datum gap is determined by the degree of dip in the strata, the dip of the fault 

 plane, the amount of throw, and the relationships between the dip and strike of 

 the strata and the fault plane. The datum gap can be worked out on a subsurface 

 structure map if there are sufficient datum points to control the general con- 

 touring of the structure and at least three wells that have penetrated the fault 

 plane in a triangular arrangement (not in a straight line) . 



Figure 24-9. Effects of unconformities on interpretation of subsurface structure. 



It is first necessary to determine the dip and strike of the fault plane from 

 the three or more wells. The method is shown in the fault-plane detail of the 

 figure cited. The procedure is exactly the same as that used in obtaining a three- 

 point dip and strike on a bed with the plane table. In the illustration, wells 

 numbered 1 to 5 have been plotted in their correct relative positions in a 

 separate drawing in order better to illustrate the method. In actual practice, the 

 determination of the dip and strike of the fault plane would be made directly 

 on the map. 



Referring again to the fault detail drawing: Wells 1, 2, and 3 penetrated 

 the fault plane at elevations 4100, 4600, and 4300 feet respectively. These wells 

 are joined by straight lines. The difference in elevation (on the fault plane) 

 between 2 and 3 is 300 feet. If the line is divided into three equal parts, the 

 difference in elevation between each of these points is 100 feet. The difference in 

 elevation between wells 2 and 1 is 500 feet; therefore, five equal spaces are laid 

 off along that line. Now, starting with the first mark on each side of the highest 

 well (4600 feet) , strike lines are drawn as shown in the figure. These lines are 

 contours on the fault plane and the rate of dip of the fault plane is revealed by 

 the spacing of the contours. This process assumes a true plane, which may not 

 actually be correct. 



When the fault-plane contours have been drawn on the map, the structural 

 contours are carefully sketched to points where they intersect fault-plane contours 

 of the same values. These intersections, as shown on the map, mark the bound- 

 aries of the datum gap. 



Although the wells used in this illustration penetrated the fault plane within 

 the datum gap, this condition has no bearing on the solution of the problem. 



461 



