920 



Subsurface Geologic Methods 



of the reservoir is known from a few wells in the region that have pene- 

 trated it. 



It can be seen in B of figure 491 that, if the folded reservoir bed were 

 flattened, its actual breadth would be greater than that shown on the 

 structure map. The first step, then, in computing the volume is to deter- 

 mine the actual area of the reservoir, rather than the area within the oil- 

 water line as it appears on the map. This can be done graphically by 

 first constructing several cross sections and one or two longitudinal sec- 

 tions, all on a natural scale: i.e., with no vertical exaggeration. 



A series of horizontal lines are drawn across tlie profile. Using the 



Figure 489. Devonian and Pennsylvanian structure maps combined. 



intersections of these lines with the line of the profile as centers, short 

 circle arcs are drawn upward from point to point, as shown in the figure. 

 The sum of the distances, a to d and a' to d', is a close approximation of 

 the surface distance over the fold. In practice, the distance over the fold 

 may be measured directly, or the profile may be drawn on profile paper. 

 Wherever the dip is constant, the extension can readily be computed since 

 the distance along the sloping surface is the hypotenuse of a right triangle 

 whose other two sides are the difference in elevation and the horizontal 

 distance between the points (scaled on the structure map) . 



When a number of zero points have thus been located on the map, 

 a new zero (oil-water) line is sketched. This is the outline of the reservoir 

 shown in C of figure 491. The bordering band of wide ruling is that por- 

 tion of the reservoir cut by the water plane. The thickness of the oil-satu- 



