Every barrel of oil starts in a source rock. Without one, the other elements of the petroleum system have nothing to store or trap. The source rock is, quite literally, the kitchen where hydrocarbons are cooked.
Source rocks are organic-rich sedimentary rocks, usually marine shales, containing kerogen — the preserved remains of plankton and algae that, when heated by burial, transform into oil and gas.
How a source rock forms
Source rocks form where large amounts of organic matter accumulate and are preserved rather than decayed. This typically happens in quiet, oxygen-poor settings such as deep marine basins, where dead plankton and algae sink and are buried under fine sediment before they can fully break down. Over time these organic-rich muds compact into shale.
The preserved organic matter becomes kerogen — a solid, insoluble organic material disseminated through the rock. Kerogen is the raw feedstock from which oil and gas are later generated.
Solid organic matter in source rock, formed from the buried remains of plankton and algae. Heat and burial break it down into liquid and gaseous hydrocarbons.
Maturation and the oil window
Generating hydrocarbons requires cooking the kerogen, and the oven is the Earth itself. As the source rock is buried deeper, temperature rises, and that heat breaks kerogen down into hydrocarbons through a process called maturation. Temperature, not just depth, is the key control:
- The oil window — roughly 60–120°C, where kerogen breaks down mainly into liquid oil. This is the sweet spot for oil generation.
- Higher temperatures — beyond the oil window, continued heating cracks the molecules further and generates mostly natural gas.
- Too cool — if the rock never reaches the oil window, the kerogen stays immature and little hydrocarbon is generated.
Once generated, the oil and gas are expelled and begin their journey via migration toward a reservoir. When hydrocarbons never leave the source rock — because it is too tight to expel them — that same rock becomes an unconventional shale resource produced directly with horizontal drilling and fracturing.
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Frequently asked
A source rock is an organic-rich sedimentary rock, usually a marine shale, that contains kerogen — solid organic matter formed from the buried remains of plankton and algae. Heat and burial convert that kerogen into oil and gas.
The oil window is the temperature range, roughly 60–120°C, in which kerogen in a source rock breaks down mainly into liquid oil. Below it the rock is immature; above it, continued heating generates mostly natural gas.
Kerogen is the solid, insoluble organic matter in a source rock, derived from the preserved remains of plankton and algae. When heated through burial, it is converted into the liquid and gaseous hydrocarbons we produce as oil and gas.