scribed is repeated. The result of this step in shown in Figure 24-19, the De- 

 vonian structure map. This figure shows the original Pennsylvanian structure 

 map superimposed on the underlying Devonian structure map in order that the 

 two may be compared directly. It should be pointed out that the closed Penns- 

 sylvanian structure is a northeasterly plunging nose at the Devonian horizon. 

 The south-plunging nose on the east side of the area is a north-plunging nose on 

 the Devonian, and if the map were extended on the south, a large closure would 

 be evident in the Devonian. 



In Figure 24-20 the combined thickness of the Pennsylvanian and Mis- 

 sissippian can be determined at any contour intersection simply by subtracting 

 the lower value from the higher, if both datums are either above sea level or 

 below sea level. Where one datum is above sea level and the other below, the 

 contour values are added to obtain the thickness. 



This method of reducing structural maps from higher to lower horizons 

 should be applied wherever the rate of convergence (in feet per mile) between 

 the structure datum and the prospective oil horizon approaches the rate of dip 

 (in feet per mile) on the flanks of the structure. 



In some regions persistent and sharply defined seismic-reflecting horizons 

 are encountered several thousands of feet above the prospective oil-producing 

 formations. Because of the fact that much of the wave energy is reflected here, 

 it is sometimes impossible for the little remaining energy of the shot to reach 



Figure 24-18. Isopach data of Mississippian superimposed on structural map (fig. 24-17) . 



473 



