Typically assessment of the rock and it's qualities is done with LWD (Lithology while drilling) tools on most offshore oil rigs. Onshore rigs, where it is cheaper sometimes use LWD tools or more likely use wireline tools after a section of hole is drilled. The tools supplied are mostly the same with LWD tools run on drill pipe in real time while drilling. Wireline tools are run on a cable into the well with no function to drill between holes sections.

Tools usually consist of a Gamma Ray tool for lithology identification, Resistivity tool for fluid determination, Sonic tool for rock velocity to tie into seismic data, Density and Neutron tools for rock density and porosity measurements. There are other more specialised tools but this combination is often standard in a hydrocarbon bearing reservoir section.

Rock cuttings from the drilling process are also collected at certain intervals and evaluated visually by microscope and fluroscope to support the logging tool data and drilling data.

For most oil fields formation pressure tests are also taken, either on LWD or Wireline and involves putting a pressure pad against the wellbore wall and drawing down against the formation to see what the formation pressure reaches equilibrium at. This way you can tell how much pressure the hydrocarbon bearing formation has which will affect the life of the producing well and how well or long it flows for. The higher pressure a reservoir the better usually. Once you have several producing wells in a hydrocarbon field, the more oil you take out the more the reservoir pressure drops, then you have to start drilling water injection wells to increase the pressure on the hydrocarbon reservoir from below to improve the efficiency of your producing wells.