Time-Temperature index (TTI) of maturity




Orcadian Basin Model

The Maturity of source rocks in a study area, particularly where no direct measurements are available, can be estimated by theoretical modelling. The depth of source rocks through geologic time (Burial History) and estimated Geothermal Gradient yields the time each element of the source has remained within a temperature interval. From this TIME - TEMPERATURE INDEX can be calculated.

The sum of all the Temperature Elements from Time of Origin to present day.  Where T is Temperature &  t is time





10 - 15

Early Oil Generation 0.56


Peak Oil Generation 0.78


Late Oil Generation 0.88


Wet Gas Generation 1.2


Dry Gas Generation  

It is clear, from the Burial History and Maturity Curves above, that for the West Mainland example the L Stromness to Rousay Flagstones are Early Mature which is consistent with the measured data of Vitrinite Reflectance (Ro%) and Tmax.


This type of analysis is only relevant for the single location. Modelled analysis becomes more complex when he spatial coordinates  extend from 2 and 3 dimensions - the difficulty is not in calculation but in representation.

The W-E Cross Sections below show the progress of maturity over a linear 120km section across Orkney in the centre

The progress of maturation can be visualised in Time and Space by downloading and playing the MPEG Video (5MB) :

Orkney Maturity X-Section.MPG

In this ANIMATION and in the sections below the Blue layer is the L Stromness Flagstones. The heavy lines are lithological boundaries the lighter lines and numbers are Vitrinite Reflectance (Ro%). The moving red area represents Early Oil Window with Ro% between 0.6% and 0.8%.

Note that over Orkney after the Carboniferous the source rocks are always in the "RED" zone, early mature, so major oil generation and migration is offshore to the East and West.

Deposition of SFB





Carboniferous & Permian Deposition





Dyke intrusion & Permian Inversion / Uplift





Slow Uplift





Stromness Flagstones Exposed at Surface