structure map, with only the contour values being changed. It is clear in the 

 cross section that the extra structural contour (oil-water line) of 660-foot 

 elevation is the same as the zero isopach contour for the saturated zone. Like- 

 wise, the 700-foot structural contour becomes the 40-foot isopach line, the 800 

 becomes the 140, and so forth. The thickest part of the zone is on the top of the 

 structure at an elevation of 1050 feet, and the thickness here is 390 feet. 

 The computation of volume from an isopach map is as follows: 

 The area contained within each contour is determined with a planimeter, 

 or, if a planimeter is not available, the area can be subdivided into rectangles and 

 right triangles as shown in C of Figure 24-21. These tracts are scaled, and areas 

 are computed according to the scaled dimensions. The outline in the figure 

 is the isopach zero line (660-foot structure contour). The same procedure is 

 repeated for the next higher contour, which in this case is the 40-foot isopach. 

 The volume of rock between these two planes is: area within zero contour + 

 area within 40-foot contour X (40 -^ 2). Inasmuch as this process gives the 

 volume for only that portion between the zero and 40-foot contours, it must be 

 repeated for the segment between the 40-foot, the 140-foot, and so on to the 

 highest contour. 



In Figure 24-22, A shows the structure just discussed, but, as indicated in 

 the cross section, the reservoir rock is uniformly 200 feet thick. The procedure 

 for determining the volume of this reservoir is considerably different from that 



Figure 24-20. Devonian and Pennsylvanian structure maps combined. 



475 



