918 



Subsurface Geologic Methods 



ture can be reduced to the prospective formation through the methods just 

 described. 



Isopach maps of oil reservoir rocks, together with porosity determi- 

 nations from cores, make it possible to calculate the volume of oil in a 

 structure. This method is most applicable to sandstone reservoirs where 

 there is little cementing or other interstitial material. Conditions of po- 

 rosity and thickness of saturation are normally less predictable in lime- 

 stone reservoirs, and for these reasons it is difficult to make accurate volu- 

 metric determinations. 



In figure 490, A shows a structure contoured on the top of the pro- 



Figure 487. Isopach data of Mississippian superimposed on structural map (fig. 486) . 



ducing formation. A few dry holes have been drilled on the flanks of the 

 structure below the oil-saturated portion of the reservoir, and by means 

 of these dry holes the oil-water contact is established at a structural ele- 

 vation of 660 feet. This oil-water contact is shown by the heavy dashed 

 line on the map and also in the cross section, B, in figure 490. 



Since the thickness of the oil column is less than the thickness of the 

 reservoir rock, the computation of the volume of saturated sandstone is 

 quite simple, because the isopach map of the saturated rock is exactly 

 the same as the 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. Likewise, the 700-foot structural contour becomes 



