Low-Saturation Gas Sensitivity

Shows why small amounts of gas (5-10%) create strong seismic anomalies. Critical insight: seismic indicates GAS PRESENCE, not saturation level.

Rock Physics Gas Detection Reserves

Input Parameters

Rock Properties

fraction
GPa
g/cc

Fluid Properties

GPa
g/cc
GPa
g/cc

Presets

Results

Theory & References

Wood's Equation

Mixed fluid bulk modulus:

1/K_fluid = Sg/K_gas + (1-Sg)/K_brine

Since K_gas << K_brine, even small Sg dramatically increases 1/K_fluid

Result: Fluid becomes much more compressible

→ P-wave velocity drops sharply

Critical Saturation

The gas saturation at which 90% of the total velocity drop has occurred

Typical values:

  • Shallow gas (low pressure): 5-10%
  • Deep gas (high pressure): 10-20%
  • Carbonates (low porosity): 15-25%

Beyond this saturation, adding more gas barely changes velocity

Implications for Reserves

The Fundamental Problem:

  • Seismic anomaly → Gas is PRESENT
  • Seismic amplitude → Cannot reliably estimate SATURATION
  • 10% Sg looks like 100% Sg seismically

Consequences:

  • Cannot map saturation from seismic alone
  • Risk of over-estimating reserves
  • Need well control for calibration
  • 4D seismic can track saturation CHANGES during production

What This Means

Good News:

  • Very sensitive gas indicator - even thin streaks visible
  • DHI analysis is powerful for gas exploration

Bad News:

  • Seismic "bright spot" amplitude ≠ gas column height
  • Cannot distinguish residual gas from commercial accumulation
  • Requires drilling to confirm volumes