Dip Distortion in Geosteering Due to Stratigraphic Thinning or
Thickening Relative to Control Well(s)
Dip distortion due to stratigraphic thinning or thickening
relative to control well(s) is common when geosteering wells. Distorted dip may
be misinterpreted as a structural anomaly. It may look like a small fault or
fold in graphic format when matching up gamma. A tell-tale sign of thinning is
dip distortion every time the particular section is cut. The direction of dip change
(up or down) should be opposite when cutting down section compared to the
direction when cutting up section.
Geosteering programs will distort the dip automatically with
thickness differences relative to control well(s) unless the dips are manually
adjusted. If dips are not manually adjusted, the TVDs given at the MDs in those
sections will not be correct. Such TVDs should not be used in subsequent
structure mapping.
Thickening relative to control well(s) is harder to detect and may manifest as more section running along with dip in the particular section than otherwise would be the case.
Deflection of the bit off of hard zones can also cause dip
distortion, often of a similar magnitude. However, this is not accompanied by cutting
through section but by changing inclination due to deflecting. In deflection
the inclination is often unexpected and it is likely that the inclination is
distorted rather than the dip, although it is typically assumed that it is dip
distortion since that is how the geosteering program must interpret it. On this
basis it should be relatively easy to distinguish between the two. Of course,
sometimes there may be genuine structural changes due to small faults so that
the expressed dip is real. Folds typically show as folds with no dip
distortion.
Below is an SES example of dip distortion. Here it looks
like the clean limey section in the target interval is thinned relative to the
control well. It will be noted that when cutting down through that section the
dip was distorted up and when cutting up through the section the dip was
distorted down. This occurred multiple times.
