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[[:file:evaluating-top-and-fault-seal_fig10-17.png|Figure 1]] shows bed cutoffs on the fault plane as well as those projected onto a vertical fault plane profile. The top of the bed is correctly plotted at a depth of [[depth::1000 ft]] along both the fault plane and the vertical profile.
[[:file:evaluating-top-and-fault-seal_fig10-17.png|Figure 1]] shows bed cutoffs on the fault plane as well as those projected onto a vertical fault plane profile. The top of the bed is correctly plotted at a depth of [[depth::1000 ft]] along both the fault plane and the vertical profile.
==Apparent thickness==
===Apparent thickness===
Projecting the base of the bed requires determining the correct apparent thickness. The true stratigraphic thickness is H. Several apparent thicknesses are involved:
Projecting the base of the bed requires determining the correct apparent thickness. The true stratigraphic thickness is H. Several apparent thicknesses are involved:
* H<sub>p</sub>—apparent thickness along the fault plane profile
* H<sub>p</sub>—apparent thickness along the fault plane profile
==Well-log thickness==
===Well-log thickness===
Use of the well-log thickness (H<sub>w</sub>) to project the base on either fault plane profile clearly gives an incorrect bed thickness at H<sub>p2</sub> in Figure 10-17. Note that H = H<sub>f</sub> only because the bed is normal to the fault plane in this specific case. The correct apparent thickness to be plotted on the fault plane profile is H<sub>p1</sub> at [[depth::1100 ft]].
Use of the well-log thickness (H<sub>w</sub>) to project the base on either fault plane profile clearly gives an incorrect bed thickness at H<sub>p2</sub> in Figure 10-17. Note that H = H<sub>f</sub> only because the bed is normal to the fault plane in this specific case. The correct apparent thickness to be plotted on the fault plane profile is H<sub>p1</sub> at [[depth::1100 ft]].
==Determining correct thickness==
===Determining correct thickness===
[[file:evaluating-top-and-fault-seal_fig10-18.png|300px|thumb|{{figure number|2}}Determining thickness.]]
[[file:evaluating-top-and-fault-seal_fig10-18.png|300px|thumb|{{figure number|2}}Determining thickness.]]
The correct thickness can be determined from trigonometry<ref name=ch10r71>Skerlec, G., M., 1990, SEALS: A short course for risking top seal and fault seal: Franklin, Pennsylvania, SEALS International, 600 p.</ref> or from a simple proportion, as illustrated in [[:file:evaluating-top-and-fault-seal_fig10-18.png|Figure 2]]. The base of the sand is plotted by using a simple proportion between the thickness in the well (H<sub>w</sub>) and the distance to the next mapped horizon in the well (D<sub>w</sub>), and between the thickness (H<sub>p</sub>) on the FPP and the distance to the next mapped horizon on the FPP (D<sub>p</sub>):
The correct thickness can be determined from trigonometry<ref name=ch10r71>Skerlec, G., M., 1990, SEALS: A short course for risking top seal and fault seal: Franklin, Pennsylvania, SEALS International, 600 p.</ref> or from a simple proportion, as illustrated in [[:file:evaluating-top-and-fault-seal_fig10-18.png|Figure 2]]. The base of the sand is plotted by using a simple proportion between the thickness in the well (H<sub>w</sub>) and the distance to the next mapped horizon in the well (D<sub>w</sub>), and between the thickness (H<sub>p</sub>) on the FPP and the distance to the next mapped horizon on the FPP (D<sub>p</sub>):
==Erroneous structural interpretation==
===Erroneous structural interpretation===
The interpretation of the juxtaposition relationships along the fault plane profile is only as good as the original maps. Older 2-D seismic surveys are inadequate unless there is abundant well control. One of the indirect benefits of routine fault seal analysis is that it focuses the explorationist on the details of fault geometry and fault-bed cutoffs that are critical to trap definition as well as to seal analysis. Modern 3-D seismic surveys are essential for reliable risk of fault seal. It is also true that using expensive 3-D seismic surveys only to make top of reservoir structure maps without fault seal analysis wastes a significant part of the information available for prospect assessment.
The interpretation of the juxtaposition relationships along the fault plane profile is only as good as the original maps. Older 2-D seismic surveys are inadequate unless there is abundant well control. One of the indirect benefits of routine fault seal analysis is that it focuses the explorationist on the details of fault geometry and fault-bed cutoffs that are critical to trap definition as well as to seal analysis. Modern 3-D seismic surveys are essential for reliable risk of fault seal. It is also true that using expensive 3-D seismic surveys only to make top of reservoir structure maps without fault seal analysis wastes a significant part of the information available for prospect assessment.