24 
OIL AND GAS; OHIO, WEST VIRGINIA, PENNSYLVANIA. 
map of the structure of the key horizon were plotted all the wells 
drilled within the area. As the elevation of the mouth of each of 
these wells had been determined, the position of the key stratum with 
reference to the mouth of the well was obtained directlv from the 
map, and with this information the distance from the key stratum 
to the oil-bearing sand at this point was obtained from the record 
of the well. 
By making this computation for each well of which a reliable 
record could be obtained, the distance from the kev horizon to the 
oil-bearing stratum was obtained in different parts of the area. Gen¬ 
erally this distance is not the same at different wells, but decreases 
in one direction or the other. 
The correction for the convergence between the key horizon and 
the oil sand is applied by means of a mechanical drawing called a 
“ convergence sheet." This drawing was made on tracing cloth by 
connecting the location of the oil wells from which reliable records 
had been obtained by straight lines. The lines were then divided 
proportionately to the amount of convergence found between the 
two wells, so that each division on the lines would represent an 
increased distance of 10 feet between the key stratum and the oil¬ 
bearing sand. 
After all the lines connecting the different wells had been thus 
divided the points that show an equal distance from the key stratum 
to the oil sand were connected, and a drawing was built up that, 
when placed over the map on which the elevation of the key stratum 
was noted at many different places, showed directly what distance 
should be subtracted from each elevation of the key stratum to make 
it equivalent to the elevation of the oil sand at that point. 
The regularity and uniformity of this mechanical drawing shows 
whether it is possible or not to make a map of the oil sand that will 
be of any practical value. If the distance between the 10-foot lines, 
which are called isochor (equal space) lines, is regular and the 
decrease is uniformly in one direction, a map of the lower sands can 
be made practically correct. If, however, the distance from the key 
horizon to the sand decreases first in one direction and then in 
another, the lines on the convergence sheet will run in circles and 
show that there is little use in trying to interpret the structure of the 
sand from a map of the surface structure. It can hardly be hoped 
that the wells used have been located at the exact point of the great¬ 
est distance between the two strata. In all probability the resulting 
convergence sheet is incorrect over limited areas. 
The amount of convergence per mile is another condition to be 
considered. If it amounts to 50 or 00 feet to the mile, there is little 
probability that the resulting map of the sand will be correct within 
