The role of a geologist in an oil company is the role of an analyst. The oil industry requires the knowledge of geologists trained in petroleum geology, which is the study of sedimentary basins.
Geologists evaluate sedimentary basins in relation to the source, reservoir, seal, trap, timing, maturation and migration within the basin to determine the presence of hydrocarbons/petroleum. In recent years with the technology of reflection seismology the role of geologists has become more accurate.
Geologists evaluate and determine the quantity of organic rock containing hydrocarbons and the potential quality of hydrocarbons for refining into petroleum products such as motor oil. They also determine how quickly hydrocarbons will flow.
Geologist determine, which seals will impede the escape of hydrocarbons such as evaporates, chalks or shales. Key among the analysis is the prevention of hydrocarbons from escaping from the reservoir and being lost.
The maturation is found by assessing the thermal history of the source rock and predicting the amount of hydrocarbons and how long it will take for the expulsion and generating of hydrocarbons. The last step is determining how the hydrocarbons move from source to reservoir in the area where the search for hydrocarbons is in motion.
There is a very good website where you can go to get additional information and a deeper understanding of the role of geology and geologists in the search and production of petroleum, which is referred to as hydrocarbons in the oil industry.