petrophysics · clay volume
Clay Volume — Larionov (Tertiary)
Vcl = 0.083 × (2^(3.7 × IGR) − 1)
click formula to derive ↑
Inputs
API
API
API
Description
Larionov's non-linear GR correction for Tertiary (younger, less consolidated) rocks gives a lower clay volume estimate than the linear gamma ray index, correcting for the tendency of young shales to have elevated GR from uranium in pore waters rather than from clay minerals alone.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Vcl | fraction | Clay Volume (Larionov Tertiary) |
| GR | API | Gamma ray reading at the depth of interest. Where uranium contamination from organic-rich shales is significant, use spectral GR (CGR = Th + K only) to avoid overestimating Vcl. |
| GRclean | API | GR baseline for 0% clay — picked from the cleanest interval in the well or from a regional clean-sand reference. |
| GRshale | API | GR baseline for 100% shale — picked from a thick, laterally continuous shale interval close to the reservoir of interest. |
Assumptions
- Rock is Tertiary or similarly young, unconsolidated to semi-consolidated — clay GR response has not matured to the linear relationship of older, more consolidated rocks
- GRclean and GRshale baselines are picked from representative intervals in the well being analyzed
Limitations
- Underestimates Vcl in old consolidated rocks — use Larionov Older Rocks correction instead
- Neither Larionov correction is appropriate when GR is dominated by uranium (organic-rich shales, U-enriched K-feldspars) — use spectral GR (CGR) if available
Use Cases
- → Gulf of Mexico Tertiary shaly-sand evaluation: Standard Vcl approach for Miocene/Pliocene sands in the GOM; the Tertiary correction avoids the net-to-gross underestimate that results from using linear IGR.
- → Comparison with vcl_gr for formation evaluation QC: Run both linear and Larionov Tertiary; where they agree closely, the sand is either very clean or very shaly; large divergence at intermediate GR is where the correction matters most.
- → Net-to-gross calculation in Tertiary basins: Apply Vcl ≤ 0.30 net pay cutoff using the Larionov Tertiary corrected Vcl to avoid over-cutting marginal pay in young, radioactive-shale formations.
Related Calculations
Region Notes
Permian Basin
Wolfcamp and Spraberry are Permian age — use Larionov Older Rocks or linear IGR rather than Tertiary correction. Tertiary correction is inappropriate for these rocks.
Gulf of Mexico
Miocene/Pliocene sands are the primary application; Vcl from Larionov Tertiary is typically 10–20% lower than linear IGR in intervals with IGR 0.3–0.5.
Anadarko Basin
Pennsylvanian-age sands are too old for the Tertiary correction; use Older Rocks form. Cretaceous sands can go either way — compare both and calibrate against core Vcl if available.
Global
Rule of thumb: use Tertiary correction for rocks younger than ~50 Ma (Eocene and younger); use Older Rocks correction for Cretaceous and older formations.
References
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