Engineering lesson
Net to gross gross thickness and net pay in volumetrics
Gross thickness describes the whole interval. Net-to-gross selects the fraction counted as net rock; net pay is the resulting thickness under the stated cutoffs. Use either net pay or gross thickness times net-to-gross, once.
By PetroCalcHub Team · Updated October 24, 2026 · Free to read and use
Make the cutoff definition visible
An interval can contain nonreservoir beds, wet rock or low-quality rock excluded by a selected interpretation. Net pay is therefore a defined interpretation rather than a universal log reading.
If 100 ft gross thickness has net-to-gross 0.5, the selected net thickness is 50 ft. Multiplying an already net 50 ft by 0.5 again produces an unintended 25 ft.
Keep averaging on the selected rock
Porosity and saturation should represent the same net interval counted in the volume calculation. Mixing a gross-interval average with net thickness can bias pore volume.
Layer-by-layer sums preserve the products of thickness, porosity and gas fraction. Multiplying separate arithmetic averages is not generally equal to that sum.
Treat sensitivity as interpretation uncertainty
The explorer varies net-to-gross while keeping gross geometry and other properties fixed. It shows the arithmetic consequence of an interpretation choice.
It does not recommend a cutoff or determine connectivity. Those questions require geological and petrophysical evidence beyond the volumetric calculator.
Explore the example
Original synthetic teaching calculation. Equations and fixed controls are stated here; these are not field measurements.
Select one of 5 recorded scenarios. Intermediate states are not interpolated.
Net-to-gross (fraction) = 0.5. Net thickness (ft): 50; Gas in place (scf): 7840800000.
- Net thickness (ft)
Worked example
Gross thickness to net thickness
160 acres and 100 ft gross thickness; φ = 0.15, Sw = 0.25, Bg = 0.005 ft³/scf. NTG is applied once.
Relation and unit basis
hnet = 100 NTG; G = 43,560 A hnet φ(1 − Sw)/Bg
NTG is a fraction under a stated interpretation cutoff, not another porosity factor.
1. Define the baseline
Net-to-gross (fraction) = 0.5
160 acres and 100 ft gross thickness; φ = 0.15, Sw = 0.25, Bg = 0.005 ft³/scf. NTG is applied once.
2. Select the relation
hnet = 100 NTG; G = 43,560 A hnet φ(1 − Sw)/Bg
NTG is a fraction under a stated interpretation cutoff, not another porosity factor.
3. Substitute the values
100 × 0.5 = 50 ft net; G = 7,840,800,000 scf on the baseline rock and fluid inputs
Retain intermediate precision. The displayed values are rounded only after evaluation.
4. Read the result
Net thickness (ft) = 50; Gas in place (scf) = 7840800000
Do not apply NTG again if the supplied thickness is already net pay.
Result and interpretation
Net thickness (ft) = 50; Gas in place (scf) = 7840800000
Do not apply NTG again if the supplied thickness is already net pay.
Net thickness is computed from the changing net-to-gross value. It is not multiplied by net-to-gross a second time.
Exact scenario data
Original synthetic teaching calculation. Equations and fixed controls are stated here; these are not field measurements.
| Net-to-gross (fraction) | Net thickness (ft) | Gas in place (scf) |
|---|---|---|
| 0.25 | 25 | 3.9204e+9 |
| 0.4 | 40 | 6.2726e+9 |
| 0.5 | 50 | 7.8408e+9 |
| 0.75 | 75 | 1.1761e+10 |
| 1 | 100 | 1.5682e+10 |
Scroll the table to see all columns. The highlighted row is the worked baseline. Downloads retain full numerical precision.
Test your understanding
Work through the question before revealing its answer. Use the stated assumptions and include units in every numerical step.
1. A model already contains 40 ft net pay. Should an NTG of 0.8 be applied again?
Show the worked answer
Only if 40 ft was actually gross thickness under the intended definition. Applying it to an already net value double counts the reduction.
2. Reproduce the baseline Net thickness (ft) without rounding intermediate values.
Show the worked answer
100 × 0.5 = 50 ft net; G = 7,840,800,000 scf on the baseline rock and fluid inputs. Net thickness (ft) = 50. Do not apply NTG again if the supplied thickness is already net pay.
3. Compare the first and last recorded scenarios. Which input changes?
Show the worked answer
Net-to-gross (fraction) changes from 0.25 to 1. Other controls remain as stated in the premise. Net thickness (ft) changes from 25 to 100.
4. What would be needed to apply this example to a different physical system?
Show the worked answer
Replace the synthetic controls with compatible documented inputs and check the relation's assumptions. NTG is a fraction under a stated interpretation cutoff, not another porosity factor. Do not apply NTG again if the supplied thickness is already net pay.
Before using the result
- Label gross and net thickness.
- Apply net-to-gross once.
- Use averages for the same selected interval.
Do not apply NTG again if the supplied thickness is already net pay.
Continue learning
Start with the broader workflow guide · Related method guide
Sources and method
The explanation and synthetic examples are original PetroCalcHub teaching material. The references below support the definitions and model context. They do not supply field validation for the synthetic scenarios.