Oil Initially In Place (OIIP)

Oil initially in place (OIIP) is the total volume of crude oil estimated to exist in a reservoir before production. It represents the quantity physically present in the subsurface, not the amount that can be recovered economically. OIIP is a starting point for reservoir evaluation and informs decisions on development, drilling and reserve classification.

How OIIP is estimated

Estimating OIIP relies on geological and petrophysical data gathered from seismic surveys and test wells. Key parameters include:
* Areal extent of the reservoir (A)
* Net pay or thickness of the hydrocarbon-bearing interval (h)
* Porosity (φ) — fraction of rock that can store fluids
* Water saturation (Sw) — fraction of pore space occupied by water
* Formation volume factor (Bo) — conversion between reservoir and surface volumes

A common volumetric formula (for stock tank oil initially in place, STOIIP) is:
STOIIP = 7758 × A × h × φ × (1 − Sw) / Bo
(where A is in acres, h in feet, and 7758 converts acre‑feet to stock tank barrels). Values for these inputs come from core samples, well logs and reservoir modeling.

Related terms

• STOIIP (Stock Tank OIIP) — OIIP expressed as surface (stock‑tank) barrels after accounting for pressure/volume changes.
• OGIP (Original Gas in Place) — the equivalent calculation for gas reservoirs.
• HCIIP (Hydrocarbons Initially in Place) — a generic term covering both oil and gas.

OIIP versus reserves

OIIP is not the same as reserves. Reserves represent the portion of OIIP that is technically and economically recoverable under current technology and market conditions. Recovery factors (the percentage of OIIP that can be produced) vary widely with reservoir properties and recovery methods.

Economic importance

OIIP indicates a reservoir’s potential scale but does not alone determine whether development is worthwhile. Operators must estimate recoverable volumes and development costs to decide whether to:
* Move volumes into proved/probable reserves,
* Delay development until prices or technology improve, or
* Invest in enhanced recovery methods to raise the recovery factor.

For example, if 50% of OIIP is currently recoverable, a field owner may defer drilling if prices are low, or pursue recovery-improving investments if economics justify them. Oil companies continuously re-evaluate OIIP, recoverable estimates and market conditions when planning operations.

Key takeaways

• OIIP measures the total oil present in a reservoir, not what can be produced.
• Estimation uses porosity, saturation, net rock volume and formation factors derived from wells and seismic data.
• Only a fraction of OIIP becomes reserves; recoverable volumes depend on geology, technology and economics.
• OIIP is fundamental for reservoir assessment and strategic decisions about exploration and development.