Fracture Indicator & AVAZ Feasibility

Analyze well log signatures to identify natural fractures and assess whether azimuthal AVO (AVAZ) seismic analysis can detect them. Scores multiple fracture indicators and evaluates seismic detectability based on acquisition geometry and reservoir properties.

Fractures Petrophysics AVAZ HTI Anisotropy

Input Parameters

Log Data (at Reservoir Depth)

μs/ft
Compressional sonic (DT or DTC)
g/cc
fraction
For porosity-sonic crossover check
ohm-m
ohm-m
For invasion profile analysis

Reference/Background Values

μs/ft
Non-fractured zone sonic
g/cc

Optional: Advanced Indicators

%
Leave blank if not available
inches
Difference from bit size

AVAZ Feasibility Parameters

m
degrees
Range of azimuths in survey (need >180° for AVAZ)
m
HTI anisotropy requires aligned fractures

Results

Theory & Guidelines

Log Signatures of Fractured Reservoirs

Key fracture indicators:

  • Porosity-Sonic Crossover: High sonic-derived porosity (from slow velocity), low density porosity (fractures add pore space but reduce density less)
  • Sonic Slowness Spike: DT increases 20-40% where fractures present (reduces P-wave velocity)
  • Resistivity Separation: Deep Rt > Shallow Rxo in oil-bearing fractures (invasion profile differs)
  • Stoneley Attenuation: Tube wave energy absorbed by permeable fractures (40-80% amplitude loss)
  • Caliper Enlargement: Breakouts and washouts common at fractured intervals

HTI Anisotropy & AVAZ

Physics: Aligned vertical fractures create Horizontal Transverse Isotropy (HTI). Seismic velocity and amplitude vary with azimuth relative to fracture strike.

AVAZ Analysis:

  • Requires azimuthal coverage >180° (ideally 360°)
  • Detectable anisotropy typically >5% amplitude variation
  • Works best with high fracture density and consistent orientation
  • Needs adequate offset for AVO measurement (>1× depth)

Processing workflow: Azimuthal binning → AVO analysis per azimuth sector → HTI inversion for fracture parameters

Seismic Detectability Criteria

Fractures are seismically detectable when:

  • Fracture density sufficient (typically >0.5 frac/m for clastic, >1 frac/m for carbonate)
  • Fractures aligned (HTI requires dominant orientation within ±30°)
  • Aperture large enough (>0.01 mm, fluid-filled)
  • Seismic wavelength appropriate (fracture spacing << wavelength)
  • Good azimuthal and offset distribution

Alternative methods if AVAZ not feasible: seismic coherence/discontinuity, curvature, ant-tracking