Oil Window Definition / Meaning
The oil window is the range of temperature, and therefore burial depth, in which the kerogen in a source rock breaks down into liquid oil. Shallower and cooler than the window the rock is immature and has generated nothing; deeper and hotter the same rock generates gas instead, and eventually nothing at all. Where a source rock sits relative to the window is one of the first questions asked about any basin, because it decides whether there is anything to find and whether it is more likely to be oil or gas.
The window is defined by temperature and time together. Heating for longer at a lower temperature does the same work as a shorter, hotter burial, which is why a very old, gently buried source rock can be mature while a young, deeply buried one is not.
Where the Window Sits
The oil window is commonly placed between roughly 60 and 120 degrees Celsius. With a typical geothermal gradient of two to three degrees per hundred metres that corresponds to burial depths of about two to four kilometres, but the depth varies from basin to basin with the gradient, and the temperature edges are gradual rather than sharp. Above the window, kerogen is immature. Below it lies the gas window, where oil already generated cracks to lighter hydrocarbons, first wet gas with condensate and then dry methane. Beyond the gas window the source rock is overmature and spent.
Stages of Maturation
| Stage | What happens to the kerogen | Products |
|---|---|---|
| Diagenesis | Low temperature; biological and chemical alteration of buried organic matter into kerogen | Biogenic methane from bacteria, no oil |
| Catagenesis | Thermal cracking with burial; the oil window and then the wet-gas window | Oil, then condensate and wet gas |
| Metagenesis | High temperature; remaining hydrocarbons crack to methane and the kerogen carbonises | Dry gas, then nothing |
How a Rock Is Placed in the Window
Maturity is measured, not guessed. Vitrinite reflectance, written Ro, is the standard: particles of a woody component reflect more light the more they have been heated, and reflectance values of about 0.6 to 1.3 percent bracket the oil window, with wet gas above that and dry gas beyond about two percent. Rock-Eval Tmax, the temperature at which pyrolysis generation peaks, gives an independent check, with values in the mid-430s to mid-460s Celsius indicating the oil window. Basin modelling then reconstructs the burial and temperature history of the rock through time to say not just where the source is now but when it passed through the window, which must be compared with when the traps formed.
Why Timing Matters
A source rock that reached the oil window before the structures above it existed sent its oil to the surface, where it was lost. A trap that formed first and a source that matured afterward is the productive combination. That question of timing is the reason exploration geologists care about the window at all: it links the source rock to the trap through the history of migration, and a basin with excellent reservoirs and seals is still empty if the kitchen cooked too early.
Usage Example
“The vitrinite data put the shale at 0.9 percent Ro under the fault block, squarely in the oil window, and the burial model has it entering the window after the anticline was already there — that is why we are drilling it as an oil prospect.”
The Window and Unconventional Plays
In shale plays the source rock is also the reservoir, so the oil window is mapped directly onto the play. Operators target the band of maturity that yields the fluid they want: the oil window for tight oil, the condensate-rich edge of it for high-value liquids, and the gas window for dry gas. A single formation can be an oil play in one county and a gas play in the next because the window is a matter of how deep the rock was buried, not what it is.