Kerogen Definition / Meaning
Kerogen is the solid, insoluble organic matter found in sedimentary rock — the altered remains of plankton, algae and land plants that were buried before they could decay. It is the substance that oil and natural gas are made from. When a kerogen-rich rock is buried deep enough and heated for long enough, the kerogen breaks down into liquid and gaseous hydrocarbons, and the rock that held it is called a source rock.
The word matters because it separates two things that are easily confused: organic matter that is still locked in the rock as a waxy solid, which is kerogen, and hydrocarbons that have already been generated from it, which are oil and gas. A rock can be rich in kerogen and have produced nothing yet.
How Kerogen Forms
Organic material that settles in oxygen-poor water — a stagnant sea floor, a deep lake, a swamp — is not fully eaten by bacteria. Mud buries it, compaction squeezes the water out, and over millions of years the mixture of proteins, fats and cellulose is transformed by low-temperature reactions into a complex, high-molecular-weight material that no solvent will dissolve. That is kerogen. The rock around it, usually a dark mudstone or shale, is a potential source rock; whether it becomes an actual one depends on what happens next.
The Four Types
| Type | Origin | Tends to generate |
|---|---|---|
| Type I | Algae in lakes; hydrogen-rich | Mostly oil, in large amounts; rare but prolific |
| Type II | Marine plankton and algae | Oil and gas; the source of most of the world’s petroleum |
| Type III | Woody land plants; oxygen-rich, hydrogen-poor | Mainly gas, and coal |
| Type IV | Reworked, oxidised or charred material | Essentially nothing; inert |
Type is inferred from the hydrogen and oxygen content of the kerogen relative to its carbon, plotted on a van Krevelen diagram, and it tells an explorer before any well is drilled whether a basin is more likely to hold oil or gas.
Measuring It
Three laboratory measurements describe a kerogen-bearing rock. Total organic carbon (TOC), in weight percent, says how much organic matter is there; a source rock generally needs at least a few tenths of a percent and a good one has several percent. Rock-Eval pyrolysis heats a crushed sample in stages and records the hydrocarbons already present (S1), those generated from the kerogen as it cracks (S2), and the temperature at which generation peaks (Tmax), which together give the hydrogen index and a maturity estimate. Vitrinite reflectance measures how much light a specific woody particle reflects under a microscope, which rises steadily with heating and is the standard yardstick of thermal maturity.
From Kerogen to Oil and Gas
Kerogen does nothing until it is heated. With burial the temperature climbs, and in the oil window the long molecules crack into liquid hydrocarbons that are expelled from the source rock and begin migrating toward a trap. Buried deeper and hotter still, the remaining kerogen and any oil left behind crack again into gas. Heated beyond that, only a carbon residue remains. The whole progression is called maturation, and a source rock is described as immature, mature or overmature according to where it sits on that path.
Usage Example
“The core came back with four percent TOC and a hydrogen index over 500, so it is a Type II kerogen that has barely entered the oil window — a good source rock that simply has not been cooked yet.”
Oil Shale Is Not Shale Oil
The two phrases describe opposite things. Oil shale is a rock rich in kerogen that was never buried deep enough to generate oil; the hydrocarbons have to be made artificially by mining the rock and heating it in a retort. Shale oil, or tight oil, is real crude that a mature source rock did generate but which never migrated out, and which is produced from the shale itself by horizontal drilling and hydraulic fracturing. Both are kerogen stories at different stages of the same process.