The best way to construct a true cross section involves the use of structural 

 and isopach maps. Incidentally, such a section is an excellent check on the 

 accuracy of a complete series of isopach and structural maps. 



Figure 24-50 consists of one structural map and three isopach maps; the 

 cross section is drawn on the basis of data taken from the maps. The method 

 employed is as follows: 



The line of section is drawn on each of the maps. In the case illustrated, 

 this line passes through two of the wells and very near a third. The wells cut 

 by the line of section can be shown in the cross section. 



From the Triassic structural map a structural profile is drawn by using the 

 elevations indicated where contours cross the line of section. The profile should 

 be plotted on a natural scale unless the structural relief is extremely low. This 

 profile is, in a sense, the structural datum of the cross section. It is also the 

 reference line from which all succeeding geologic boundaries are drawn. 



Now, from the Jurassic isopach map, thicknesses of the Jurassic along the 

 line of section are obtained where isopachs cross the line. The thicknesses are 

 plotted above the structural profile. The top of the Jurassic is then drawn 

 through these points. The same procedure is followed with the Triassic and 

 Permian isopach maps to draw the contacts stratigraphically below the structural 

 reference line. The surface profile can be taken from a topographic contour map. 



Figure 24-51^4 is a log map. It consists of well logs plotted to any adaptable 

 scale in their respective locations on the map. The example shows the bases of 

 the logs at the map location, but they may be plotted with the tops of the logs 

 or any selected continuous stratigraphic horizon on the logs at the respective 

 map points. When the logs are plotted, they may be joined by formational cor- 

 relation lines, as shown. 



Figure 24-515 is a panel map of the same area as that shown in the log 

 map. In the panel map it is possible to show changes in lithologic facies, pinch- 

 outs, and other stratigraphic conditions occurring between the wells. 



As only the front panels are shown in their entirety, they should be drawn 

 first. In other words, the lowermost panels on the page are drawn, then the 

 next higher, and so on to the top of the drawing. Panels joining wells along 

 north and south lines are omitted, for they would appear only as single lines on 

 the map. 



The stratigraphic isometric projection is a special adaptation of the panel 

 map. Figure 24-52^4 is a base map with a few principal streams and well loca- 

 tions. Figure 24-52Z? is the isometric projection made from this map. 



In order to construct an isometric projection, it is necessary to have the 

 map contained in a rectangular grid, unless the land lines, as in the case illustrat- 

 ed, provide such a grid. This grid, which may be drawn to any scale, regardless 

 of the scale of the map, serves only the temporary purpose of placing map 

 features correctly on the perspective drawing. Instead of a grid, coordinate 



512 



