The Gamma Ray Log

Part 3, Chapter 3: Gamma Ray and Spontaneous Potential

Learning objectives

Explain what the gamma ray measures and why it flags shale
Set clean and shale baselines on a gamma ray curve
Compute the gamma ray index IGR
Recognize how baseline errors bias the shale volume

The Clay Counter

The first curve of the core suite is the gamma ray. It counts the rock's natural radioactivity, which comes from potassium, uranium, and thorium, and those elements concentrate overwhelmingly in clay. So the gamma ray runs high in shale and low in clean sand or carbonate, and it is the suite's primary shale indicator. It works in any hole, cased or open, oil-based or water-based mud, which is why it is on nearly every log ever run.

Baselines and the Index

To make the raw API curve quantitative, pick two baselines: a clean line through the lowest readings (the cleanest sand) and a shale line through the highest (the hottest shale). The gamma ray index is then how far a bed sits between them,

I_{GR} = \frac{GR - GR_{clean}}{GR_{shale} - GR_{clean}}

running from 0 in clean rock to 1 in full shale. Drag the two lines in the figure and the shale-volume track moves with them.

Picking Baselines Well

Those two picks are the first real judgment call in log analysis. Drop the clean line too low and clean rock looks shaly, dragging the shale volume up and the net pay down; raise it too high and shaly rock looks deceptively clean, overstating the pay. Choose the clean baseline in the most quartz-rich sand you can find and the shale baseline in a thick, uncaved shale, and re-check them zone by zone. The index they produce is the input to the shale-volume models of the next section.

References

Asquith, G. and Krygowski, D. (2004). Basic Well Log Analysis, 2nd ed. AAPG Methods in Exploration 16.
Rider, M. and Kennedy, M. (2011). The Geological Interpretation of Well Logs, 3rd ed. Rider-French.