Geosteering is the optimal placement of a wellbore based on the results of realtime downhole geological and geophysical logging measurements rather than three-dimensional targets in space.
The objective is usually to keep a directional wellbore within a hydrocarbon pay zone defined in terms of its resistivity, density or even biostratigraphy.
In mature areas, geosteering may be used to keep a wellbore in a particular reservoir section to minimize gas or water breakthrough and maximize economic production from the well.
Prior to this, Gamma ray gave some bed information, but was rarely used to dynamically adjust the well path to the best oil saturation and porosity.
The development of the Troll Oilfield by Norsk Hydro (Later Statoil and Equinor) would not have been possible without the ability to precisely geosteer within a 4-meter thick horizon to avoid gas above and water below.