The Photoelectric Factor

Part 12, Chapter 12: Lithology and Mineralogy

Learning objectives

Explain why the photoelectric factor reads the matrix mineral
State the Pe values of quartz, calcite, dolomite, and anhydrite
Show that Pe is nearly porosity-independent while density is not
Recognize that Pe alone cannot separate calcite from anhydrite

A Log That Reads the Mineral

Every porosity log so far has mixed lithology with porosity: a given bulk density could be a tight sand or a porous limestone, and you could not tell which from the density alone. The photoelectric factor breaks that tie. The $P_e$ curve (in barns per electron) measures the average atomic number of the formation, which is fixed almost entirely by the matrix mineral: quartz reads about 1.8, dolomite 3.1, and calcite and anhydrite about 5.

Why Porosity Barely Moves It

The reason $P_e$ ignores porosity is that it mixes through the volumetric cross section $U = P_e,\rho_e$ , and the logged value is

P_{e,\text{log}} = \frac{U_{\text{log}}}{\rho_{e,\text{log}}}, \quad U_{\text{log}} = P_{e,ma}\,\rho_{ma}(1-\phi) + P_{e,fl}\,\rho_{fl}\,\phi.

The pore fluid has a tiny photoelectric factor (water 0.36, oil 0.12, gas near zero), so it barely dilutes the matrix term. A limestone reads $P_e \approx 5$ whether it is tight or thirty percent porous, while the bulk density over that same range sweeps from 2.71 down past 2.0, straight through the grain densities of other minerals. So $P_e$ answers which mineral and $\rho_b$ answers how much pore.

The Limits

The photoelectric factor is robust but not omniscient. Calcite and anhydrite both read about 5, so $P_e$ alone cannot separate them, and the density log has to finish the job. A barite-weighted mud, full of high-atomic-number barium, swamps the measurement and makes $P_e$ useless. And clay carries a moderate $P_e$ that pulls a shaly reading off the clean-mineral value. $P_e$ is the first and cleanest lithology indicator; the crossplots that follow combine it with the density and neutron to resolve genuine mineral mixtures.

References

Ellis, D. V. and Singer, J. M. (2007). Well Logging for Earth Scientists, 2nd ed. Springer.
Schlumberger (2009). Log Interpretation Charts. Schlumberger Educational Services.