PP fracture detection and characterization
An inline (below) from a full-azimuth full-offset 3D survey, processed through azimuthal PSDM. The OVT tiles were formed, after PSDM, into four azimuth sectors, and four angle stacks per azimuth sector. The interpretation was performed on limited-azimuth limited-angle volumes.
The yellow pick is the Top/Carbonate reservoir.
The grey crosshairs indicate a CMP bin and time of interest, specifically, the known N20E fracture azimuth in the Top/Carbonate, where large fracture density, and wide apertures seen in the horizontal well’s resistivity image log.
Fracture parallel (“N45E”) had a mappable reflector on the far angle stack; fracture perpendicular (“N125E”) showed a brighter amplitude.
From Lynn and Goodway, Azimuthal P-P prestackamplitudes in the presence of oil-filled aligned porosity (fracture porosity),
Interpretation, Nov. 2020,
The same inline, angles 6-15 degrees stacked.
The yellow pick is the Top/Carbonate reservoir.
The grey crosshairs indicate a CMP bin and time of interest, specifically, the known N20E fracture azimuth in the Top/Carbonate, where large fracture density, and wide apertures seen in the horizontal well’s resistivity image log.
Fracture parallel (“N45E”) had a very bright reflector on the near angle stack; fracture perpendicular (“N125E”) showed a very dim amplitude.
These azimuthal amplitudes were modelled in the Lynn and Goodway (2020) paper.
The AVO gradient varies by cosine 2theta (over azimuth),
... for Top/Carbonate Reservoir. Same location as prior two figures.
The fracture parallel azimuth is the most negative AVO gradient azimuth; the fracture perpendicular azimuth is the most positive AVO gradient.
The fracture-parallel direction has “minimal” impact of the aligned fractures; but the fracture perpendicular azimuth has “maximal” impact of the aligned fractures.
These results were matched by models.
The crossplot of Near-Angle amplitudes and AVO gradient for the bin of interest (prior 3 slides).
Model (ORT) that matches field data shown here.
An azimuthal gather at a different bin of interest. Primary sort word: azimuth. Secondary sort word: angle of incidence (0-40).
The pale-green line on the Top/Carbonate Reservoir has a positive AVO gradient at 0(North), but a flat or negative gradient at 90 (east). The near angle amplitude varies from dim to bright (0, 90); the far angle (here used to 25 degrees) varies from bright (0) to brighter (90). The AVO gradient changes by azimuth: with an important part played by the near angle (5 to 15 degree) amplitudes.
Aligned porosity (fracture porosity in one set of fractures)
- alters the visible porosity signature by azimuth: the near (5-15 degrees) amplitudes are typically used to estimate porosity, for a known (shale-carbonate) interface.
The dimmest near-angle amplitude azimuth has the most positive AVO gradient;
The brightest near-angle amplitude azimuth has the most flat (or negative) AVO gradient.
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