What formula does this calculator use?

STOIIP (STB) = 7758 × Area (acres) × h (ft) × φ × (1 − Sw) / Boi. Recoverable reserves = STOIIP × Recovery Factor. The constant 7758 converts acre-feet to barrels.

What is a typical recovery factor for oil?

Primary recovery factors range from 10–30%. With secondary (water/gas injection) and tertiary (EOR) methods, overall recovery can reach 30–60% depending on reservoir type and drive mechanism.

What is formation volume factor (Boi)?

Boi is the ratio of reservoir oil volume to surface (stock-tank) oil volume at initial conditions. It accounts for oil shrinkage when pressure drops at surface. Typical values range from 1.05 to 1.8 res bbl/STB.

What does water saturation mean?

Water saturation (Sw) is the fraction of pore space occupied by water. Oil saturation is (1 − Sw). A higher water saturation means less oil in place. Irreducible water saturation typically ranges from 15–40%.

What is a realistic porosity range?

Most oil reservoir rocks have effective porosity between 10% and 30%. Sandstone reservoirs typically range from 15–25%; carbonates from 5–20%.

Does this include free gas or gas cap?

No. This calculator estimates oil-phase reserves only using the standard oil volumetric equation. Gas cap and dissolved-gas reserves require separate calculations.

Oil Reserves Calculator

Name: Oil Reserves Calculator
Availability: InStock
Author: Nham Vu

Estimate recoverable oil reserves (in barrels) using the volumetric method with six reservoir parameters.

Reservoir Parameters

Reservoir Area (acres)

Net Pay Thickness (ft)

Porosity (%)

Water Saturation (%)

Formation Volume Factor (Boi) (res bbl/STB)

Recovery Factor (%)

Enter reservoir parameters and click Calculate to see results.

Summary

Estimate recoverable oil reserves (in barrels) using the volumetric method with six reservoir parameters.

How it works

Enter the reservoir area in acres.
Enter the net pay thickness in feet.
Input average porosity as a percentage (e.g., 20 for 20%).
Input average water saturation as a percentage (e.g., 30 for 30%).
Enter the formation volume factor (Boi) in res bbl/STB.
Enter the recovery factor as a percentage to get recoverable reserves.

Use cases

Preliminary resource estimation for exploration prospects.
Comparing recoverable reserves across different recovery factor scenarios.
Quick sanity-check of volumetric estimates before detailed reservoir simulation.
Teaching petroleum engineering students the volumetric method.
Evaluating acquisition targets with known reservoir parameters.
Sensitivity analysis on porosity and water saturation uncertainty.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu