Reading a Well: The Log Display
Learning objectives
- Read the standard depth-down, multi-track log display
- Identify shale and clean sand from the gamma ray
- Use deep resistivity to tell hydrocarbon from water
- Connect the log display to the interpretation chain on the Ogbon-1 well
The Display
Everything so far comes together in the way logs are actually presented. Depth runs down the page, and the curves are grouped into side-by-side tracks, each carrying one or more measurements on a fixed scale. The eye learns to read the tracks together: a clean sand here, a shale there, a resistive streak that might be pay. The figure is your first look at the Ogbon-1 well, the same well the rest of the course evaluates end to end. Drag the marker and read, at any depth, what the tools see and what the rock is.
The Three Looks
Track one is the gamma ray: it reads high in shale (clay is radioactive) and low in clean sand, so it is the quick-look shale flag. Track two is deep resistivity on a logarithmic scale: hydrocarbon resists electric current and reads high, while water and shale conduct and read low, so a resistive clean sand is the classic pay signature. Track three overlays bulk density and neutron porosity: the two curves sit together in clean wet rock and split apart in shale, and together they give porosity and a clue to lithology.
From Logs to Rock
Reading the display is the start of the chain you met in the first section. The gamma ray will become shale volume, the density will become porosity, and the resistivity, with that porosity, will become water saturation. The flag strip on the right is where it all lands: the net pay. Every later chapter sharpens one of these readings, and the capstone returns to this exact well to turn its logs into a complete evaluation.
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.