Structural Cross-Sections
Once the PG has electric logs from two or more wells, he usually will make a "Cross-Section". With a "structural" (STRUK-sure-all) cross-section, he will attempt to show the various positions of the rock formations as they actually look underground.
The PG knows the elevation above sea level for each of the wells. He wants to show a model of the way the wells would line up if they could be viewed from the side! Like viewing the layers in a cake that has been cut.
The PG may choose a "datum" (day-dum), 
or an artificial elevation, that is 500 feet below sea
level (-500'). The PG wants to take his electric logs and "hang" them so
that they are all "hung", or lined up, on the same datum, 500 feet (-500') below
sea level.
The PG has 4 wells to work with:
Well #1 has a ground level elevation of +1000 feet (1000 feet above sea level. Well #2 has a ground level elevation of +1050 feet. Well #3 has a ground level elevation of +1045 feet. Well #4 has a ground level elevation of +1060 feet.
The PG takes each well's electric log and determines where the -500-foot datum will be located on each one. Well #1 has a ground level elevation of +1000 feet. In order to get to -500 feet, the PG must go down the log to a drilled depth of 1500 feet (+1000 minus 1500 feet equals -500 feet). The PG makes a mark on the first log at a depth of 1500 feet. He follows the same method for each of the remaining logs, until they each have a mark on them at the -500-foot datum...wherever it lies on the well log.
Then, the -500 datum is drawn as a horizontal straight line on paper. See
the datum at 500' below sea level on the cross-section to the right. The PG
places each log on the paper so that the mark on
the log lines up with the datum line on the paper.
After the logs are placed correctly, the PG draws lines between the formations he believes are the same in each well. On the cross-section at the right, the PG has picked a "Marker Bed" and an "A Sand". He draws pencil lines connecting the Marker Bed and the top and bottom of the A sand.
Notice how the formations picked on the cross-section are tilted.
This
shows the actual way the formations are tilted in the earth. Rock formations below
the surface of the earth are almost never perfectly flat. It is the tilting and and
folding of the rock layers that creates so many oil and gas traps for the PG to find.
Finally, a
very large structural cross-section is shown at right. This type of cross-section is
more like a "cartoon", and shows rock layers over a horizontal distance of
several hundred miles
Most cross-sections are now generated using computers:
Finally, the green and yellow areas on this structural cross-section (hung on the sea-level datum) represent "pay zones". These "pay zones" are intervals that the petroleum geologist believes may produce oil or gas. The red bars show where some of the zones have been perforated for production.
Large cross-section from "Pennsylvanian Sandstones of the Mid-Continent"