You start by "spotting", or drawing,  all the wells on a map.  In a large part of the United States, the Township and Range System is used to spot wells.

Let's assume we are making a map in one the western states that uses the Township and Range system.  In this area, a "section" of land is one mile on each side (one mile square).  This also happens to be exactly 640 acres.

Here's a section of land to the right, with wells spotted on it.

Next, you look at the electric logs for each well.  We are going to make an "isopach" (eye-so-pak) map.  An isopach map is a map that shows the thickness of something.  In this case, it is the thickness of a sandstone formation.  

Count the number of feet of sand in the zone you are interested in.  Then, put the number of feet of sand below each well spot.

Start contouring with the highest values.  Use a "contour interval" that is slightly less than the biggest values.  In this case, the first line drawn is the 40-foot contour. 

As you draw your line, look carefully when you pass between two wells.  Try to use your eye like a "ruler", and position the line at the proper distance between the two wells.  Notice the well with a footage of "38".  Since 38 is very close to 40 (the contour we are drawing, you should "pull" the 40-foot contour over close to the 38-foot line, like I did here.

If there is a "40" on the map (there is on this one), you will draw the 40-foot line right through that well.

Now draw the 30-foot contour. "Eyeball" the map and be sure to leave the proper amount of room for the remaining contour lines.  Use a pencil, because you have to erase a lot!!

Finish the map by drawing the 20-foot, 10-foot, and zero contour lines.  Label the "contour interval" you used at the bottom of the map.  In this case, the contour interval is 10 feet.

You're done!  It takes quite a bit of practice to draw decent contour maps by hand.  The one we just did was a "quickie".  Normally, you would draw this in pencil and then go back and make it better with ink. 

It's important to know how to contour a map, but these days, geologists use computers to do a lot of their contouring.  The computer can't do it all, however.  It still requires quite a bit of "help" from the petroleum geologist to make a decent-looking map.