Logging If oil and gas is present in the pores in the rock it affects the physical properties of the rock in subtle ways. These changes can be identified and quantified by measuring devices called logging tools. These are lowered into the well on a "wire line" or as we can done now days "incorporate them in the drilling assembly" "the wire line" measure various rock properties as they are slowly pulled back up to surface. The resulting readouts are called logs and these are interpreted to attempt to quantify how thick the layers of oil and gas are, whether the shows are residual and what the porosity and permeability might be. The interpretations are usually fairly accurate at the two ends of the scale i.e. if it is a very good reservoir with good oil or gas saturations or if it is poor and essentially non producible. In between there is often some uncertainty so to resolve any uncertainty and also, in the case of a good reservoir, to see how much the flow rate might be, it is necessary to obtain fluid samples or test the well.