Sometimes, PG's want to get a better look at the rocks deep in a well than they can get by looking at the small chips called well samples.  In this case, the PG will order that a "core" be taken from the well.

A core is a solid cylinder of rock about 3 inches in diameter, and it can be anywhere from a few feet to over a hundred feet long. Often, a PG will order several cores in a well, so the total amount of cored rock may amount to hundreds of feet.  

Taking a core requires that the regular drill bit be removed from the hole.  It is replaced with a "core bit", which is capable of grinding out and retrieving the heavy cylinder of rock.  The core bit is usually coated with small, sharp diamonds that can grind through the hardest rock.  A core bit cuts very 

At the left is a picture of a coring bit.  The gold-colored part is studded with natural diamonds.  These diamonds, the hardest substance known to man, grind away the rock in tiny chips.  Notice the large hole in the center of the bit.  This hole contains the core.  As the bit moves down through the rock, the bit and the drilling pipe above it encase the core inside the steel core "barrel".

The next picture shows three core bits of a different type. 

Coring is very expensive, because of the slowness of coring and the expensive diamond bits that must be used. So, PG's only take cores when it is absolutely necessary.

Cores are very valuable sources of information.  The big hunks of rock let the PG find out exactly what rocks are present, instead of making educated guesses, as must be done with well samples.  The PG can clearly see the boundaries between sandstone, limestone, and shale.  If one of the formations contains oil, the PG can easily tell by looking at the core. 

The core is taken back to a laboratory and tested in many ways.   Accurate measurements of the actual porosity of the rock, as well as the amount of oil and water in the pores, can be obtained.  If desired, parts of the core can be sliced thinner than a hair and viewed under a microscope to determine the exact rock type and microfossils present. This is all very precise data that is very useful to the PG.
 

Here is a picture of a "whole core", just as it comes from the core barrel, loaded into storage boxes.  The core is carefully labeled as to depth.  The core can be examined by the petroleum geologist at this time, but the core is usually "slabbed" first.

A "slabbed" core is simply a core that is sawn down the middle by a powerful diamond-encrusted rock saw.  Cutting the fresh, flat, surface allows the PG to see things in the core much more clearly.  He is usually looking for indicators that will tell him what environment the rock was formed in, such a beach, a sand bar, or a river system.  He will also note grain sizes, small fossils, and burrows caused by worms or other marine life.  He will look very closely for the presence of oil in the rock or any hint of gas bubbles.  A slabbed core is shown at the left.

Information provided by: http://www.abix.com