When oilmen drill for core samples, they pull up cores from the rocks. A core is a long cylinder of rock. After everything has been tried, the only way to find out for sure is to drill a wildcat well. If you don't know if an area has oil or not, the you drill a wildcat. Don't mix up a regular well with a well that is taking core samples.
Most drilling rigs chew up the rock into rock chips, which come up with the mud.
These rock chips are strained out of the mud and can be studied. Core sampling rigs bring up long cylinders of rock, no small chips. So even if the oil company finds a huge oil deposit underneath your farm, you won't get rich like the Beverly Hillbillies. The oil company will pay you rent though. This is money to make up for the inconvenience of having all those men and machines on your land.