Structural analysis of borehole images

Borehole images can yield highly detailed data-sets that provide a 1D sample of feature orientations along the well trajectory. Structural analyses up-scale these data and thereby allow a geological model to be built and extended away from the wellbore. The principle elements of structural analysis are structural zonation and the derivation of representative structural tilts by zone, fracture and fault characterisation and the assessment of present-day in-situ stress. Throughout the interpretation process, we use the attitude software application developed by our sister company Task Geomodelling specifically for this purpose.

Quick-look structural analysis

• Filtering of manual dip data to extract indicators of the palaeohorizontal during deposition.
• Display of such data on tadpole plots, azimuth plots and rose histograms.
• Vector analysis (including azimuth walkouts and cumulative dip magnitude plots) to identify orientation changes between structural zones.
• Stereographical analysis of filtered bedding data by zone to generate representative structural dips defining post-depositional (generally tectonic) rotation.
• Identification of probable faults by direct observation, bedding drag and fractures (damage zones).
• Presentation of fracture and fault data by type as stereoplots.
• Examination and interpretation of structural zone boundaries into e.g. unconformities, faults, palaeohorizontal changes.

Standard structural analysis

As above, plus:
• Curvature analysis of progressive bedding dip rotations. Axes of curvature are helpful in defining large-scale structural elements such as fault strike, palaeoslope in slumped successions and axial trends in folded sequences.
• Assessment of structural style to characterise gravity transport deposits.
Identification and interpretation of fracture sets by character and orientation.
• Generation of fracture frequency and spacing data along the wellbore and correction to mitigate borehole sampling bias. Presentation of fracture frequency curves by constant interval, structural zone and supplied unit.
• Fault kinematic analysis, including modelling of drag geometry to estimate sense of offset and minimum throw, and the description of damage zones.
• Presentation of structural cross-sections through transverse sections.
• Interpretation of present-day in-situ stress indicators such as borehole breakout, drilling-induced tension fractures, oriented calipers and acoustic transit time to assess the orientation of the present-day stress field at the wellbore.

Advanced fracture analysis

As above, plus:
• Integration of dynamic data such as well test, pressure, temperature and spinner data to assess reservoir performance and hence understand barriers, baffles and conduits. Evaluation of mud log records against fractured intervals.
• Modelling of critically stressed fractures using the present day in-situ stress field orientation.

Structural correlation

• Evaluation of geological models against borehole image structural information.
• Generation of structural cross-sections.
• Mapping of horizons based on borehole-derived structure.
• Advanced curvature analysis of key surfaces.
• Inter-well correlation of structural styles and features.
• Geometrical modelling of faults cut in multiple well sections.
• Comparison of dip data to seismic, both in cross-section and plan views.

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