petrophysics · clay volume
Clay Volume — Clavier Correction
Vcl = 1.7 − √(3.38 − (IGR + 0.7)²)
click formula to derive ↑
Inputs
API
API
API
Description
The Clavier correction is an alternative non-linear GR-to-Vcl transform that gives estimates intermediate between the linear IGR and the Larionov corrections. It is derived from a different empirical dataset and is sometimes preferred in Middle East carbonates and North Sea settings where the Larionov corrections were not originally calibrated.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Vcl | fraction | Clay Volume (Clavier) |
| GR | API | Gamma ray reading at depth. Use spectral CGR where uranium contribution from organic matter may inflate the GR above the true clay-mineral response. |
| GRclean | API | GR baseline for a clean (0% clay) interval — the minimum reading that represents zero clay content. |
| GRshale | API | GR baseline for 100% shale — picked from the thickest, most uniform shale interval near the reservoir. |
Assumptions
- GR response is primarily from clay minerals; K-feldspar or uranium enrichment will bias Vcl upward
- Baselines (GRclean, GRshale) are representative of end-members in the formation being evaluated
Limitations
- No single GR-to-Vcl correction is universally superior — always calibrate to core clay volume (XRD) where available
- As with all GR-based methods, insensitive to clay type: illite, smectite, and kaolinite have different impacts on resistivity and porosity but similar GR response
Use Cases
- → Middle East carbonate formation evaluation: In mixed carbonate-shale sequences, Clavier is often used as an alternative to Larionov where the calibration dataset is closer to Middle East depositional environments.
- → Cross-check against Larionov methods: Run Clavier alongside Larionov Tertiary and Older Rocks corrections; the envelope of Vcl estimates from all three methods quantifies the uncertainty in clay volume.
- → Net pay sensitivity analysis: Apply different Vcl cutoffs (0.25, 0.30, 0.35) with different GR transforms to bracket the net-to-gross ratio uncertainty for reserve range estimates (P10/P50/P90).
Related Calculations
Region Notes
Permian Basin
Wolfcamp: Clavier gives Vcl 5–15% lower than linear IGR but slightly higher than Larionov Older for typical IGR 0.3–0.5. Use alongside Larionov Older to bracket uncertainty.
Gulf of Mexico
GOM Miocene/Pliocene: Clavier gives values between Larionov Tertiary and linear IGR — more conservative correction than Tertiary but still well below linear.
Middle East Carbonates
Clavier is a common choice in Arabian Platform carbonate-shale sequences; typical GRclean 10–20 API, GRshale 90–130 API in the Cretaceous-Jurassic reservoirs.
North Sea
Brent Group: Clavier and Larionov Older typically agree within 3–5 pu Vcl for Jurassic sandstones; both are acceptable; calibrate to core XRD where possible.
Global
No single non-linear Vcl correction is universally best. Run all three (Larionov Tertiary, Larionov Older, Clavier) and use core data to select the most appropriate one for the basin.
References
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