D

Delineation Well
A well drilled in an unproven area adjacent to a proven well to determine the extent of the reservoir; also referred to as a "stepout" well.  

Distillation 
Method of fractionation. Distillation is based on the difference in boiling point of the liquids in the mixture to be separated. Successive vaporisation and condensation of crude oil in a fractioning column will separate out the lighter fractions, leaving a residue.

Downhole
A term used in exploration and production to describe tools, equipment and instruments used in the well bore. Also, it refers to conditions or techniques applying to the well bore.