1. The Volumetric STOIIP Equation (Oilfield Units)
In standard oilfield engineering units (acres, feet, stock tank barrels), the volumetric formula for oil initially in place is:
Where:
7,758 = Conversion constant (barrels per acre-foot: 43,560 sq ft/acre × 1 bbl/5.61458 cu ft)
A = Reservoir drainage area (acres)
h = Net pay thickness (feet)
φ = Average reservoir effective porosity (fraction, 0 < φ < 1)
S_wi = Initial irreducible connate water saturation (fraction)
B_oi = Initial oil formation volume factor (reservoir barrels per stock tank barrel - rb/STB)
2. Petrophysical Parameter Breakdown
- Net Pay (h): Derived from wireline logs (gamma ray, resistivity, density-neutron) by applying cutoffs for minimum porosity, maximum shale volume (Vsh), and maximum water saturation.
- Effective Porosity (φ): Interconnected pore volume capable of storing and transmitting fluid. Typically ranges from 8% (tight sandstones) to 28% (unconsolidated sands).
- Formation Volume Factor (B_oi): Crucial thermodynamics parameter from PVT laboratory analysis. Because oil contains dissolved gas that flashes out when brought to the surface, 1 stock tank barrel of oil occupies more than 1 barrel inside the pressurized reservoir (typical B_oi = 1.15 to 1.65 rb/STB).
3. Recoverable Reserves & Recovery Factor (RF)
STOIIP represents the total oil in the rock, not the amount that can be commercially extracted. Recoverable reserves equal STOIIP multiplied by the Recovery Factor (RF):
Typical Primary Recovery: 10% - 25% (Depletion drive)
Secondary Recovery (Waterflood): 30% - 50%
Tertiary Recovery (EOR - CO2, Chemical, Thermal): 50% - 65%
An exploration discovery well penetrates a clastic sandstone reservoir with the following petrophysical evaluation:
- Reservoir Area (A): 1,280 acres (2 sections) • Net Pay (h): 65.0 feet
- Porosity (φ): 20.0% (0.20) • Water Saturation (S_wi): 30.0% (0.30 → 70% oil saturation)
- Oil FVF (B_oi): 1.32 rb/STB • Estimated Recovery Factor (RF): 32.0%
- Step 1: Calculate Reservoir Rock Volume
Rock Volume = 1,280 acres × 65.0 ft = 83,200 acre-feet - Step 2: Calculate Pore Volume & Hydrocarbon Volume
Hydrocarbon Pore Volume (HCPV) = 83,200 × 0.20 × (1 - 0.30) = 11,648 acre-feet - Step 3: Convert to Surface Barrels (STOIIP)
STOIIP = (7,758 × 11,648) / 1.32 = 90,365,184 / 1.32 = 68,458,473 STB (68.46 Million Barrels) - Step 4: Calculate Estimated Recoverable Reserves
Commercial Reserves = 68,458,473 × 0.32 = 21,906,711 STB (21.91 Million Barrels).
Frequently Asked Questions
Why is Monte Carlo simulation preferred over deterministic STOIIP estimation?
Reservoir parameters (especially net pay, porosity, and area) vary across vast underground geology. Monte Carlo assigns probability distributions to each input, generating a full cumulative probability curve (P90 Proved, P50 Probable, P10 Possible) compliant with SPE PRMS guidelines.
What is the equivalent formula for Gas Initially in Place (OGIP)?
For gas reservoirs: OGIP (G) = [ 43,560 × A × h × φ × (1 - S_wi) ] / B_gi (in standard cubic feet - SCF), where B_gi is the gas formation volume factor.