While we are open to specialized dip type classifications and definitions based on your needs, HEF does apply a consistent set of bedform and fracture fabric definitions when we manually interpret borehole image features with the following typical designations:
Dip Types (Standard Structural and Fracture Interpretations)
Bed (formerly referred to as COMPUTED)
• Bedding features that are laid down horizontally, or close to it
• The main guide for interpreting primary structural features (faults and folds)
• In well bedded zones the bedding surfaces are usually very distinct features on the image log
BED_LOW_CONF
• Lower confidence bedding where the image is poor quality, is distinct or shattered by fractures,
or otherwise obscured by other textures
CROSS BEDDING
• Bedding surfaces with stratigraphic dip relative to surrounding structural bedding
• Wispy texture like brush strokes on the image log
• Good indicators of paleocurrent, energy and depositional environment
NODULAR EVENT
• Irregular contact such as from a concretion
• Features that are lens or irregularly shaped and should not be included as structural or
stratigraphic bedding, or truncations of any significance
• Nodular events are included only in the HEF image plot, so that the orientation of these features
can be known at a glance, but do not pollute other data sets
TRUNCATION
• Contacts between different rock types, or a significant stratigraphic break
• Well defined surfaces that are oblique to the bedding (BED) above or below (or both)
• Questionable or unclear structural breaks, such as possible faults that have no associated fracturing
• HEF avoids using the term TRUNCATION for the many bed-set boundaries within a cross-bedded
sandstone sequence
FRACTURE
• Open, dark, conductive (or acoustically non-reflective) feature (dark because the mud almost always
has a lower resistivity than the surrounding rock)
• Intersects more than 50% of borehole circumfrence
• Note: The fracture aperture on an image log appears orders of magnitude larger than the actual
aperture (HEF can estimate the aperture using mud and formation resistivities along with the trace
width
SHORT FRACTURE
• Same characteristics as an open fracture
• Intersects less than 50% of the borehole circumfrence
• Lower confidence feature
• Usually only picked in acoustic and oil-based images
LARGE FRACTURE
• Open fractures that appear over the full circumfrence of the borehole
• Have an appreciable thickness (aperture) on the image
HEALED FRACTURE
• Fractures with no open aperture space
• Electric image logs: light-colored traces or traces that are dark on one side and light on the other
• Acoustic image logs: often cannot be imaged or appear as light colored (acoustically reflective)
• Filling material is generally mineral cements or sometimes low-porosity gouge developing along the
fracture, particularly common along small offset shear fractures that occur in high-porosity
sandstones
• Fractures infilled with clay cements will appear dark, and are nearly impossible to distinguish from
open fractures
BREAKOUT
• Normally as a result of under balanced drilling, the sides of the wellbore that are in the direction
of the minimum horzontal stress may spall away
• Breakout, or spalling appears as dark, parallel vertical features on opposite sides of the borehole
• Good indicator of present-day principal horizontal stress direction
DRILL.IND.FRAC.
• As a result of over balanced drilling, tensile fractures (drilling induced fractures) can form and
propagate in the direction of the maximum horizontal stress
• They appear as a pair of borehole-parallel straight conductive cracks (vertical, en echelon or
otherwise stress-delimited) that are non-sinusoidal
• Good indicator of present-day principal horizontal stress direction
OPEN SHEAR
• Open fracture that appears to have some amount of fault displacement
• This term is reserved for features that have direct or indirect indications of shear offset
• Generally shear fractures or small faults, though sometimes they may be the surfaces of
larger faults or fault zones
• May appear as a) an offset or mismatch of the bedding across a fracture-type trace or b) the
appearance of a "breccia" texture along a zone. Open shears are often associated with significant
flow zones
SEALED SHEAR
• Closed or resistive-material-filled fracture that appears to have fault displacement
• Similar to OPEN SHEAR, but filled with fault gouge and/or healed with resistive minerals (similar
to HEALED FRACTURES)
Dip Types (Oilsands)
Scour Surface
• Truncational surfaces where the overlying sand is different in dip magnitude and/or direction from
the underlying bedding
• Bedding surfaces with “pebbly” rip-up clast features (uneven surfaces) in the sands
• Muddy units with an overlying sand body. Contact is generally clear
• Lower and upper contacts of Mud Breccias
Erosional Surface
• The contact between sand and overlying thick mud (> 0.5m), signifying a flooding event
• Boundaries within an IHS (Inclined Heterolithic Strata) unit where there is a clear, abrupt change in
the lateral accretion direction
Unconformable Bed Boundary
• Only at the top of the Carbonate/McMurray boundary
Lateral Accretion
• Packages of alternating thin sand/shale beds (cm’s thick) which are inclined (3-15 deg) with similar
dip magnitude and direction
• Generally occur in IHS and mudstone, but can carry on into marginal sands and mudstones
• Units can be several metres thick
Cross Bedding
• Very thin, sometimes faint, wispy beds in clean sands (<10% VSH)
• Bedding inclined between 10-35 degrees
• Good paleocurrent cross-beds occur generally between 15-35 deg
• Cross-beds commonly bounded by scour surfaces
Bedding
• Any other bedding surface that doesn’t fit the above definitions
• Stratigraphically flat bedding contrast
Convoluted (Bedding)
• High-angled bedding (>10 deg)
Nodular Event
• Discontinuous lenses or concentrations of harder sediment which disrupt the surrounding bedding
• Generally occur in siderite bands in the caprock
Large Fracture
• Open fractures that appear over the full diameter of the borehole and have an appreciable
thickness (aperture) on the image (see above)
Fracture
• Open conductive fracture that occurs across more than 25% of the borehole
• Fractures appear darker than the surrounding lithologys
• In rare cases, the resistivity of the mud filtrate (RMF) value is higher than the openhole resistivity
curve, resulting in a fracture that is lighter than the surrounding lithology (see above)
Healed Fracture
• Sealed or mostly sealed fracture filled with resistive material
• Healed fractures appear lighter than the surrounding lithology (see above)
Open Shear
• Open fracture that appears to have some amount of fault displacement (see above)
Sealed Shear
• Closed or resistive-material-filled fracture that appears to have fault displacement (see above)
Drill.Ind.Frac
• Parallel vertical cracks in the plane of horizontal principal stress (see above)
Breakout
• Parallel vertical features where the borehole wall is enlarged perpendicular to the principal
horizontal stress direction (see above)
· The main guide for interpreting primary structural features (faults and folds)