Invisible Reservoir Screener

Screen gas reservoirs for seismic invisibility due to competing effects: gas decreases velocity (more compressible) but also decreases density (lighter). If impedance changes are small, the reservoir won't produce a strong reflection and could be "invisible" on seismic.

Rock Physics Gas Detection Impedance DHI

Input Parameters

Shale/Seal Properties

m/s
Typical shale: 2500-3500 m/s
g/cm³
Typical shale: 2.2-2.5 g/cm³

Brine-Saturated Sand Properties

m/s
Brine sand: 3000-4000 m/s
g/cm³
Brine sand: 2.2-2.5 g/cm³

Gas-Saturated Sand Properties

m/s
Gas sand: 2200-3200 m/s
g/cm³
Gas sand: 1.9-2.3 g/cm³

Detection Settings

%
Minimum impedance contrast for detection (typically 5%)

Quick Presets

Results

Theory & References

Gas Effects on Rock Properties

Velocity: Gas reduces P-wave velocity (Vp) by increasing compressibility. The effect is strongest at low pressures and high porosities.

Density: Gas reduces bulk density (ρ) because gas is much lighter than water or oil.

Impedance: Acoustic impedance Ip = Vp × ρ. The net effect depends on which change dominates.

ΔIp = (Ip_gas - Ip_brine) / Ip_brine × 100%

When Reservoirs Become Invisible

  • Density Compensation: When velocity decrease is offset by density decrease, impedance barely changes
  • Low Porosity: Less pore space means weaker gas effect on both Vp and ρ
  • High Pressure: At high effective stress, gas becomes stiffer and has less velocity impact
  • Carbonate Reservoirs: Often have lower porosity and weaker fluid effects than clastic rocks
  • Same Polarity: If brine and gas sands both have same sign contrast vs shale, hard to distinguish

Detection Strategies

When impedance contrast is weak, consider:

  • AVO Analysis: Gas may show distinctive gradient behavior even with weak intercept
  • Acoustic Inversion: Quantitative impedance can detect subtle changes
  • Multi-Attribute Analysis: Combine amplitude with frequency, phase, and other attributes
  • Well Control: Calibrate seismic response at known gas wells
  • Seismic Resolution: Ensure reservoir is thick enough relative to tuning thickness