Vitrinite Reflectance Calculator: Thermal Maturity from Tmax and Ro

If you have ever sat in a prospect review and heard someone ask “but is the source rock even cooked?”, this is the number they were asking about. Vitrinite reflectance, written Ro, measures how much light reflects off polished vitrinite particles in a rock sample. Vitrinite is woody plant material, and as burial temperature climbs over geologic time it becomes progressively more reflective in a way that never reverses. That one-way behavior is what makes Ro such a trusted maturity recorder: it remembers the maximum temperature the rock ever reached, even if the basin later cooled or was uplifted.

The practical questions Ro answers are blunt ones. Has this shale generated oil yet? Has it gone past oil into gas? Is it burned out entirely? A source rock with world-class organic richness is worthless as an oil source if it never got hot enough, and equally disappointing if it got far too hot. Maturity is the gate that organic richness has to pass through.

Getting Ro from Rock-Eval Tmax

Measuring Ro directly under a microscope is careful, slow work, and plenty of wells only have Rock-Eval pyrolysis data instead. Fortunately Tmax, the temperature at which the S2 peak reaches its maximum during pyrolysis, tracks maturity closely enough that a simple linear conversion works well for most marine and coaly source rocks:

Ro = 0.0180 x Tmax – 7.16

with Tmax in degrees Celsius. This is the widely used correlation published by Jarvie and coworkers, originally calibrated on Barnett Shale data and since applied far beyond it. The calculator above accepts either input: give it a Tmax and it estimates Ro, or give it a measured Ro and it shows the equivalent Tmax alongside the maturity classification.

A quick worked example

• A shale sample returns Tmax = 445 deg C from Rock-Eval pyrolysis
• Ro = 0.0180 x 445 – 7.16 = 0.85%
• 0.85% sits inside the 0.55 to 0.9 early oil band, right at the doorstep of peak oil generation
• Interpretation: this rock is actively generating oil but has not yet reached maximum expulsion

Reading the maturity windows

The band boundaries are conventions rather than hard physical switches, but they are remarkably consistent across basins. Below about 0.55% Ro the kerogen is immature and whatever hydrocarbons exist are biogenic or migrated in from elsewhere. From 0.55 to about 0.9% oil generation ramps up. The main oil window, roughly 0.9 to 1.3%, is where expulsion peaks and most conventional oil accumulations trace their charge to. From 1.3 to 2.0% the remaining oil cracks to condensate and wet gas, and beyond 2.0% only dry gas survives. Push past roughly 4% and even methane generation is finished; the rock is overmature and essentially spent.

Where the conversion gets shaky

A few honest caveats before you lean on any converted number. The Tmax correlation was built mostly on Type II marine and Type III terrestrial kerogen; Type I lacustrine source rocks follow a different trajectory and the formula can misplace them by a full maturity window. Tmax itself also becomes unreliable when the S2 peak is small, which happens in lean or overmature samples where there is barely any kerogen left to crack. And suppressed vitrinite, common in hydrogen-rich source rocks, can read 0.1 to 0.2% lower than the true maturity. None of this makes the conversion useless, it just means a converted Ro should be treated as a solid first estimate that a measured profile can later confirm or correct.

One habit worth borrowing from basin modelers: never hang an interpretation on a single sample. Maturity should increase smoothly with depth, so a handful of samples spanning a depth range tells you far more than one number, and any sample that breaks the trend is usually flagging caved material, reworked vitrinite, or a bad pick rather than real geology.

Related Calculators and Reference

If you are working up a source rock story, the natural next steps on this site are the STOIIP / GIIP calculator for volumes once you believe charge happened, the water saturation calculator for testing whether the reservoir actually caught any of it, and the net-to-gross calculator for trimming gross rock down to pay. For the science behind maturity indicators, the AAPG Wiki page on thermal maturity is a solid, freely available reference.