petrophysics · net pay
Net-to-Gross Ratio
NTG = Net Thickness / Gross Thickness
click formula to derive ↑
Inputs
ft
ft
Description
Computes the net-to-gross ratio — the fraction of a gross interval that meets reservoir-quality cutoffs — as net pay thickness divided by gross interval thickness. Net thickness is the sum of depth points flagged net pay by the net pay cutoffs calculator; this calculator does not itself apply cutoffs, it summarizes an already-completed cutoff analysis into a single reservoir-quality ratio.
Variables
| Symbol | Unit | Description |
|---|---|---|
| NTG | fraction | Net-to-Gross Ratio |
| hn | ft | Cumulative thickness of intervals flagged as net pay by the porosity/Sw/Vcl cutoffs, summed over the logged interval. |
| hg | ft | Total thickness of the interval under evaluation, before any reservoir-quality cutoffs are applied. |
Assumptions
- Net thickness has already been derived from a completed net pay cutoff analysis (porosity/Sw/Vcl) applied consistently across the interval
- Gross and net thicknesses are measured over the same vertical interval and depth datum
Limitations
- A single NTG value represents an average over the interval — it does not capture where within the interval the net pay is concentrated, which matters for completion design
- NTG derived at a single well may not be representative of the field average; areally varying reservoir quality requires a map-based or multi-well NTG
Use Cases
- → Volumetric input for net rock volume: Feed NTG directly into Geology's net rock volume calculator to scale gross rock volume down to reservoir-quality rock volume.
- → Completion quality benchmarking: Compare NTG across offset wells in the same formation to rank reservoir quality and guide landing zone or perforation strategy.
- → Reserve booking support: Report NTG alongside net pay thickness as a standard, defensible input to PRMS-compliant reserve volume calculations.
Related Calculations
Region Notes
Permian Basin
Wolfcamp/Bone Spring unconventional benches commonly show NTG 0.3-0.6 due to interbedded shale and carbonate stringers within the target interval.
Gulf of Mexico
Clean, blocky Miocene/Pliocene channel and bar sands can reach NTG 0.8-1.0; heavily interbedded turbidite sequences fall well below that.
Eagle Ford
Carbonate-rich mudrock benches often show NTG 0.5-0.8 once appropriately relaxed clay-volume cutoffs are applied, since conventional cutoffs would exclude most of the productive interval.
References
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