source rocks.
• Type III (H/C < 1.0): land-plant lignin — gas-prone (this is coal's cousin).

The oil window: thermal cracking under the Arrhenius clock

As burial continues and the geothermal gradient (typically 25–30 °C/km) raises the temperature past ~60 °C, kerogen begins to break apart thermally. This is catagenesis, and the temperature band from about 60 to 120 °C is the oil window. Chemically, it is homolytic cracking of C–C and C–heteroatom bonds via a free-radical chain, closely analogous to industrial thermal cracking but running at a thousandth of the temperature over a billion times the duration:

• Initiation: R–CH₂–CH₂–R′ → R–CH₂• + •CH₂–R′ (a weak C–C bond, ~350 kJ/mol, breaks homolytically)
• Propagation (β-scission): R–CH₂–CH₂–CH₂• → R–CH₂• + CH₂=CH₂; and H-abstraction R• + R″H → RH + R″•
• Termination: R• + R′• → R–R′ (radical recombination)

The kerogen loses its heteroatoms and its cross-links, shedding smaller, hydrogen-rich, mobile molecules — first heavy asphaltenes and resins, then progressively lighter alkanes, cycloalkanes (naphthenes), and aromatics that make up crude oil. Because this is a first-order-ish thermal process, geochemists model it with the Arrhenius equation, k = A·e^(−Eₐ/RT), using activation energies Eₐ of roughly 200–250 kJ/mol distributed across many parallel bonds. The critical insight is the time–temperature trade-off: rock held at 100 °C for 100 million years generates the same oil as rock flashed briefly to a much higher temperature. Maturity is tracked by vitrinite reflectance (%Rₒ): oil generation runs from Rₒ ≈ 0.55 to 1.35%, peaking near 0.9%.

Cook it too long and you get natural gas

The oil window has a hard upper edge. Push past ~120–150 °C (Rₒ > ~1.35%) and you enter metagenesis, the gas window. Here the liquid hydrocarbons already generated are themselves cracked — every remaining C–C bond is a target, and cracking always drives toward the most thermodynamically stable small molecule, methane:

• C₁₀H₂₂ (decane) → C₇H₁₆ + C₃H₆, and ultimately long chains → CₙH₂ₙ₊₂ → (n) CH₄ + carbon-rich residue

Cracking a big alkane into methane is entropically favoured (many small molecules from one large one, ΔS > 0) and becomes spontaneous (ΔG < 0) at high T. The carbon that methane can't carry off — because CH₄ is the most hydrogen-rich hydrocarbon per carbon — is left behind as pyrobitumen, a solid, insoluble, increasingly graphitic residue. Above ~200 °C the system is overmature: even methane is gone or destroyed, and the kerogen has been reduced to graphitic carbon. This is why very deep or very old, deeply buried basins hold only dry gas or nothing at all — the oil has been thermally destroyed. It also explains the depth zonation of a producing basin: shallow biogenic gas, then oil, then wet gas, then dry gas, then barren, top to bottom.

Migration and the trap: why oil pools where it does

Generating oil is only half the problem; oil forms inside a low-permeability source rock (an organic-rich shale or marl) where it can't be extracted. It has to migrate. Newly formed hydrocarbons are less dense than the pore water (oil ≈ 0.8–0.9 g/cm³ vs brine ≈ 1.0–1.2 g/cm³), so buoyancy drives them upward and sideways through connected pore space and micro-fractures — primary migration out of the source, then secondary migration through a permeable carrier bed like sandstone or fractured limestone. The journey ends at a trap: a geometry where an impermeable seal or cap rock (evaporite salt, dense shale, anhydrite) blocks further ascent. Classic traps include:

• Structural traps — anticlines (upfolds) and fault-sealed blocks, where oil collects at the crest.
• Stratigraphic traps — pinch-outs, reefs, and unconformities where reservoir rock is sealed by a lithology change.
• Salt-dome traps — buoyant salt diapirs that pierce and drape strata (the Gulf of Mexico's signature).

In the reservoir, oil, gas, and water separate by density: a free gas cap on top, oil in the middle, and formation brine below, filling only the pore space of the rock — there is no underground 'lake' of oil. Without the coincidence of source, maturity, migration path, reservoir, seal, and trap all in the right place at the right time, no field forms. That is why petroleum is geographically rare and economically concentrated.

The planetary bookkeeping: a slow carbon leak we're reversing in a century

Petroleum formation is one arm of the long-term (geological) carbon cycle — the slow burial of reduced organic carbon that, over hundreds of millions of years, pulled CO₂ out of the atmosphere and left free O₂ behind (every carbon buried as kerogen instead of respired leaves one O₂ un-consumed). This organic-carbon burial, together with silicate weathering, is part of what has kept Earth's surface habitable. The scale is staggering but the rate is glacial: the sedimentary organic-carbon reservoir holds ~10¹⁶ t C, but it accumulated at a net rate equivalent to a tiny fraction of a percent of photosynthesis. Humans now burn conventional oil at ~100 million barrels per day (~4.5 billion tonnes/year), returning in a single year the carbon that took hundreds of thousands to millions of years to bury. Burning it reverses the original chemistry:

• 2 C₈H₁₈ (octane, a proxy) + 25 O₂ → 16 CO₂ + 18 H₂O, ΔH ≈ −5,470 kJ/mol octane

That is the crux of climate change: fossil-fuel combustion is the reversal of ~10⁸ years of biological pump and burial chemistry, run in ~200 years. Atmospheric CO₂ has climbed from ~280 ppm preindustrial to over 420 ppm, and roughly a third of that has dissolved into the ocean, driving surface pH down ~0.1 units (from ≈8.2 to ≈8.1) via ocean acidification. Petroleum is, quite literally, buried sunlight and buried carbon; using it un-buries both. Because kerogen maturation takes millions of years, oil is not renewable on any human timescale — the reservoirs we drain do not refill.

The three maturation stages: what happens to buried organic carbon as depth and temperature rise

Stage	Temperature	Dominant chemistry	Main product
Diagenesis	< ~50 °C	Microbial decay, dewatering, loss of O & N; polymerisation to kerogen	Kerogen + biogenic CH₄
Catagenesis (oil window)	≈ 60–120 °C	Thermal cracking of C–C bonds; H redistribution	Liquid crude oil + wet gas
Metagenesis (gas window)	≈ 120–200 °C	Further cracking; aromatisation; graphitisation of residue	Dry gas (CH₄) + pyrobitumen
Overmature	> ~200 °C	Total cracking; kerogen → graphitic carbon	Only CH₄, then barren