The Time-Temperature index (TTI) of maturity is a theoretical measure of maturation and oil generation. One needs to be able to model the geological burial history of the area (Depth v Time) and be able to estimate the geothermal history. Allowance has to be made for uplift and erosion as well.
The empirical relationship between vitrinite reflectance and petroleum formation was utilized by Lopatin (1971) to develop a simple method of using both time and temperature to calculate the thermal maturity of the organic matter in sediments.
Lopatin took a burial history diagram and superimposed on it a grid of isotherms at 10°C intervals as shown in the above diagram. He then determined the thermal exposure of the source rock in each time- temperature interval and summed these up to give the total exposure since the time the source rock was originally deposited.
This was called the time-temperature index (TTI) of maturity. The thermal exposure was calculated by multiplying each time interval by a temperature factor based on the old chemical rule that reaction rates double for each 10°C rise in temperature. This is why isotherms at 10°C intervals are used.
Lopatin's technique worked well despite the reaction rates increasing by more than a factor of two with rising temperatures. Although Lopatin's initial calibration for the oil window was based on coals and Type III kerogens, Waples extended the method to other kerogen types. It is now a widely used technique for defining the oil window in sedimentary basins.
Text books give details of how this is done plus more complex integration methods and use of activation energies to yield theoretical maturities in terms of Ro% and TTI, resulting models all yield the same general picture of Hydrocarbon Generation.