Dipmeter Patterns On Arrow Plots
Trying to analyze a dipmeter without knowing which rocks are shales and which are reservoirs is pointless. Don't ignore the evidence from the other available logs. Don't try to analyze dipmeters in isolation with a blindfold hiding the other logs; use an integrated approach incorporating all available data.
Since some structural patterns can be confused with some stratigraphic ones, you may need to review the stratigraphic patterns before settling on the final interpretation. Remember that structural dips are found at bed boundaries and inside shales. Stratigraphic dips are found inside the sandstone or carbonate reservoirs, not outside them.
The illustrations and text in this section are from "Diplog Interpretation", published by Dresser Atlas (now Baker Atlas) in 1984.
Regional dip is indicated by the dips recorded in these shale sections. The sands are cross-bedded and the limestone fractured, giving readings which cannot be interpreted as regional dip. In many instances, only one of each 15 or 20 dips reflects actual structural dip.
anticline well drilled through the crest of an anticline or the
trough of a syncline would exhibit low angle dips. these dips
can be low enough to give low angle dip scatter. Wells drilled
on either flank of this fold will indicate larger and more consistent
dips. These dips will reflect the structural dip at the point
where the well cuts the formation.
The direction of dip in this asymmetrical anticline decreases until the crestal plane of the structure is encountered then increases to nearly vertical as the well bore cuts the formations essentially parallel to the bedding planes. Other overturned anticlines will produce different Diplog patterns depending upon the amount of overturning present.
Recumbent syncline bed "A" is the youngest bed. Beds "B", "C", "D", etc., become progressively older. A recumbent anticline would be the same except that bed "A" would be the oldest bed with the others becoming progressively younger.
Recumbent anticline dip increases to 90 degrees where the borehole crosses the vertical section of the fold. Below this the dips are reduced and would usually dip in a direction approximately 90 degrees to that above the axial plane of the fold.
fault with no bedding plane distortion. Fault is not apparent
from Diplog and must be located by other methods. If only one
well is drilled in an area, this type of fault will normally not
be found. Bed "E" has been cut out where the borehole
crosses. If this is the zone of interest, the well must be sidetracked
or re-drilled to encounter the objective horizon.
Normal fault with fault plane dipping same direction as formation bedding planes and exhibiting a small drag zone near the fault plane.
fault with fault plane dipping opposite to the dip of the formations
illustrating drag into the fault plane.
Normal fault with rollover. This is a pattern typical of growth faults which frequently exhibit reverse drag or rollover. These rollover anticlines are important hydrocarbon traps along the U.S. Gulf Coast.
Reverse fault with no bedding plane deformation. Beds "D" and "E" ad parts of "C" and "F" have been repeated. Repetition of beds is good evidence that a fault is a reverse fault.
Reverse fault with fault plane dips in the same direction as the dip of the beds. Drag zone in both fault blocks.
Reverse fault with fault plane dips opposite the dip of the formations. Drag zone in both fault blocks. Sandstone is assumed to be free from cross-bedding in this illustration.
Thrust fault with marked bedding plane distortion on both sides of the fault plane. This same pattern could be generated by a recumbent or overturned fold. In some areas, both thrust faulting and overturned folds are commonly encountered. Obviously, the correct interpretation of this arrow pattern depends to a considerable extent upon knowledge of the regional and local geology.
fault with drag on bottom block and little or no deformation of
the beds above the fault plane. This type of response is shown
by the Lewis overthrust which has formed Chief Mountain in Glacier
National Park, Montana.
should study these patterns carefully, comparing patterns from
various structures to define differences and similarities.
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