Subsurface Maps and Illustrations 



915 



Although the subsurface maps representing drilled thicknesses are 

 commonly 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 



Figure 484. Subsurface structural map on top of Pennsylvanian. 



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 the structures. A third common use is in estimating 

 the elevation on a datum bed below the total depth of a well that has 

 penetrated a higher known stratigraphic horizon. But there are many 

 other practical uses, some of which are described below. 



Figure 484 is a subsurface structural map on the top of the Penn- 

 sylvanian. In the northwest quadrant is an anticline with somewhat more 

 than 100 feet of closure. The southward-plunging anticline on the east 

 side is open on the north end. These structural contours are shown as 

 dotted lines in figure 485. The thickness of the Pennsylvanian is- shown 



