In a nutshell, geoscience is the study of features on and under the surface of the earth. In the oil and gas industry, the principle purpose is to find reservoirs of hydrocarbons which a company can then analyse and decide whether to proceed with a project to extract the reserves.
Drilling and completion go hand-in-hand with geosciences, to access the reservoirs you need to drill into them, from the initial surveys and exploration where a potential reservoir might be identified, through to monitoring the reservoir during production to decide if any intervention is required, to finally planning the plugging and abandonment of the well.