Quick Look Techniques

Screw Faults

A screw fault is a fault that laterally reverses its direction of dip. This is a mapping contrariety that cannot exist in extensional areas and has only a questionable possibility in compressional areas.

Consider Fault B in Figure 1 which is a completed structure map on the AA Sand. Follow the fault trace from West to East. Notice to the West that the contours indicate a fault downthrown to the South. Consider the area between the -8,400-ft and -8,350-ft contours downthrown to the fault in the eastern region. At this position the map indicates that the fault changed its direction of dip from South to North. Observe all the contours placed downthrown to the fault from Location A to B as in contrast to only one contour (-8,400-ft) upthrown. Fault B is a screw fault.

Also, consider Location D (Figure 1) on Fault A. Notice that Fault A has reduced in size (Vertical Separation) from 300 ft at Location C to approximately 50 ft at Location D. Fault A is working toward becoming a screw fault.

Fig. 1

A screw fault is indicative of an incorrect fault interpretation. Figure 2 is the correct interpretation for the area mapped and is significantly different. The prospective areas are totally different.

Fig. 2

A map with a recognized screw fault in an area of interest should be rejected until the problem can be resolved.