where the well is drilled. Obviously, a correct interval is obtained in a straight 

 hole only where the beds are level. Since most wells are drilled on structures, 

 there are many opportunities for wells to penetrate formations where appreciable 

 dips do exist; and, if the dips are quite steep, the error in interval may be large 

 enough to affect the regional aspects of the isopach map. There is little doubt 

 that the apparent thinning of the section on the tops of some structures is only 

 the result of this condition. If core dips are available, the true stratigraphic 

 thicknesses can be determined by Busk's or some similar method for obtaining 

 stratigraphic thicknesses in inclined strata. 



Although the subsurface maps representing drilled thicknesses are com- 

 monly called isopach maps, a more precise term is isochore. An isochore map 

 is one that shows by contours the drilled thicknesses of the formations, without 

 regard to the true stratigraphic thicknesses. The term is not ordinarily used 

 but is mentioned here simply because it does come up occasionally in geologic 

 literature. 



Isopach maps are interesting to draw and frequently reveal intriguing and 

 perplexing problems, but too often their many practical uses are not fully realized 

 or employed in subsurface work. Isopach maps are generally used for the 

 purpose of predetermining drilling depths to specific horizons in wildcat wells. 

 They are also used as a means of locating buried structures in regions where 

 formations habitually become thinner over the crests of structures. A third 



Figure 24-16. Isopach data of Pennsylvanian superimposed on structural map (fig. 24-15) . 



471 



