Engineering lesson
Reservoir barrels versus stock tank barrels
Reservoir barrels describe oil volume at reservoir conditions. Stock-tank barrels describe the associated stock-tank oil. Oil formation volume factor Bo is the ratio between those specified volumes.
By PetroCalcHub Team · Updated October 24, 2026 · Free to read and use
Write the volume ratio with units
If Bo = 1.2 reservoir bbl/STB, 1,000 STB corresponds to 1,200 reservoir bbl on that property basis. Dividing a reservoir volume by Bo recovers stock-tank oil volume.
Both quantities use a barrel-sized unit, but their fluid states and dissolved-gas contents differ. Treating them as the same volume can create a material-balance error.
Retain the pressure and temperature state
Bo depends on fluid composition and state. A saturated correlation below bubble point and an undersaturated correction above bubble point answer different property questions.
A single Bo copied across an entire depletion history is an assumption. It is acceptable for a controlled exercise but should not be mistaken for a measured PVT table.
Separate shrinkage from geometric conversion
Converting bbl to ft³ changes units at the same state. Applying Bo relates reservoir oil to stock-tank oil. These are different operations.
The example holds the stock-tank amount fixed and varies Bo. It is a volume-basis exercise rather than a separator-train simulation.
Explore the example
Original synthetic teaching calculation. Equations and fixed controls are stated here; these are not field measurements.
Select one of 5 recorded scenarios. Intermediate states are not interpolated.
Bo (reservoir bbl/STB) = 1.2. Reservoir oil volume (bbl): 1200; Stock-tank oil volume (STB): 1000.
- Reservoir oil volume (bbl)
Worked example
Reservoir and stock-tank oil volumes
Stock-tank amount fixed at 1,000 STB. Each Bo is a synthetic state assumption.
Relation and unit basis
Vreservoir = Bo Vstocktank
Bo in reservoir bbl/STB; this is not a geometric barrel-to-cubic-foot conversion.
1. Define the baseline
Bo (reservoir bbl/STB) = 1.2
Stock-tank amount fixed at 1,000 STB. Each Bo is a synthetic state assumption.
2. Select the relation
Vreservoir = Bo Vstocktank
Bo in reservoir bbl/STB; this is not a geometric barrel-to-cubic-foot conversion.
3. Substitute the values
1.2 reservoir bbl/STB × 1,000 STB = 1,200 reservoir bbl
Retain intermediate precision. The displayed values are rounded only after evaluation.
4. Read the result
Reservoir oil volume (bbl) = 1200; Stock-tank oil volume (STB) = 1000
The same stock-tank amount corresponds to different reservoir volumes under different Bo assumptions.
Result and interpretation
Reservoir oil volume (bbl) = 1200; Stock-tank oil volume (STB) = 1000
The same stock-tank amount corresponds to different reservoir volumes under different Bo assumptions.
The stock-tank amount stays fixed. Only the stated reservoir-to-stock-tank volume factor changes.
Exact scenario data
Original synthetic teaching calculation. Equations and fixed controls are stated here; these are not field measurements.
| Bo (reservoir bbl/STB) | Reservoir oil volume (bbl) | Stock-tank oil volume (STB) |
|---|---|---|
| 1 | 1,000 | 1,000 |
| 1.1 | 1,100 | 1,000 |
| 1.2 | 1,200 | 1,000 |
| 1.3 | 1,300 | 1,000 |
| 1.5 | 1,500 | 1,000 |
Scroll the table to see all columns. The highlighted row is the worked baseline. Downloads retain full numerical precision.
Test your understanding
Work through the question before revealing its answer. Use the stated assumptions and include units in every numerical step.
1. What stock-tank volume corresponds to 1,500 reservoir bbl at Bo = 1.25?
Show the worked answer
1,500/1.25 = 1,200 STB on the specified property basis.
2. Reproduce the baseline Reservoir oil volume (bbl) without rounding intermediate values.
Show the worked answer
1.2 reservoir bbl/STB × 1,000 STB = 1,200 reservoir bbl. Reservoir oil volume (bbl) = 1200. The same stock-tank amount corresponds to different reservoir volumes under different Bo assumptions.
3. Compare the first and last recorded scenarios. Which input changes?
Show the worked answer
Bo (reservoir bbl/STB) changes from 1 to 1.5. Other controls remain as stated in the premise. Reservoir oil volume (bbl) changes from 1000 to 1500.
4. What would be needed to apply this example to a different physical system?
Show the worked answer
Replace the synthetic controls with compatible documented inputs and check the relation's assumptions. Bo in reservoir bbl/STB; this is not a geometric barrel-to-cubic-foot conversion. The same stock-tank amount corresponds to different reservoir volumes under different Bo assumptions.
Before using the result
- Label reservoir bbl and STB.
- Record the Bo state.
- Keep geometric and PVT conversions separate.
The same stock-tank amount corresponds to different reservoir volumes under different Bo assumptions.
Continue learning
Start with the broader workflow guide · Related method guide
Sources and method
The explanation and synthetic examples are original PetroCalcHub teaching material. The references below support the definitions and model context. They do not supply field validation for the synthetic scenarios.