geology · stratigraphic
Average Water Saturation (Multi-Zone)
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Inputs
Paste one row per line — separate columns with a comma or tab: Water Saturation (fraction), Porosity (fraction), Thickness (ft)
⚠ Needs at least 2 valid rows to calculate — 0 so far.
Description
Averages water saturation across multiple zones using the standard doubly-weighted reservoir-engineering convention — weighted by both porosity and thickness, Σ(Sw·φ·h)/Σ(φ·h) — rather than a simple arithmetic mean. This is the correct way to combine per-zone Sw values into the single representative Sw a volumetric calculation (ooip/giip, pore_volume_hcpv) expects, since a zone's contribution to hydrocarbon pore volume scales with its own φ·h, not with an unweighted count.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Swavg | fraction | Average Water Saturation |
| Sw, φ, h | — | One row per zone: water saturation, porosity, and thickness. Averaging is doubly-weighted by porosity AND thickness — Σ(Sw·φ·h)/Σ(φ·h) — so a zone with little pore volume (low φ·h) contributes proportionally less to the average than a simple arithmetic mean of Sw would give it. |
Assumptions
- Each row is a genuinely distinct zone over the same averaging interval — not overlapping or double-counted thickness.
- Sw and porosity values are already computed (e.g. from sw_archie and a porosity log) for each zone — this calculator only combines them, it does not derive either from raw logs.
- Porosity-and-thickness weighting is the correct convention for the intended downstream use — the same convention this platform's own ooip/giip volumetric calculators implicitly assume for their single Sw input.
Limitations
- A simple arithmetic mean of Sw across zones would give a materially different (and less physically meaningful) result than this doubly-weighted average — always confirm a downstream comparison value uses the same φ·h-weighting convention.
- Does not itself validate that the pasted zones are non-overlapping or drawn from a consistent depth basis — garbage in, garbage out for zone selection.
- If total Σ(φ·h) is zero (all-zero porosity or thickness rows), the average is undefined and reported as 0 with a warning rather than silently producing NaN.
Use Cases
- → Feeding a volumetric estimate: Combine per-zone Sw values from Archie or another saturation method into the single representative Sw that ooip/giip or pore_volume_hcpv expects as a scalar input.
- → Multi-zone reservoir characterization: Roll up several net-pay zones' individually computed water saturations into one field- or well-representative value, correctly weighting each zone by its own pore volume contribution.
Related Calculations
Region Notes
Global
The Σ(Sw·φ·h)/Σ(φ·h) doubly-weighted average is standard reservoir-engineering practice for combining multi-zone saturation data into a single volumetric input — always confirm which averaging convention a downstream reserves report or reservoir model expects before comparing values directly.
References
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