What it is
A T2 distribution is the primary output of an NMR measurement. After an RF pulse excites nuclear spins in a magnetic field, the signal decays back to equilibrium over a range of characteristic times. These times — T2 relaxation times — depend on the molecular environment of the nuclei. Fluids in small pores and tightly bound to surfaces relax faster (short T2). Fluids in large pores or free in solution relax slower (long T2).
By inverting the observed signal decay into a distribution across relaxation times, you can infer the relative proportions of different fluid environments in the sample. This distribution is sometimes visualized as a T2 histogram with the short-T2 peak representing bound water and the long-T2 peaks representing free water and hydrocarbons.
Why it matters
In petrophysics, the T2 distribution is the core diagnostic for:
- Distinguishing bound from producible fluids in a reservoir
- Estimating irreducible water saturation
- Inferring pore-size distribution
- Identifying hydrocarbon presence in challenging formations (organic shales, carbonates, clay-rich rocks)
For remote subsurface workflows, the analogous interpretation is at a much coarser scale — classifying saturation states across polygons rather than per-depth-point within a wellbore — but the underlying signal physics is the same.
Common pitfalls
T2 distributions are sensitive to:
- Paramagnetic and ferromagnetic minerals. Iron-rich formations distort the signal and require corrections.
- Signal-to-noise ratio. Noisy acquisitions produce unreliable distributions regardless of interpretation sophistication.
- Calibration. The interpretation of relaxation cutoffs is formation-specific and requires regional calibration against drilled ground truth.