What The Patterns Mean
The second is to use a catalog or cook book of typical patterns to compare your pattern with those already described. As mentioned earlier, regional dip removal can change a pattern, so the cook book approach is not too useful unless dip removal has been done. Also, the cook book patterns presume that dip directions shown on logs are always parallel to your cross section direction. This is not always true so it becomes necessary to rotate dips to get the "best" patterns. Both transverse and longitudinal cross sections should be visualized when analyzing dip patterns.
Before you start sketching patterns, review the basic structural features described earlier in this Chapter. Then draw a sketch of the dipmeter data. Take a piece of graph paper or blank well log paper and draw a vertical line to represent the wellbore. A log print with gamma ray, SP, and resistivity is also a good place to draw your diagram.
Select the interval you wish to analyze and mark some depth lines to orient your data. For structural analysis, a compressed scale of 1 or 2 inches to 100 feet or smaller is appropriate. Transfer the position of the black patterns to your sketch. These represent breaks in the geologic sequence, such as unconformities or tops and bottoms of sedimentary structures. Use the gamma ray curve or a computed lithology log and the well history data as guides to major erosional surfaces.
Stick plot of regional dip
Next, choose regional dip in each major rock unit. At this point you have to decide on the direction of cross section that your sketch will represent. Usually, for structural analysis, it is chosen to be the regional dip direction, although another sketch drawn at right angles to the first may be useful in many cases.
For example, if regional dip is to the south east, the cross section should run from north west to south east. Draw short hash marks on the well bore at an angle representing the actual dip shown on the log. Some vertical exaggeration may be appropriate. An example of this simple case is shown above.
Stick diagram for a normal fault with drag
Next, position representative samples of the dip from the blue and red patterns onto your sketch. You are really creating your own stick plot. It may be helpful to include a sketch of the dipmeter log itself on the same piece of paper. If the red and blue patterns are contained within a sand body, they are stratigraphic dips and should not be used in a structural interpretation.
Now comes the hard part. Extend the hash marks to represent the bedding planes of a structure. Basically you are only dealing with regional dip, anticlines, synclines, unconformities, and faults. To propose a fault, there should be some evidence from the well history, some scattered dips at the break between the red and blue patterns, a change in direction of dip, some missing or repeated section, or drag and rollover features. The red pattern is usually connected to the green pattern above it.
Stick diagram for overthrust fault
Reverse faults and overturned anticlines can have similar patterns - the anticline is distinguished by dips approaching 90 degrees, whereas reverse faults seldom do this. Overthrust faults will usually show an abrupt change in dip direction near the fault plane
Normal faults, overthrust faults, channels, unconformities, and disconformities can have similar patterns - repeat section indicates the overthrust case. The lithology and dips indicating crossbedding help distinguish channels. Choose the model which suits the local geology the best. Most patterns can be interpreted without imposing a fault and the most common error in dipmeter analysis is the suggestion of too many faults.
Growth faults, contemporaneous with deposition, usually show rollover, which is a dip pointing toward the upthrown block. Post-depositional faults usually show drag, which is a dip pointing toward the downthrown block. Hybrid faults can exhibit complex patterns. Some faults show no rollover or drag. Remember the effect of regional dip on these patterns.
your own stick diagram and interpreting a plausible geologic section
to match it takes practice, patience, and a good grasp of 3-D
space. A good knowledge of geology doesn't hurt. The effect of
hole deviation must also be considered. Although the dips presented
on the log are true dips, with hole deviation taken into account,
their position in space may not have been corrected for true vertical
depth or the track of the borehole.
Page Views ---- Since 01 Jan 2015
Copyright 1978 - 2019 E. R. Crain, P.Eng. All Rights Reserved