The Passey Delta-logR Method

Part 16, Chapter 16: Unconventional and Source-Rock Petrophysics

Learning objectives

Overlay resistivity and a porosity log to coincide in a lean baseline
Read the separation in resistivity decades as Delta-logR
Convert Delta-logR to TOC, scaled by the maturity (LOM)
See that the same separation gives different TOC at different maturity

Two Logs You Already Have

The beauty of the Passey method is that it needs no special tool, only the resistivity and a porosity log, usually the sonic. Scale the two so they overlie in a lean, non-source interval. Now read the rest of the well: wherever the rock is organic-rich the two curves pull apart, and the size of the gap is the organic richness.

The Separation Is Delta-logR

Measured in resistivity decades, the separation is

\Delta\log R = \log_{10}\!\frac{R}{R_{base}} + 0.02\,(\Delta t - \Delta t_{base}).

Both terms add: the resistivity rises and the sonic slows, and each pushes its curve away from the other. Where the rock is lean the two terms are zero and the curves lie on top of each other.

Maturity Sets the Scale

The same separation does not always mean the same TOC, because

TOC = \Delta\log R \times 10^{(2.297 - 0.1688\,LOM)},

where LOM is the level of organic maturity. An immature rock converts a given separation into a higher TOC; a mature, gas-window rock into a lower one. So you need a maturity estimate, the subject of the next section, to turn the separation into a number.

References

Passey, Q. R. et al. (1990). A practical model for organic richness from porosity and resistivity logs. AAPG Bulletin, 74(12), 1777-1794.
Crain, E. R. Crain's Petrophysical Handbook.