reservoir · volumetrics
Oil in Place (OOIP)
OOIP = (7758 × A × h × φ × (1−Sw)) / Bo
click formula to derive ↑
Inputs
acres
ft
fraction
fraction
RB/STB
Description
Estimates the original oil in place using the standard volumetric equation — drainage area times net pay times porosity times hydrocarbon saturation, converted from reservoir to stock-tank conditions via the oil formation volume factor. This is the foundation for resource assessment before a recovery factor is applied.
Variables
| Symbol | Unit | Description |
|---|---|---|
| OOIP | STB | Oil in Place |
| A | acres | Areal extent of the drainage area or reservoir compartment. 259 ha (≈640 acres / 1 section) is a common single-well drainage assumption for unconventional spacing. |
| h | ft | Net (hydrocarbon-bearing, reservoir-quality) pay thickness, after applying porosity/Sw/Vcl cutoffs from the net pay calculation. |
| φ | fraction | Average effective porosity of the net pay interval — typically a thickness-weighted average from log analysis. |
| Sw | fraction | Average water saturation of the net pay interval. (1−Sw) gives the hydrocarbon saturation used in the volumetric calculation. |
| Bo | RB/STB | Oil formation volume factor — reservoir barrels of oil (plus dissolved gas) per stock-tank barrel at surface. Typically 1.1-1.5 for black oil, higher for volatile oil. |
Assumptions
- Porosity, Sw, and net pay are representative averages of the drainage area (not just the wellbore)
- Bo is representative of the reservoir fluid at initial reservoir pressure and temperature
- Drainage area and net pay are reasonably well constrained by seismic, offset wells, or analogy
Limitations
- A single-point volumetric estimate does not capture reservoir heterogeneity — use a distribution (P10/P50/P90) for reserves reporting
- Does not account for recovery factor — multiply by RF (often 15-40% primary, higher with EOR/waterflood) to get recoverable reserves
- Sensitive to the area and net pay estimates, which are often the largest sources of uncertainty early in field life
Use Cases
- → Resource size estimation: Early-stage estimate of hydrocarbons in place before detailed reservoir simulation, used to screen prospects and rank opportunities.
- → Economic screening: Combine with a recovery factor and oil price to perform first-pass economic screening of a prospect or development area.
- → Reserve booking support: Provide the volumetric basis for SEC/PRMS reserve bookings, typically run probabilistically across the input ranges.
Related Calculations
Region Notes
Permian Basin
Unconventional spacing units of 1280 ac (DSUs) are common; per-well drainage area is often modeled as 1 section (≈259 ha) or less depending on well spacing within the DSU.
Gulf of Mexico
Bo for GOM Miocene oils is often 1.2-1.6 RB/STB; deepwater fields with higher GOR can have Bo above 1.8.
North Sea
Brent/Forties crude typically has Bo≈1.3-1.4 Rm³/Sm³; drainage areas are constrained primarily by 3D seismic structure mapping rather than well spacing.
Global
Always report OOIP alongside the recovery factor assumption used to derive recoverable volumes — OOIP itself is not a reserve.
References
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