METHOD OF INVESTIGATION. 
25 
a limit of 20 or 30 feet. If, however, the convergence is only 10 or 
20 feet to the mile, the resulting map should be of the same degree 
of accuracy as the map of the surface structure. 
In making maps of subsurface strata in areas that have not been 
productive, most of the records used for making a convergence sheet 
must be taken from u wild-cat wells.” In certain cases it is difficult 
to procure the records of such wells, and often the best- that can be 
obtained is the depth, from memory, at which the sand was found. 
Here is a source of serious error, for a mistake in this distance may 
make the resulting map incorrect for a considerable distance about 
the well. 
In making a subsurface map, full knowledge should be had of the 
well records used for constructing the convergence sheet, and if any 
reliable records have been thrown out whose distances would change 
the convergence sheet the reason for discarding them should be 
given. In selecting the records for the construction of a convergence 
sheet it is desirable to consider wells from which a good record is 
obtainable and those that are located near the outcrop of an easily 
recognized surface stratum. 
MAP OF OIL SAND. 
With the convergence sheet completed, the operation of making a 
contour map of the oil sand was very simple. The tracing was placed 
over the map on which are noted the elevations of the key horizon. 
From each of these elevations the amount shown by the convergence 
sheet was subtracted. This gave the elevation of a point on the oil 
sand. Bv connecting the points of equal elevation by lines a contour 
map of the oil-bearing sand was made. 
