Reconcile stock changes before calling them physical flows

Write an inventory identity with named terms

For a fixed boundary, ending stock equals beginning stock plus inflows minus outflows plus other adjustments. The identity is simple, but each term needs matching units, dates and coverage. A flow measured per day must be multiplied by its period before it can be added to a stock measured in total units.

The other-adjustments category is not a license to hide unexplained differences. It should identify reclassification, revised reporting scope or another supported mechanism where possible. An unresolved residual remains unresolved. Inventing a physical flow solely to force the identity to balance would create data rather than explain the source.

Separate reclassification from injection

Take invented working gas of 2,000 Bcf at the first date and 2,030 Bcf at the next. The level change is plus thirty Bcf. Suppose source evidence also identifies five Bcf reclassified from base to working gas. The implied physical addition after that adjustment is twenty-five Bcf, not thirty.

EIA’s weekly storage methodology distinguishes reportable reclassifications from implied flow. The guide’s five-Bcf example is synthetic and illustrates the accounting direction. A real reconciliation must use the source’s reported adjustment and applicable definition; the existence of a classification mechanism does not justify assuming one whenever a result seems unusual.

Keep levels, changes and revisions distinct

Suppose the first-date stock is later revised from 2,000 to 2,004 while the second remains 2,030. The revised level difference becomes twenty-six Bcf. That change in the historical difference need not mean a new physical event occurred; it can result from improved information about the earlier stock.

Retain both vintages if the question concerns what a release showed at the time. If the question instead concerns the best currently retained historical estimate, use one consistent current vintage and say so. Mixing an original prior-week value with a revised current-week value can create a change measure that the agency never published.

Check reporting boundaries and units

A storage field entering or leaving a survey frame can affect comparable coverage. A regional reassignment can move stock between regions while leaving a national total unchanged. The source methodology and footnotes matter because the same numerical movement can have different interpretations under different boundaries.

For a synthetic oil example, an increase of 700 thousand barrels over seven days corresponds to 100 thousand barrels per day. It does not equal 700 thousand barrels per day. A bridge table should display stock units and rate units explicitly so a valid identity is not defeated by a thousand-fold or seven-fold scaling error.

Use the bridge to guide investigation

Start with published levels, then add verified flow and adjustment information. Reconcile the remainder and inspect timing, rounding and definitions before assigning an economic explanation. EIA’s petroleum balance material provides a related example of independently measured components requiring a balancing item; that residual is not automatically a single hidden demand source.

The lab’s monthly means summarize stock levels and do not reconstruct all weekly physical flows. Keep that limit visible when using its output in a dossier. An aggregate seasonal gap can motivate a deeper reconciliation, but it cannot substitute for the source-specific bridge needed to explain injections, withdrawals or inventory transfers.

Keep in the decision record

• A level difference is not automatically a physical flow.
• Reclassification and revisions need separate bridge terms.
• Match stock units, rate units and time intervals.

Source notes

1. EIA — Weekly Underground Natural Gas Storage methodology, updated 10 September 2018
2. EIA — The Basics of Underground Natural Gas Storage
3. EIA — Crude Oil Supply and Disposition definitions

Source dates and scope matter. Historical inventory observations are retained snapshots; forecasts and synthetic curve assumptions are labelled separately. No investment outcome or product coverage is implied.

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