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Quick Look Techniques |
Implied Fault Analysis on Structure MapsStructure maps in faulted areas are often prepared without the use of fault surface maps (Tearpock and Bischke, 1991). Instead, isolated fault cut data from well logs or seismic lines are used to draw a fault trace on a completed structure map. If a prospect map you are reviewing has been prepared in this manner, there are two primary concerns regarding the map: (1) are the faults mapped correctly and (2) are the fault traces positioned correctly? The position of fault traces on a completed structure map is directly related to five (5) factors: the amount of missing section or vertical separation, the strike and dip of the fault, and the strike and dip of the formation. To check a completed structure map for accuracy we use a quick look technique called "Implied Fault Analysis". If an implied fault surface is geologically unreasonable either with regard to fault dip or strike, it follows that the fault trace representation on a completed structure map will also be unreasonable. This may lead to the conclusion that the faults are not interpreted correctly or that the traces of the fault are positioned incorrectly on the completed map. Either problem can greatly impact a prospect. An implied fault surface map is constructed by recording where each structure contour, of a given elevation, intersects the fault trace on a completed structure map. Where the structure contour intersects the fault trace, the elevation of the fault surface is implied at that point. The elevation of the surface of the fault at each intersecting location is equal to the elevation of the intersecting structure contour. Figure 1A is a two-dimensional map view of the top of a horizon. The map shows two fault traces and a fault gap. On Figure 1A we see that at Point A the 7000-ft structure map contour in the downthrown block intersects the 7000-ft contour on the fault surface. The same contour on the fault surface intersects the 7000-ft structure map contour on the upthrown block at Point B. It is obvious from Figure 1B that the intersection of the structure contours, with the fault surface contours of the same elevation, define the position of the upthrown and downthrown fault traces. The upthrown and downthrown intersections also delineate the width of the fault gap (Fig. 1B).
 Fig. 1a
 Fig. 1b Figure 2 shows a structure map in a faulted area. Let us apply Implied Fault Analysis to this map. In the south, the 4100-ft contour on the upthrown block can be connected to the 4100-ft contour in the downthrown block. Based on the implied fault analysis, the fault strikes east-northeast and dips to the north-northwest. If the same method is applied to the 4400-ft contour in the central portion of the map the implied fault strike direction changes to the northwest. And in the north, if the 4600- ft upthrown and downthrown contours are connected, the same fault now strikes north-south and dips to the east. This fault is a Screw Fault as described in the previous article in this series. Therefore, the map is geologically impossible.
 Fig. 2 Figure 3 is an exercise for you to try. Can you justify the fault as presented on the structure map?
 Fig. 3 Implied fault analysis is a powerful Quick Look Technique (QLT) that can be employed very rapidly to evaluate a fault on a completed structure map. The structure and fault surface contours of the same elevations must intersect at a point located on the fault trace. Simply connect the upthrown and downthrown points of equal elevation to construct the implied fault surface map. The use of this technique to check a fault interpretation on a completed structure map is a must when you know a fault surface map was not prepared and integrated with the final structure map.
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