Not all long-term assets are created equal. Natural resources — oil reserves, timber tracts, mineral deposits — are consumed physically, requiring a specific accounting method called depletion. Libby's Chapter 8 extends the asset consumption model beyond depreciation to cover resources, and uses Southwest Airlines' aircraft fleet as a case study for how companies apply the units-of-production method in practice.
Natural resources are long-term assets representing the right to extract or harvest physical resources from the earth: oil and gas reserves, coal deposits, mineral ore bodies, timber stands, gravel quarries. Unlike PP&E, which wears out with use and time, natural resources are physically extracted — the resource literally leaves the ground or is cut down. The balance sheet records the acquisition cost of the rights (plus exploration, development, and site preparation costs) and depletes this balance as units are extracted.
| Category | Nature | Cost Allocation Method | Examples |
|---|---|---|---|
| Property, Plant & Equipment (PP&E) | Tangible assets that wear out with use and time | Depreciation over useful life | Buildings, machinery, vehicles, computers |
| Natural Resources | Tangible assets physically extracted or harvested from nature | Depletion based on units extracted | Oil and gas reserves, coal mines, timber tracts, mineral deposits |
| Intangible Assets | Non-physical assets with legal or contractual value | Amortization over useful life (or impairment for indefinite-life) | Patents, trademarks, customer lists, goodwill |
Natural resources are initially recorded at cost: the purchase price of the mineral rights plus all costs to prepare the site for production (drilling, tunneling, access roads, drainage systems). These capitalized costs form the depletion base — the total amount to be expensed over the life of the resource through the depletion process.
Book Value Over Time — Straight-Line vs Double Declining Balance
Example: $100,000 asset · $10,000 salvage value · 5-year life
Start
Year 1
Year 2
Year 3
Year 4
Year 5
↑ $10K salvage floor — neither method goes below
Straight-Line — Smoother earnings
Equal $18,000 expense every year. Predictable, easy to explain, preferred for financial reporting.
$18K
$18K
$18K
$18K
$18K
DDB — Front-loaded expense
Heavy early, light late. Tax benefit: deduct more now, pay less tax today. Common for MACRS tax depreciation.
$40K
$24K
$14.4K
$8.6K
$3K
Total depreciation over 5 years is identical under both methods: $90,000 (cost $100K − salvage $10K). The difference is only timing.
Figure 2.1 — DDB front-loads $40K of depreciation in Year 1 vs SL's $18K — a $22K difference in reported pre-tax income. The totals equalize by Year 5. Most companies use SL for reporting and DDB for taxes.
Depletion is almost always calculated using the units-of-production method — the natural choice because resources are consumed in discrete, measurable units (barrels of oil, tons of coal, cubic feet of timber). The depletion rate per unit is calculated based on total estimated recoverable reserves; as units are extracted, the cost is transferred from the asset to COGS.
Depletion Rate Per Unit
Depletion Rate = Total Capitalized Cost ÷ Estimated Total Recoverable Units
Annual Depletion Expense = Depletion Rate × Units Extracted During the Year
An oil company acquires a lease for $45M. Estimated recoverable reserves: 15 million barrels. Development costs: $15M. Total depletion base = $60M. Depletion rate = $60M ÷ 15M barrels = $4.00 per barrel. Year 1: extract 800,000 barrels → depletion = 800,000 × $4.00 = $3,200,000. Year 2: extract 1,200,000 barrels → depletion = $4,800,000. Balance sheet: Resource asset falls from $60M by $3.2M in Year 1 to $56.8M, then by $4.8M in Year 2 to $52M. Note: if reserve estimates are revised, the depletion rate changes prospectively from the revision date.
Depletion calculations are only as accurate as the estimated recoverable reserves — and those estimates are inherently uncertain and subject to revision. A downward revision (fewer recoverable reserves than previously estimated) increases the per-unit depletion rate, accelerating expense recognition. An upward revision decreases it. For energy and mining companies, the reserve engineer's report is a critical document: aggressive reserve estimates reduce current-period depletion expense and inflate income. SEC rules require companies to use standardized assumptions in reserve estimation to limit manipulation.
Before oil or gas can be extracted, it must be found. Exploration is expensive and highly uncertain — most wells are dry. Accounting for these costs is controversial because they represent real cash expenditures that may have no future value (dry holes) or enormous future value (major discovery). U.S. GAAP allows two methods for oil and gas companies:
| Method | Treatment of Dry Holes | Treatment of Successful Wells | Income Statement Effect | Balance Sheet Effect |
|---|---|---|---|---|
| Successful-Efforts Method | Expensed immediately when a well is classified as dry | Capitalized and depleted over productive life | More volatile — large charges in periods of unsuccessful drilling | Lower assets — only successful well costs on balance sheet |
| Full-Cost Method | Capitalized with all other exploration costs; depleted over all reserves | Capitalized with all exploration costs | Smoother — dry hole costs are deferred and spread over time | Higher assets — all exploration costs capitalized regardless of outcome |
Two exploration-stage oil companies can have identical exploration programs and identical results — but one using successful-efforts reports dramatically lower earnings and assets during periods of high dry-hole rates, while the other (full-cost) shows smooth, growing assets. When comparing energy companies: (1) note which method each uses; (2) check the DD&A (depletion, depreciation, and amortization) rate per unit to compare extraction costs; (3) examine the reserve engineer's report for reserve quality and revision history.
Libby uses Southwest Airlines as a case study in applying the units-of-production method to a non-natural resource — aircraft. Southwest chose to base aircraft depreciation on actual flight cycles (takeoffs and landings) rather than time, because aircraft airframes and engines wear out primarily through pressurization cycles, not simply through the passage of time.
A Boeing 737 costs $50M and is expected to complete 75,000 flight cycles over its useful life, with a $5M residual value. Depreciable base = $50M − $5M = $45M. Depreciation rate = $45M ÷ 75,000 cycles = $600 per cycle. In a year when this aircraft flies 4,200 cycles: depreciation = 4,200 × $600 = $2,520,000. In a year when it flies only 2,100 cycles (reduced schedule): depreciation = $1,260,000 — exactly half. The method ties depreciation directly to actual usage, not the calendar.
Southwest's choice reflects a genuinely superior matching principle application: aircraft cost is caused by flight cycles, so it should be allocated to the periods that generate those cycles. During COVID-19, when Southwest's fleet flew far fewer cycles, its aircraft depreciation declined automatically — accurately reflecting reduced consumption of the asset's useful life. An airline using straight-line depreciation would have reported the same depreciation regardless of whether planes flew or sat idle.
A company's choice of depreciation method — straight-line, accelerated, or units-of-production — implicitly encodes management's view of how the asset's economic value is consumed. Straight-line: time is the primary consumption driver. DDB: heavier early use and faster obsolescence. Units-of-production: physical use drives wear and value loss. When comparing companies in capital-intensive industries, always verify the depreciation method — two companies using the same asset with different methods will report different margins even with identical operations.
Key Takeaways
A mining company acquires mineral rights for $30M and spends $10M on development costs. Estimated recoverable ore: 5 million tons. In Year 1, it extracts 600,000 tons. What is depletion expense for Year 1?