Coal, oil and natural gas are nonrenewable resources: they store energy captured from sunlight by living things millions of years ago. Where and how they form follows from depositional environments and the geothermal gradient.
Why it works
Most oil and gas began as microscopic algae and plankton that settled into seafloor mud where oxygen was low, so they did not fully rot. Buried kilometers deep, the organic-rich mud (now shale, the source rock) warms along the geothermal gradient. Between about $60$ and $120^\circ\text{C}$ (the oil window) the organic matter breaks down into oil; hotter, it cracks into natural gas. Oil and gas are lighter than water, so they seep upward through the rock until something stops them. An oil field needs a porous, permeable reservoir rock (sandstone or limestone), an impermeable seal above it (shale or salt), and a trap that gathers the oil, such as the crest of an anticline. Coal has a different origin: plants in swamps, buried as peat and turned by heat and pressure into lignite, bituminous coal and anthracite.
Key idea
Oil and gas form from buried plankton and algae cooked at about 60 to 120 °C, then migrate up into porous reservoir rock and collect under a seal in a trap. Coal forms from buried swamp plants.
Worked examples
Worked example: Depth of the oil window
In a basin, the surface is $15^\circ\text{C}$ and the gradient is $30^\circ\text{C/km}$. Between what depths does the oil window lie?
- Top ($60^\circ\text{C}$): $(60 - 15) \div 30 = 1.5\,\text{km}$.
- Bottom ($120^\circ\text{C}$): $(120 - 15) \div 30 = 3.5\,\text{km}$.
Answer: From about 1.5 to 3.5 km deep.
Worked example: Name the parts
In an oil field, oil fills the pores of a sandstone under a layer of salt, at the top of a dome-shaped fold. Name the reservoir, seal and trap.
- Reservoir: the porous sandstone that holds the oil.
- Seal: the salt, which oil cannot pass through.
- Trap: the dome (an anticline in every direction), which gathers the oil at its top.
Answer: Sandstone reservoir, salt seal, dome trap.
Common mistake
A popular idea is that oil comes from dinosaurs. Most oil formed from tiny algae and plankton buried in seafloor mud; there were never nearly enough dinosaurs to make it, and many oil fields are far older or younger than the dinosaurs. Check the source rocks: they are fine-grained marine shales rich in microscopic organic matter.