Coal, Oil & Natural Gas: Formation & Traps
Ancient sunshine trapped in stone — but coal and petroleum form through completely different geologic stories.
The gasoline in your car and the electricity from a coal plant are both fossil fuels, but they form through completely different geologic stories. Coal is compressed plant matter that never left the swamp. Oil and gas are fluids that migrate through pore spaces like water through a sponge, until a sealing cap stops them. Understanding the difference matters for finding, extracting, and managing these resources.
Two different stories
Coal and petroleum are both fossil fuels, but they form through unrelated processes. Coal is a solid organic rock made of compressed plant matter. Petroleum (oil and natural gas) is a fluid hydrocarbon mixture that forms from microscopic organisms buried in sediment. Confusing the two leads to the common error of applying petroleum-system logic to coal or vice versa.
Coal — from swamp to anthracite
Coal begins in oxygen-poor swamp environments where dead plant matter accumulates faster than it can decay. This raw organic mush is called peat. As burial deepens, pressure and temperature drive off water and volatiles, increasing the carbon content:
- Peat → ~60% water, lowest energy content.
- Lignite ('brown coal') — still soft, high moisture, low heat.
- Sub-bituminous — intermediate rank, common for power generation.
- Bituminous ('soft coal') — harder, higher carbon, widely used.
- Anthracite ('hard coal') — metamorphic-grade, >86% fixed carbon, highest heat content.
Progressing from peat to anthracite is a one-way journey of increasing carbon content, decreasing volatiles, and rising rank. You cannot 'un-cook' a coal.
The petroleum system
Petroleum forms from microscopic plankton and algae buried in fine-grained sediment on ancient seabeds. If buried deeply enough (typically 2–5 km) and heated to 60–150 °C over millions of years, organic matter transforms into liquid oil and natural gas. Higher temperatures favour gas generation; beyond ~200 °C most hydrocarbons break down. But formation is only the beginning — four elements must come together:
- Source rock — organic-rich fine sediment (usually shale) where hydrocarbons are generated.
- Reservoir rock — porous and permeable rock (usually sandstone or limestone) that can hold and transmit fluids.
- Seal / cap rock — impermeable rock (usually shale or salt) that prevents hydrocarbons from escaping upward.
- Trap — a geologic structure (fold, fault, salt dome, or stratigraphic pinch-out) that concentrates the petroleum.
Without all four, you have no producible petroleum accumulation.
Conventional vs unconventional reservoirs
Conventional reservoirs have good porosity and permeability, so oil and gas flow freely to a well. Unconventional reservoirs are trickier:
- Shale oil/gas — the source rock itself is the reservoir, but permeability is extremely low. Hydraulic fracturing ('fracking') creates artificial cracks to release hydrocarbons.
- Tar sands / oil sands — extremely viscous oil mixed with sand. Surface mining or steam injection is needed.
- Coal-bed methane — natural gas adsorbed onto coal surfaces, released by pumping out water.
- An anticline can form a structural trap if a porous sandstone reservoir is capped by impermeable shale.
- Oil seeps indicate an active petroleum system — source, migration, and escape — so a sealed trap nearby may have accumulated oil.
- The crest of the anticline is the highest point of the reservoir; buoyant oil migrates upward and would pool there beneath the seal.
- Recommendation: Yes, the anticline crest is the most logical drilling target.
- Total carbon increase = 95% − 25% = 70 percentage points.
- Sample increase = 75% − 25% = 50 percentage points.
- Fraction = 50 ÷ 70 = 0.71 (rounded). This coal is roughly bituminous rank.
- Pore volume = 1,000 m³ × 0.20 = 200 m³.
- Oil volume = 200 m³ × 0.40 = 80 m³.
- Even in a good reservoir, most of the rock volume is solid grain material, not oil.
Check your understanding
- Coal forms from compressed plant matter in swamps, progressing through peat → lignite → sub-bituminous → bituminous → anthracite.
- Petroleum requires four elements: source rock, reservoir rock, seal/cap rock, and trap.
- Oil and gas occupy pore spaces in reservoir rock, not underground lakes.
- Conventional reservoirs flow freely; unconventional reservoirs (shale, tar sands, coal-bed methane) need special extraction techniques.
🎓 Go deeper: university courses & trusted references
Handpicked external material for this module — for when you want the full university treatment of earth resources & environmental geology.
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