petrophysics · porosity
Multi-log Average Porosity
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Inputs
Paste one row per line — separate columns with a comma or tab: Porosity (fraction)
⚠ Needs at least 2 valid rows to calculate — 0 so far.
Description
Reconciles porosity estimates from multiple logs or methods (neutron, density, sonic, NMR, core, etc.) at the same depth/zone into a single representative value using the standard non-gas-zone log-analysis practice of a plain arithmetic mean. This is distinct from phi_nd's root-mean-square neutron-density combination (a specific 2-log, gas-effect-aware formula) and from average_thickness_porosity/average_water_saturation (which average across depth ZONES, weighted by thickness, not across different measurement METHODS at one depth).
Variables
| Symbol | Unit | Description |
|---|---|---|
| φavg | fraction | Average Porosity |
| φ | — | One row per porosity estimate at the SAME depth/zone from a different log or method — e.g. phi_nd, phi_den, phi_sonic, NMR, or core. A plain, unweighted mean does not depend on which method produced which value, so rows are not individually labeled; only the set of φ values matters. Paste at least 2 rows. |
Assumptions
- Every row is an independent porosity estimate of the SAME formation interval from a different logging tool or method, not a different depth or zone — mixing the two collapses this into a meaningless number.
- A plain, unweighted mean is the correct convention when no gas effect or major lithology-driven bias is suspected between methods; where one is suspected, a wide spread should be investigated rather than averaged through.
- Rows are not individually labeled by source method — since the formula is an unweighted mean, the result depends only on the set of φ values, not on which method produced which value.
Limitations
- This is a plain, unweighted arithmetic mean — it does not itself diagnose or correct for gas effect, bad-hole conditions, or a genuinely wrong reading from one method; phi_nd's RMS/crossover approach or phi_effective's shale correction remain the right tools when those specific effects are suspected.
- The general averaging practice here is attributed to Asquith & Krygowski (2004) on the strength of it already being this platform's cited source for the same discipline's other porosity calcs — attribution-corroborated, not page-checked this session, the same disclosed-gap standard already applied to gross_rock_volume/net_rock_volume/average_thickness_porosity/average_water_saturation.
- Does not itself validate that every pasted row shares a common depth and represents the same interval — garbage in, garbage out for row selection.
Use Cases
- → Reconciling log-derived porosity into one number: Combine phi_nd, phi_den, phi_sonic, NMR, and/or core porosity estimates at a given depth into a single representative porosity for a non-gas, non-anomalous interval.
- → Quick QC check across methods: A wide spread between pasted method estimates is flagged automatically, surfacing a likely gas effect, lithology mismatch, or bad log reading worth investigating before the value is used downstream.
Related Calculations
Region Notes
Global
Simple multi-method averaging is standard practice in non-gas, non-anomalous intervals, but always check the spread between methods first — a wide spread in a gas-bearing zone is exactly when phi_nd's RMS/crossover formula, not a plain mean, is the appropriate tool.
References
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