Gamma Ray & SP Logs

Using natural radioactivity and spontaneous voltage to flag shales, spot permeable beds, and hint at brine salinity.

Petroleum EngineeringFormation EvaluationFree preview
⏱️ About 14 min
Gamma Ray & SP Logs — illustration
Illustrative image (AI-generated).

How can a single curve tell shale from clean reservoir rock - and hint at permeability - without ever sampling it?

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The big idea: The gamma ray log reads natural radioactivity to flag shales and estimate shale volume, while the spontaneous potential log deflects opposite permeable beds to reveal permeability and brine salinity.
🎯 By the end, you'll be able to
  • Explain why shales are radioactive and clean sands are not
  • Compute the gamma ray index $I_{GR}$ and use it to estimate shale volume $V_{sh}$
  • Describe how the SP log deflects opposite a permeable bed
  • State how SP is used qualitatively to detect permeability and infer water resistivity
📎 Helpful to know first

The Gamma Ray Log

The gamma ray (GR) log measures the natural radioactivity emitted by the formation, in API units. Shales are radioactive because their clay minerals adsorb uranium, thorium, and potassium. Clean sands and carbonates lack these clays, so they read low on GR. A single GR curve therefore separates shale (high API) from clean reservoir rock (low API) at a glance.

Shale Volume from the GR Index

To quantify how shaly a zone is, we normalize the GR reading between clean and shale endpoints. The linear gamma ray index is $I_{GR} = (GR - GR_{clean})/(GR_{shale} - GR_{clean})$, taken as the first-pass shale volume $V_{sh} = I_{GR}$. Here $GR_{clean}$ is the minimum GR (clean sand or carbonate) and $GR_{shale}$ is the maximum (a nearby shale). Nonlinear corrections (Steiber, Clavier) reduce $V_{sh}$ in older, consolidated rocks, but the linear index is the starting point.

✨ The SP Log

The spontaneous potential (SP) log records a natural voltage generated between the borehole and formation, with no active current source. Opposite a permeable bed the SP deflects from the shale baseline - normally negative when the formation water is saltier (more conductive) than the mud filtrate. That deflection marks permeable rock, and its magnitude relates to the salinity contrast between mud filtrate and formation water, which is the basis for qualitatively estimating $R_w$.

📝 Worked example: A zone reads GR = 75 API. From a nearby clean sand $GR_{clean} = 15$ API, and from a nearby shale $GR_{shale} = 135$ API. Estimate the shale volume $V_{sh}$ using the linear GR index.
  1. Write the linear index: $V_{sh} = I_{GR} = (GR - GR_{clean})/(GR_{shale} - GR_{clean})$.
  2. Substitute: $V_{sh} = (75 - 15)/(135 - 15) = 60/120$.
  3. Evaluate: $60/120 = 0.50$.
  4. Interpret: $V_{sh} = 0.50$ means the zone is 50% shale - moderately shaly.
✓ $V_{sh} = 0.50$ (50% shale). The zone sits halfway between the clean (15 API) and shale (135 API) endpoints, so it is half shale by the linear index.

Check your understanding

1. A clean sand reads GR = 15 API and a nearby shale reads GR = 135 API. A reservoir zone reads GR = 75 API. What is the linear shale volume $V_{sh} = (GR - GR_{clean})/(GR_{shale} - GR_{clean})$?
$(75 - 15)/(135 - 15) = 60/120 = 0.50$, so $V_{sh} = 0.50$.
2. Why do shales read high on the gamma ray log?
Clays adsorb naturally radioactive elements, making shales radioactive; clean sands and carbonates lack these clays and read low.
🔎 Related lessons
Resistivity Logs & Archie's Equation · soon
✅ Key takeaways
  • The GR log flags shales (high API) versus clean reservoir rock (low API) by natural radioactivity.
  • The linear GR index $I_{GR} = (GR - GR_{clean})/(GR_{shale} - GR_{clean})$ gives a first-pass $V_{sh}$.
  • The SP log deflects opposite permeable beds and hints at formation-water salinity and $R_w$.
➡️ Now we use resistivity to turn rock and fluid properties into a water-saturation number via Archie's equation.
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