Oil reserves: separate discoveries, purchases and revisions
Reserve replacement measures additions relative to production, but its interpretation depends on what counts as an addition. A producer can replace more than its annual production by buying another company’s reserves, even when its own exploration adds little. Reconstruct the movement before describing a replacement percentage as operating success.
Separate the physical movement categories
The EIA distinguishes revisions, extensions and discoveries, purchases and sales, and production in explaining proved-reserve changes. It also distinguishes organic additions from purchased reserves. The agency’s analysis is context; the issuer’s actual reserve statement supplies the company-specific quantities and applicable reporting basis.
Do not mix resources with proved reserves or use a volume conversion as an economic-value equivalence. A barrel-of-oil-equivalent convention combines energy-related quantities, not identical margins or realizable prices. Record commodity, geography, working interest and whether the reserves are developed or undeveloped.
Hypothetical reserve bridge
Assume opening proved reserves of 500 million BOE. During the year, extensions and discoveries add 40, improved recovery adds 5, purchases add 80, revisions subtract 20, sales subtract 10 and production consumes 50. Ending reserves are 545.
| Reserve worksheet | Million BOE | Interpretation |
|---|---|---|
| Opening proved reserves | 500 | Same ownership and reserve basis |
| Extensions, discoveries and improved recovery | +45 | Defined organic additions |
| Purchased reserves | +80 | Transfer from other owners |
| Revisions | −20 | Inspect price and technical causes |
| Sold reserves | −10 | Portfolio disposal |
| Production | −50 | Resource depletion |
| Ending proved reserves | 545 | Reconcile all movement lines |
An editorial organic replacement ratio is 45 / 50 = 90%. A ratio including purchases but excluding revisions and sales is 125 / 50 = 250%. Ending reserve growth is 9%. These are different descriptions, so publish the numerator definition beside each percentage instead of selecting the most impressive label.
Investigate the revisions
Revisions can arise from prices, technical information, costs or other assumptions. A higher commodity-price basis can make more volume economically recoverable without a new discovery. A negative technical revision raises a different question about reservoir performance. Do not put both into an unexplained “other” bucket if the filing provides detail.
Record reserve-price assumptions and reporting dates. Reserve classification under the issuer’s applicable rules may differ from a geological estimate in a presentation. When comparing companies, check both definitions rather than assuming all publicly disclosed reserve totals use the same conditions.
Connect additions with spending and development
If the thesis concerns exploration effectiveness, compare organic additions with compatible exploration and development activity. An acquisition’s purchase price cannot be treated as exploration spending. Equally, undeveloped additions may require future capital before production; a larger reserve base does not establish that funding is available.
Add a development calendar with expected project sequence, required investment and permitting status where disclosed. Preserve management’s schedule as an expectation until supporting milestones occur. A reserve-life calculation using ending reserves divided by current production is a static ratio, not the date at which operations must stop.
At the next annual review, follow the original undeveloped population and distinguish conversion to developed reserves from genuinely new additions. Moving a quantity between categories should not increase the total twice. The completed bridge tests the source of reserve replacement and does not forecast commodity prices, production or investment returns.
FAQ
Can acquired reserves count in replacement?
They can under a stated definition. Label them separately so readers can distinguish acquisition-led replacement from new organic resource additions.
Does a reserve-life ratio forecast mine or field closure?
No. Future production rates, investment, discoveries and revisions can change the ratio. It is a snapshot under the stated assumptions.