Stochastic Volumetrics Calculator

Monte Carlo simulation for probabilistic reserves estimation. Input P10/P50/P90 ranges for all parameters and generate full probability distributions.

Volumetrics Monte Carlo Reserves

Input Parameters

Fluid Type

Area (km²)

Net Pay Thickness (m)

Porosity (fraction)

Water Saturation (fraction)

Net-to-Gross Ratio

Formation Volume Factor

Bo (Oil FVF, rb/stb)

Monte Carlo Settings

Presets

Results

Theory & References

Volumetric Equation

Oil (STOIIP):

STOIIP = (Area × h × φ × NTG × (1-Sw)) / Bo

Gas (GIIP):

GIIP = (Area × h × φ × NTG × (1-Sw)) / Bg

Where units are consistent (area in m², thickness in m, volumes in reservoir m³)

Monte Carlo Method

Probabilistic approach that accounts for uncertainty in all parameters:

  • P90: 90% probability reserves exceed this value (conservative)
  • P50: 50% probability (best estimate)
  • P10: 10% probability (optimistic)

Runs thousands of calculations, each time randomly sampling from the input distributions.

Distribution Types

  • Triangular: Simple, bounded by min/mode/max
  • PERT: Beta distribution, more realistic for geological parameters
  • Lognormal: Skewed right, good for naturally varying parameters

PERT is recommended for most subsurface applications.

Best Practices

  • Use geologically reasonable ranges - too wide reduces confidence
  • P90/P10 ratio typically 2-5× for mature plays, up to 10× for exploration
  • Check for parameter correlation (e.g., porosity and Sw often correlate)
  • Mean ≠ P50 for skewed distributions
  • Report P50 for planning, P90 for conservative reserves booking