Faults are usually picked from seismic data but wells should be checked for evidence of faults. This is because smaller faults detected from wells can have as much impact upon field development as larger seismic-scale faults.

The first step is to check thickness variations in correlation panels of your well logs. This can be TVD thickness but TST (true stratigraphic thickness) is needed in more steeply dipping beds. The most common fault-related explanations are shown in Figure 1 and 2.

Figure 1. (a) Missing section where a well fails to hit some of the stratigraphy - in this case most of the bright orange unit. (b) The thicker light orange unit is a result of deposition of this unit as the normal fault is active.

Figure 2. Repeat sections - Upper one is caused by a reverse fault, Lower one is where a high angle well intersects a normal fault

As faults can rotate bedding, they can also be picked on cumulative dip plots. In the post “Think about Fault Drag” we showed how bedding dip and azimuth are commonly displayed as tadpoles on log tracks.

You can take the following steps,

Export bedding dips and azimuth (from most industry softwares) to a spreadsheet.
- On the spreadsheet (Figure 3), measured depth (metres) in Column A, Bedding Azimuth in Column B and Bedding dip (degrees) in Column C.
Populate Cumulative Dip in Column D (Figure 3) using the formula in Figure 3.
Create a scatter chart (Figure 4) from Column D. This will need some formatting and switching of axes etc. I’ve also added some arrows and numbers to show structural domains.
Azimuth (column) be can be used to create a walkout plot (later blog).

The separation of the 4 structural domains is an interpretation and other lines of evidence should also be considered when making a fault interpretation. Check if the lithology changes as the boundary may be an unconformity (Figure 5). Note also that these plots will not detect faults that are boundaries between beds of similar dip but different azimuth - this is where an azimuthal walkout plot is useful.

Figure 3. Data input to cumulative dip plot

Figure 4. Cumulative dip plot with 4 structural domains picked, interpreted to be separated by faults.

Figure 5. Unconformity between the Upper and Lower Devonian, Arbroath Scotland (see post “Scottish Unconformities: Arbroath”)