Core plugs give laboratory porosity, permeability and grain density on real rock. Logs give continuous but indirect readings. Calibrating the log interpretation to core is what makes a petrophysical model defensible, and it fails quietly if the two datasets are not at the same depth.
Why depths disagree
Driller depth is measured along drill pipe; logger depth along a wireline that stretches. Core can be lost, or expand once recovered. Offsets of a metre or more between core depth and log depth are common, and they are rarely constant down the well.
The usual workflow
- Run a core gamma scan and match its character to the log gamma-ray curve.
- Apply the depth shifts interval by interval, and record them.
- Compare core porosity with log-derived porosity at matched depths, allowing for the difference in sample volume.
- Adjust interpretation parameters until the log model reproduces the core, then apply it to the uncored intervals.
Skipping the first two steps tends to produce a calibration that looks poor for reasons that have nothing to do with the rock.
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