Quick Look Techniques

Additive Property of Faults

In an area of intersecting faults, the vertical separation (the missing or repeated section) of the individual faults should be additive, or very close to additive, across the intersection where the two faults merge into one. Vertical separation or missing section is discussed in the previous issue of this Bulletin. Figure 1 shows two faults downthrown to the south that merge laterally to the east. The vertical separation for the eastern portion of the fault is equal to the sum of the vertical separations for the two smaller faults.

When checking the additive property, we are not referring to the fault gap or overlap widths being additive, but the vertical separations (the missing or repeated section seen in a well log). The vertical separation is checked by calculating the vertical difference in contour values across the fault in the strike direction of the contours (Figure 1). An easy way to review the additive property of faults is to go clockwise around a fault (see dashed circle on Figure 1) adding the vertical separations for the faults dipping in a clockwise direction and subtracting the vertical separations for the faults dipping in a counter-clockwise direction. If the additive property of intersecting faults is honored correctly, the additions and subtractions should nearly equal after crossing all the faults. Be sure to check this property as close to the intersection as possible. Faults can change in size laterally; therefore, at some distance from the intersection the vertical separations may not be additive. Typically, the closest contours to the intersection are used to check this property.

Fig. 1

Figure 2 shows a completed structure map prepared from seismic data with intersecting faults that do not honor the additive property. This is a direct indication that the map is incorrect. There is a 1500 foot bust on this map. Because of the magnitude of this error, it is likely that the problem is the result of a seismic mistie across one or more faults. The data must be reviewed again and the map redone before any decisions can be made with regard to hydrocarbon potential.

Fig. 2

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