the lower horizons and be reflected back to the seismometers in sufficient 

 strength to produce usable records. Therefore, a detailed seismic structure map 

 may be obtained on the upper horizon, but sparse data or none on the lower. 

 Now, if a few deep wells provide the necessary convergence data, an isopach 

 map can be constructed from the subsurface information; and, by means of this 

 map, the seismic structure can be reduced to the prospective formation. 



Isopach maps of oil reservoir rocks, together with porosity determinations 

 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 porosity and thickness of saturation 

 are normally less predictable in limestone reservoirs, and for these reasons it is 

 difficult to make accurate volumetric determinations. 



Figure 24-21,4 shows a structure contoured on the top of the producing 

 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 elevation of 660 feet. This 

 oil- water contact is shown by the heavy dashed line on the map and also in cross 

 section B in Figure 24-21. 



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 



Ficure 24-19. Subsurface structural map of top of Devonian. 



474 



