oil shows at snatcher 2...., page-12

Coring

Drilling with a core barrel is like augering post holes. A cylindrical bit cuts through the rock and instead of breaking it up in chips like a normal bit, it preserves a cylinder of relatively undisturbed rock. This 'core' is extremely valuable for determining: porosity, permeability, fracturing, depositional enviroment and even fracture strength. The core is then compared to the wireline logs to help interpret wells that do not have core.
A core on the first appraisal to a discovery without core is quite common.

The tricky part is that you must decide to core before you see the rocks so it is common to drill a few meters of what you hope is your reservoir and then 'trip' out the bit and run in with the core barrel.

Coring is much slower than regular drilling and therefore more expensive so most companies only core important reservoirs and then usually only once.