GR & Volume Estimator

Calculate reservoir gross rock volume (GRV), pore volume, and hydrocarbon volumes (STOIIP/GIIP). Includes Monte Carlo uncertainty analysis for P10/P50/P90 estimates.

Reserves Volume STOIIP Monte Carlo

Input Parameters

Fluid Type

Reservoir Geometry

km²
m
fraction
Typical: 0.5-0.9 (clean sand), 0.3-0.6 (shaly sand)

Rock & Fluid Properties

fraction
fraction
rb/stb
Typical: 1.1-1.5 (oil)
fraction
Typical: 0.25-0.45 (primary+secondary)

Monte Carlo Uncertainty (Optional)

Quick Presets

Results

Theory & Formulas

Volumetric Equations

GRV = Area × Thickness

NRV = GRV × Net-to-Gross

Pore Volume = NRV × Porosity

HC Pore Vol = PV × (1 - Sw)

STOIIP & GIIP

STOIIP = (PV × (1-Sw)) / Bo

GIIP = (PV × (1-Sw)) / Bg

  • STOIIP: Stock Tank Oil Initially In Place
  • GIIP: Gas Initially In Place
  • Bo: Oil Formation Volume Factor (rb/stb)
  • Bg: Gas Formation Volume Factor (rb/scf)

Recoverable Reserves

Recoverable = STOIIP/GIIP × Recovery Factor

  • Oil RF: Typically 25-45% (primary+secondary)
  • Gas RF: Typically 70-85%
  • Enhanced recovery can increase oil RF to 50-70%

Monte Carlo Analysis

Uses random sampling within parameter ranges to generate probability distribution:

  • P90: 90% chance reserves exceed this value
  • P50: 50% chance (median, best estimate)
  • P10: 10% chance reserves exceed this value
  • Assumes triangular or uniform distributions

Assumptions & Limitations

  • Assumes homogeneous reservoir properties
  • Single-phase hydrocarbon (oil or gas)
  • Uniform thickness and lithology
  • Does not account for trap geometry complexities
  • Formation volume factors assumed constant