Original synthetic study
How input rounding propagates into a volumetric estimate
Input rounding propagates through the product of net rock volume, porosity and gas fraction and through division by Bg. The effect depends on which inputs were rounded and in which direction.
By PetroCalcHub Team · Updated October 24, 2026 · Free to read and use
Make the unrounded case the reference
The synthetic study fixes geometry and uses a porosity with additional decimal places. Each scenario rounds that porosity to a different precision while preserving the other inputs.
Relative output error is measured against the unrounded calculation. The exercise is deterministic rounding analysis, not a statement about geological measurement uncertainty.
Use the product to explain the response
When only porosity changes, Gnew/Gbase = φnew/φbase. When Bg is rounded instead, it appears in the denominator and its effect reverses direction.
If several inputs change, calculate the full ratio rather than adding percentage changes exactly. A first-order sum can be useful for small perturbations but is an approximation.
Separate display precision from knowledge
Retaining intermediate precision prevents avoidable arithmetic drift. It does not make an uncertain geological average known to many decimal places.
Report sensible output precision and preserve full values in the calculation record. The download includes the baseline, rounded inputs and relative errors so the result can be reproduced.
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.
Porosity decimal places = 4. Rounded gas in place (scf): 8034206400; Relative error (%): 0; Rounded porosity: 0.1537.
- Rounded gas in place (scf)
Worked example
Porosity rounding propagated through volumetrics
160 acres, 50 ft net pay, Sw 0.25, Bg 0.005 ft³/scf. Reference porosity 0.1537; rounding precision varies.
Relation and unit basis
Grounded/Greference = φrounded/0.1537
Only decimal rounding changes. This is arithmetic error, not a geological uncertainty distribution.
1. Define the baseline
Porosity decimal places = 4
160 acres, 50 ft net pay, Sw 0.25, Bg 0.005 ft³/scf. Reference porosity 0.1537; rounding precision varies.
2. Select the relation
Grounded/Greference = φrounded/0.1537
Only decimal rounding changes. This is arithmetic error, not a geological uncertainty distribution.
3. Substitute the values
Reference G = 8034206400 scf at φ = 0.1537; rounded φ at four decimals remains 0.1537, so error = 0%
Retain intermediate precision. The displayed values are rounded only after evaluation.
4. Read the result
Rounded gas in place (scf) = 8034206400; Relative error (%) = 0; Rounded porosity = 0.1537
Display precision does not create geological knowledge, but premature rounding can add avoidable arithmetic error.
Result and interpretation
Rounded gas in place (scf) = 8034206400; Relative error (%) = 0; Rounded porosity = 0.1537
Display precision does not create geological knowledge, but premature rounding can add avoidable arithmetic error.
Only the porosity rounding choice changes. Other sources of uncertainty are held outside this experiment.
Study data and reproduction
Original synthetic teaching calculation. Equations and fixed controls are stated here; these are not field measurements.
| Porosity decimal places | Rounded gas in place (scf) | Relative error (%) | Rounded porosity |
|---|---|---|---|
| 1 | 1.0454e+10 | 30.12362 | 0.2 |
| 2 | 7.8408e+9 | -2.407287 | 0.15 |
| 3 | 8.0499e+9 | 0.1951854 | 0.154 |
| 4 | 8.0342e+9 | 0 | 0.1537 |
| 5 | 8.0342e+9 | 0 | 0.1537 |
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. If porosity rises 1% relatively with every other input fixed, what happens to G?
Show the worked answer
It rises exactly 1% in this volumetric model. That proportionality is an independent check on the rounding study.
2. Reproduce the baseline Rounded gas in place (scf) without rounding intermediate values.
Show the worked answer
Reference G = 8034206400 scf at φ = 0.1537; rounded φ at four decimals remains 0.1537, so error = 0%. Rounded gas in place (scf) = 8034206400. Display precision does not create geological knowledge, but premature rounding can add avoidable arithmetic error.
3. Compare the first and last recorded scenarios. Which input changes?
Show the worked answer
Porosity decimal places changes from 1 to 5. Other controls remain as stated in the premise. Rounded gas in place (scf) changes from 10454400000 to 8034206400.
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. Only decimal rounding changes. This is arithmetic error, not a geological uncertainty distribution. Display precision does not create geological knowledge, but premature rounding can add avoidable arithmetic error.
Before using the result
- Use the unrounded result as the reference.
- Retain intermediate calculation precision.
- Distinguish rounding error from geological uncertainty.
Display precision does not create geological knowledge, but premature rounding can add avoidable arithmetic error.
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.