petrophysics · clay volume
Clay Volume — Larionov (Older Rocks)
Vcl = 0.33 × (2^(2 × IGR) − 1)
click formula to derive ↑
Inputs
API
API
API
Description
Larionov's non-linear GR correction for older, more consolidated rocks (Cretaceous and older) gives lower Vcl than the linear IGR, similar to the Tertiary correction but with less aggressive downward scaling. It is appropriate for Paleozoic and Mesozoic formations where clay minerals have been diagenetically transformed and their GR contribution is more representative of true clay content.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Vcl | fraction | Clay Volume (Larionov Older) |
| GR | API | Gamma ray reading at depth. Use CGR (spectral, Th+K only) in organic-rich formations to remove uranium contribution. |
| GRclean | API | GR baseline for clean sand (0% clay). Pick from the cleanest representative sand interval in the well. |
| GRshale | API | GR baseline for 100% shale. Pick from a thick, laterally continuous shale close to the reservoir. |
Assumptions
- Rock is Cretaceous age or older, where clay-mineral diagenesis has established a more mature relationship between GR and actual clay volume
- GR baselines are picked from representative intervals in the same formation being evaluated
Limitations
- Over-corrects (gives too-low Vcl) if applied to Tertiary-age rocks — use Larionov Tertiary form for young formations
- GR is not selective for clay type — arkosic sands with K-feldspar can give high GR with low true clay content; use spectral GR where K-feldspar is suspected
Use Cases
- → Permian Basin Wolfcamp and Spraberry evaluation: Wolfcamp is Permian age; Larionov Older is appropriate here. Gives Vcl estimates 15–25% lower than linear IGR for productive silty intervals with IGR 0.3–0.5.
- → Appalachian Basin Devonian and Pennsylvanian sands: Older Rocks correction is the appropriate Vcl model for these Paleozoic formations; linear IGR consistently overestimates clay in these mature, well-cemented sands.
- → Net pay cutoff calibration against core: When core clay volume (from XRD or petrography) is available, plot core Vcl vs. IGR to determine whether Larionov Older or Tertiary better matches — direct calibration beats any default rule.
Related Calculations
Region Notes
Permian Basin
Wolfcamp (Permian age): Larionov Older gives Vcl 10–20% below linear IGR for typical productive intervals with GR 40–80 API; this is the preferred Vcl model for Wolfcamp formation evaluation.
Anadarko Basin
Pennsylvanian sands (Atoka, Morrow): use Older Rocks form; typical clean GR 15–25 API, shale GR 100–140 API; Vcl in productive pay < 0.25.
Appalachian Basin
Devonian sands (Berea, Oriskany): very clean sands (GR < 30 API) with shale GR 100–150 API; Larionov Older gives Vcl < 0.10 in best pay — consistent with core data.
North Sea
Brent Group Jurassic sands: Larionov Older is appropriate; typical clean GR 20–35 API, shale GR 100–130 API. Siderite and chlorite cement can affect density log more than GR.
Global
Both Larionov corrections give lower Vcl than linear IGR. The Older Rocks form is always between the Tertiary (lowest) and linear (highest) estimates — it is a moderate correction for mature rocks.
References
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