Log Net Pay & Cut-off Sensitivity

Calculate net pay from well logs with comprehensive sensitivity analysis. Determine pay vs. non-pay intervals using petrophysical cut-offs (Vsh, φ, Sw) and understand how cut-off variations affect net-to-gross and reserve estimates.

Net Pay Cut-offs Petrophysics Sensitivity

Input Parameters

Well Log Readings

API
g/cm³
fraction
ohm-m

Formation Parameters

API
API
g/cm³
2.65 (sandstone), 2.71 (limestone)
g/cm³
1.00 (water), 0.85 (oil)
ohm-m

Net Pay Cut-offs

fraction
Pay if Vsh < cutoff
fraction
Pay if φ > cutoff
fraction
Pay if Sw < cutoff
m
Total interval thickness

Cut-off Presets

Results

Theory & References

What is Net Pay?

Net Pay is the portion of a reservoir interval that meets minimum quality standards for commercial hydrocarbon production. It is determined by applying cut-offs to key petrophysical properties:

  • Vshale: Shale volume must be low enough (typically <50%)
  • Porosity (φ): Must exceed minimum for storage (typically >8%)
  • Water Saturation (Sw): Must be low enough for hydrocarbon saturation (typically <65%)

Net Pay: Vsh < Vsh,cutoff AND φ > φcutoff AND Sw < Sw,cutoff

Industry Standard Cut-offs

Play Type Vsh max φ min Sw max
Conventional 0.50 0.08 0.65
Tight Sand/Gas 0.40 0.06 0.70
Carbonate 0.30 0.05 0.60

Note: Cut-offs should be calibrated to local geology, core data, and well test results.

Cut-off Impact on Reserves

Cut-off selection significantly affects reserve estimates:

  • Conservative cut-offs: Higher confidence, lower volumes (P90-like)
  • Relaxed cut-offs: Higher volumes, more uncertainty (P10-like)
  • Sensitivity analysis: Quantifies uncertainty due to cut-off assumptions

Best Practice: Use core calibration, production tests, and analog data to justify cut-offs. Document rationale and perform sensitivity analysis for reserve reporting (SEC, SPE-PRMS).

Calculation Methods

Shale Volume (Vshale):

Vsh = 0.083 × (2^(3.7×IGR) - 1) [Larionov, Tertiary]

IGR = (GR - GRclean) / (GRshale - GRclean)

Porosity (φ):

φ = (ρma - ρb) / (ρma - ρfl) [Density]

Water Saturation (Sw):

Sw = [(a × Rw) / (φ^m × Rt)]^(1/n) [Archie]