petrophysics · clay volume
Clay Volume (GR Linear)
IGR = (GR − GRclean) / (GRshale − GRclean)
click formula to derive ↑
Inputs
API
API
API
Description
Estimates clay (shale) volume from the gamma ray log using a linear gamma ray index between clean and shale baselines. This Vcl drives shaly-sand water saturation corrections and net-to-gross calculations.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Vcl | fraction | Clay Volume (Linear) |
| GR | API | Gamma ray reading at the depth of interest. Use a corrected/spectral curve (CGR) where uranium contamination from organic-rich shales is a concern. |
| GRclean | API | Gamma ray value representative of a 0% clay (clean sand or carbonate) baseline, picked from the cleanest interval in the well. |
| GRshale | API | Gamma ray value representative of 100% shale, picked from a thick, clean shale interval nearby. |
Assumptions
- GRclean and GRshale are representative baselines picked from the well being analyzed (or an offset well in the same formation)
- Gamma ray response is dominated by clay content rather than other radioactive minerals (e.g., uranium in organic-rich source rock, K-feldspar, mica)
- No major borehole or casing effects on the GR curve (corrected, calibrated log)
Limitations
- Linear Vcl overestimates clay volume relative to the Larionov non-linear corrections, especially at intermediate IGR values
- Organic-rich mudrocks (high uranium) can produce GR readings unrelated to clay content — use a spectral GR (CGR) or alternative clay indicator
- Single-log methods are less reliable than multi-mineral log analysis in complex lithologies
Use Cases
- → Net-to-gross determination: Apply a Vcl cutoff (e.g., Vcl ≤ 0.35) across the interval to compute the net-to-gross ratio for volumetrics.
- → Shaly-sand Sw correction input: Feed Vcl into Simandoux or Indonesia-equation water saturation models to correct for clay-bound water conductivity.
- → Net pay cutoff screening: Combine with porosity and Sw cutoffs to flag net pay intervals in the net pay cutoffs calculator.
Related Calculations
Region Notes
Permian Basin
Wolfcamp shale benches typically use GRclean≈20-30 API and GRshale≈140-180 API; Larionov Tertiary correction is commonly applied to Spraberry/Wolfcamp intervals.
Eagle Ford
High uranium content in organic-rich facies inflates raw GR — use spectral GR (CGR, with uranium removed) for Vcl rather than total GR where available.
Haynesville
GRshale baselines often exceed 200 API in this over-pressured, organic-rich shale; pick local baselines per well rather than using generic defaults.
Global
Always re-pick GRclean/GRshale per well or field — these baselines are not portable between basins or even between wells with different GR tool generations.
References
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