petrophysics · net pay
Net Pay Cutoffs
Net: φ ≥ φcut AND Sw ≤ Swcut AND Vcl ≤ Vclcut
click formula to derive ↑
Inputs
fraction
fraction
fraction
fraction
fraction
fraction
Description
Applies porosity, water saturation, and clay volume cutoffs to a depth point and flags it as net pay only if all three criteria pass. Cutoff values must be calibrated to local formation data and core measurements — do not use default values blindly.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Net | — | Net Pay Flag |
| φ | fraction | Effective or total porosity at this depth, from a porosity log or calculation. |
| Sw | fraction | Water saturation at this depth, from Archie or another Sw model. |
| Vcl | fraction | Volume of clay/shale at this depth, from a GR or multi-mineral clay model. |
| φcut | fraction | Minimum porosity for a zone to be counted as reservoir rock — calibrate to core/log data for the formation. |
| Swcut | fraction | Maximum water saturation for a zone to be counted as productive — typically set near Swirr plus a margin. |
| Vclcut | fraction | Maximum clay volume for a zone to be counted as reservoir — eliminates shale baffles and laminated intervals. |
Assumptions
- Input logs (porosity, Sw, Vcl) have already been environmentally corrected and are representative of in-situ formation properties
- Cutoff values have been calibrated against core data, production logs, or analog wells in the same formation rather than left at generic defaults
- Each input curve is sampled at the same depth and resolution so that the AND logic is evaluated on consistent data
Limitations
- Sharp cutoffs create step-function pay/non-pay boundaries that do not capture gradational reservoir quality — consider a continuous net-pay-quality index for marginal zones
- Cutoffs derived for one field or formation may not transfer to another with different rock fabric, clay type, or fluid properties
- This calculator evaluates a single depth point — net pay thickness requires summing the flagged intervals over the full logged interval
Use Cases
- → Reserve booking (PRMS-compliant): Generate a defensible net pay flag curve to support reserve volume calculations consistent with PRMS reporting standards.
- → Perforation interval selection: Identify which depth intervals meet reservoir quality criteria for completion design.
- → Pay flag generation for log analysis: Produce a net pay flag curve for display alongside porosity, Sw, and Vcl on a standard log analysis plot.
Related Calculations
Region Notes
Permian Basin
Wolfcamp/Bone Spring unconventional benches often use φcut≈0.06, Swcut≈0.55-0.60, and Vclcut≈0.40-0.50 due to higher background clay content than conventional sands.
Gulf of Mexico
Unconsolidated Miocene/Pliocene sands typically use higher cutoffs, φcut≈0.15-0.20 and Swcut≈0.50-0.60, reflecting higher reservoir quality and lower Swirr.
Eagle Ford
Carbonate-rich mudrock benches often use φcut≈0.04-0.06 and Vclcut≈0.50-0.60, since conventional clay-volume cutoffs would exclude most of the productive interval.
References
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