petrophysics · clay volume
Thomas-Stieber Shale Distribution
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Inputs
fraction
fraction
fraction
fraction
fraction
fraction
Description
Estimates the laminated shale volume from a neutron-density crossplot position, following the Thomas-Stieber method: a straight 'clean sand line' is drawn between a picked clean-sand point (0% shale) and a picked shale point (100% shale) in neutron-density space, and the logged data point's fractional position along that line — averaged from both the density and neutron axes — gives Vsh. This is distinct from GR-based Vsh methods (vcl_gr, Larionov, Clavier): it uses only density and neutron porosity, giving an independent shale-volume estimate and a QC cross-check against the GR-derived value.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Vsh | fraction | Laminated Shale Volume |
| φN_ss | fraction | Neutron porosity picked from the cleanest (0% shale) sand interval nearby — the clean-sand-line endpoint on the neutron axis. |
| φD_ss | fraction | Density porosity picked from the same clean-sand interval as φN_ss — the clean-sand-line endpoint on the density axis. |
| φN_sh | fraction | Neutron porosity picked from a thick, clean, adjacent shale interval — the shale-point (100% shale) endpoint on the neutron axis. Typically elevated relative to sand due to clay-bound water. |
| φD_sh | fraction | Density porosity picked from the same shale interval as φN_sh — the shale-point endpoint on the density axis. Typically low, reflecting the matrix density assumption's mismatch with shale. |
| φN | fraction | Neutron porosity at the depth of interest, the same depth as the density porosity reading below. |
| φD | fraction | Density porosity at the depth of interest, the same depth as the neutron porosity reading above. |
Assumptions
- The clean-sand and shale points are picked from genuinely representative, laminated (not dispersed or structural) intervals in the same well or a directly analogous offset well
- The neutron-density relationship between the clean-sand and shale points is approximately linear (the laminated-shale mixing model) rather than following a dispersed- or structural-shale trend
- Both φN and φD are corrected for environmental effects and represent the same depth
Limitations
- Only models the laminated shale-distribution case — Thomas & Stieber's original method also distinguishes dispersed and structural shale via separate trend lines, which requires additional chart interpretation not implemented here; a large density/neutron disagreement (flagged above) is a sign the laminated assumption may not hold
- Sensitive to the clean-sand and shale endpoint picks — poorly chosen or non-representative endpoints propagate directly into Vsh
- Should be cross-checked against a GR-based Vsh (vcl_gr or a Larionov variant); systematic disagreement between the two methods usually indicates either endpoint miscalibration here or a GR response not purely driven by clay content
- The original Thomas & Stieber (1975) SPWLA paper was not directly accessible during research, and no independently published numeric worked example with a stated Vsh result was located — the formula chain above is well-established and documented in secondary sources (e.g., Asquith 2004), but this calculator's golden vector is verified-tier (hand-derived from the documented formula), not matched against a published example. Same disclosure pattern as sp_rw, waxman_smits_sw, and pore_compressibility_newman.
Use Cases
- → Independent Vsh cross-check: Compare this neutron-density-derived Vsh against a GR-based Vsh (vcl_gr, vcl_larionov_tertiary) — a genuine independent cross-check since it uses entirely different log curves.
- → Shale distribution screening: A large spread between the density- and neutron-axis estimates flags intervals where shale is likely dispersed or structural rather than laminated, warranting closer log review.
- → Net-to-gross QC in laminated reservoirs: In thinly laminated sand-shale sequences, use this Vsh alongside net_to_gross to sanity-check a GR-based net pay flag against an independent porosity-log-based estimate.
Related Calculations
Region Notes
Gulf Coast
Classic thinly laminated Miocene/Pliocene sand-shale sequences are the setting the Thomas-Stieber method was originally developed for — laminated shale is the dominant distribution type here.
Permian Basin
Wolfcamp/Spraberry mixed carbonate-mudrock sequences often show dispersed rather than purely laminated shale — expect larger density/neutron disagreement and treat the averaged Vsh as approximate.
Global
Always re-pick the clean-sand and shale endpoints locally for each field or formation — endpoints borrowed from an unrelated area are a common source of systematic Vsh bias.
References
Primary source
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