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 of the reservoir is known from a few 

 wells in the region that have penetrated it. 



It can be seen in Figure 24-225 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 determine 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 sections, all on a natural scale. 



A series of horizontal lines is drawn across the profile. Using the intersec- 

 tions of these lines with the line of the profile as centers, one draws short-circle 

 arcs upward from point to point, as shown in the figure. The sum of the distances, 

 atod 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 

 Figure 24-22C The bordering band of wide ruling is that portion of the reservoir 

 cut by the water plane. The thickness of the oil-saturated reservoir in this band 

 increases from zero at the outer edge to 200 feet at the inner. The volume is, 

 therefore, the area (in square feet) X 100. The closely ruled area is that part 

 of the reservoir where the thickness is everywhere 200 feet, and the volume 

 within that area is the area (in square feet) X 200. 



The two examples will serve to illustrate the use of isopach maps in special 

 adaptions to determine volumes. There are instances where the plane of the 

 oil-water interface is inclined, and others where the reservoir bed varies in 

 thickness across the structure; but it is necessary only to show these variations 

 by isopachs to compute the volume of the reservoir. These irregular conditions 

 require some adjustment in procedure, but not in principle. 



When the volume of the reservoir rock is obtained, the volume of the 

 contained fluid is determined by multiplying by the percentage porosity, as 

 ascertained from laboratory tests on representative cores. 



From the foregoing it is evident that the volume of any stratum, such as a 

 coal seam or a bed of gypsum, can easily be calculated from an isopach map 

 of that stratum. Other uses of isopach maps will be discussed in connection with 

 paleogeologic and facies maps. 



478 



