In its most general sense, the term diesel fuel means any energy-producing fluid that can be used to power diesel engines. In most cases this means a petroleum derivative.
However, the oil that comes out of the ground is a soup of various chemicals that must be separated to get a usable product. These chemicals evaporate at different temperatures, and the process of evaporating and removing them is called refinement. In a refinery the oil is heated to very high temperatures so its constituents will evaporate. Since each chemical has its own evaporation temperature, it is possible to remove them one by one. Diesel fuel is one of the steps in this process. It has had some of the chemicals removed, but some have also been left in. Therefore it is referred to as a “”partial distillate.”"
The beginning of the process is the location of a promising area to drill. Sometimes this decision is based on government surveys indicating the presence of hydrocarbons, the organic molecules that make up petroleum. In other cases, the selection of a drilling spot is determined by its proximity to other sources that have already proven productive. A third basis for the selection can be the fact that the area was once a productive oil well that went out of production, but which can now be drilled deeper because of advances in technology since it was closed.
Once a potentially productive spot has been found, surveys are conducted to confirm the presence of oil. One of the most common ways to do this is seismic mapping, where an explosion or some other strong impact is applied to the ground and its echoes are studied to determine what types of matter the sound waves have passed through. The purpose is not just to make sure oil is present, but that it is in a location where it can be extracted with relative ease. The depth of the petroleum deposit is determined, as well as the amount and type of rock between it and the surface. If it is shown that the oil is extractable, drilling begins.
Oil exploration was originally conducted on land since underwater drilling presents enormous technical difficulties. However, these onshore sources are becoming increasingly depleted, which has spurred the advancement of technology for offshore oil operations. Because of this meeting of necessity and method, oil exploration is increasingly becoming an offshore activity.