STOIIP Volumetric Equation
Stock tank oil initially in place is the headline number of any discovery, computed by multiplying down from gross rock volume to the oil that would sit in surface tanks.
The equation
STOIIP = A h φ (1 – Sw) / Bo
- A is area, h is net pay thickness
- φ is porosity, Sw is water saturation
- Bo shrinks reservoir barrels to stock tank barrels
Worked example
A 2 square kilometer accumulation with 15 meters of net pay, porosity 0.22, water saturation 0.30, and Bo of 1.2 holds 3.85 million cubic meters, about 24.2 million barrels in place. Recover a third of it and the field produces some 8 million barrels.
When to use it
Every input is uncertain, and they multiply, so ranges matter more than the base case. That is exactly why probabilistic volumetrics exist: run the distribution, not the single number.
Related Calculators and Reference
- Mud Weight and Pressure Gradient Calculator: convert between ppg, SG, and psi per foot.
- Probabilistic STOIIP Calculator: volumes with uncertainty ranges.
- PetroWiki by SPE: the industry reference for petroleum engineering.