Subsurface Maps and Illustrations 923 



rated reservoir in this band increases from zero at the outer edge lo 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 adaptations to determine volumes. There are cases 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 isopachous contours to compute the volume of the 

 reservoir. These irregular conditions require some adjustment in pro 

 cedure, but not in principle. 



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

 the contained fluid is determined by multiplying by the percentage po- 

 rosity, 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 fiom an 

 isopach map of that stratum. 



Other uses of isopach maps will be discussed in connectioTi vvitli 

 paleogeologic and facies maps. 



Paleogeologic Maps 



A geologic or areal geologic map is one that shows the present dis- 

 tribution of consolidated rocks at the surface and immediately below the 

 soil or unconsolidated mantle. A paleogeologic map shows the distribu- 

 tion of formations at a surface which existed at some specific time in the 

 geologic past. Such a surface is shown in the lower block of figure 472. 

 In figure 492, A is the areal geologic map of the upper block and li is 

 the paleogeologic map of the pre-Jurassic surface in the lower block. 



As might be presumed, paleogeologic maps are constructed from in- 

 formation supplied by wells. The four wells shown in the block diagram 

 mentioned penetrate pre-Cambrian, Devonian, and Pennsylvanian beds be- 

 neath the pre-Jurassic unconformity. The remaining twenty wells of B in 

 figure 492 encounter rocks of different ages beneath the unconformity, and 

 it is upon this type of information that the map is constructed. 



Several factors control the relative breadth of the bands or areas 

 that appear on the paleogeologic map. Among these are the relative thick- 

 nesses of the formations, the rates of thinning, the relative rates of dip in 

 the different formations and the actual degree of dip, the character of the 

 eroded surface, and the amount and character of folding subsequent to 

 truncation. It is well to keep these conditions in mind when drawing a 

 paleogeologic map, because it may be necessary to interpolate several 

 geologic boundaries between two control points, and any one of the condi- 

 tions listed above might have a pronounced effect on the map position 

 of the boundary lines. 



