Subsurface Maps and Illustrations 



919 



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 

 1,050 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 plani- 

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

 rectangles and right triangles ,as shown in C of figure 490. 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 



Figure 488. Subsurface structural map on top of Devonian. 



case is the 40-foot isopachous contour. The volume of rock between these 

 two planes is 



Area within zero contour + area within 40-foot contour X (40 ^ 2) . 



Since this gives the volume for only that portion between the zero and 

 40-foot contours, the process must be repeated for the segment between 

 the 40-foot and 140-foot, and so on to the highest contour. 



In figure 491, 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 previously described. In this example it is assumed 

 that the elevation of the oil-water contact plane is known and has been 

 mapped according to the dashed line on the map. The uniform thickness 



