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Quick Look Techniques |
Mapping Throw In Place of Vertical Separation: A Costly Subsurface Mapping MisconceptionPerhaps the most costly subsurface mapping error made by interpreters is to map vertical separation as if it is throw. The two terms are often confused; vertical separation is called throw, or throw is substituted for vertical separation. These two terms are, however, not identical. They are measurements of two distinctly different geometric fault components. The following discussion is a simplified explanation of correct mapping procedures. For a complete discussion of proper mapping techniques and methods please consult Tearpock and Bischke (1991). Terminology, DefinitionsThe term "throw" is commonly used in structural literature to describe a fault displacement component. Throw is illustrated in Figure 1. It is important to understand that throw, in contrast to vertical separation, is a fault slip or displacement component. Throw is defined in the A.G.I. Glossary as the "vertical component of the net slip", which is AC in Figure 1.  Fig. 1 Throw, which is related to fault slip or displacement, cannot be directly measured from electric logs, as is discussed below. Throw is not normally used in subsurface mapping techniques. Vertical separation (AE) in Figure 1 can be defined as the distance that a bed has been vertically displaced during faulting. This distance is important to mappers, as vertical separation is directly recorded by (or readily calculated from) electric logs. To illustrate this point consider the following hypothetical example of a structure that dips uniformly to the west (Fig. 2). Two wells are drilled into this structure, which contain the SP logs shown in Figure 2. The dashed line in Figure 2 represents a normal fault, which displaces such that the hanging wall portion of Well No. 1 is juxtaposed over the footwall portion of Well No. 2 (Fig. 3).
 Fig. 2 From the geometric configuration shown in Figure 3 the following observations can be made. As Well No. 1 is displaced above the footwall portion of Well No. 2, the top of Bed B in the hanging wall is brought into contact with the top of Bed C in the footwall. Therefore, the missing section in Well No. 1 corresponds to the vertical interval from the top of Bed B to the top of Bed C. In Figure 2 the sequence of Beds A, B, and C in Well No. 1 becomes the sequence A and C in Figure 3. Closer inspection of the electric logs reveals that the missing section in Well No. 1 is represented by the coarsening upward sequence and its lower shale that are present in the hanging wall portion of Well No. 2. Observe that throw is not equal to missing section. This hypothetical example clearly shows that throw is not equal to missing section. Furthermore, the missing section is equal to the fault component Vertical Separation.  Fig. 3 One Common Error A common mapping mistake is to measure the vertical separation in electric logs and then apply the vertical separation as if it is throw when mapping across faults. An incorrect method for mapping across faults is shown in Figure 4. In Figure 4 we employ the -8000-ft contour as an example and assume 1200 ft of missing section caused by a fault that strikes NE-SW. Data from the upthrown block is contoured, and the contours are projected to the upthrown fault trace. At the intersection of the -8000-ft contour with the upthrown trace, the -8000-ft contour is projected across the fault gap perpendicular to the trace or the strike of the fault. At the intersection of the projected -8000-ft contour with the downthrown fault trace the contour is assigned the value of -9200-ft = -8000-ft + (-1200 ft of missing section). The -9200-ft contour is then drawn in to honor the well control in the downthrown block. We have just applied the missing section as if it is throw. The correct method for applying log data in mapping across faults is to map the vertical separation across the fault. Vertical separation is the vertical drop across the fault in the strike direction of the contour, as shown in Figure 5. In Figure 5 the -8000-ft contour in the upthrown block becomes the -9200-ft contour in the downthrown block. This vertical drop or the amount that the bed has been vertically displaced is the vertical separation.  Fig. 4 Quickly Checking Maps For ErrorsHow do you check a map to determine if it is constructed using missing section as vertical separation and not throw? First we must know the value of the missing section, as observed in the wellbore. Again assume that the missing section is 1200 ft. Take a completed structure map and project a contour across a fault gap along its strike vector from the upthrown block to the downthrown block as shown in Figure 5. In Figure 5 the -8000-ft contour in the upthrown block is projected across the fault gap to the -9200-ft contour in the downthrown block. The amount that the bed is vertically separated is 1200 ft, which corresponds to a missing section of 1200 ft. The map has been correctly contoured using missing section as vertical separation.  Fig. 5 References:Tearpock, D.J., and R.E. Bischke, 1991, Applied Subsurface Geological Mapping, Prentice Hall, Englewood Cliffs, N.J., 648 p.
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