DEPOSIT - An accumulation of oil or gas capable of being produced commercially.
DERRICK - A wooden or steel structure built over a wellsite to provide support for drilling equipment and a tall mast for raising and lowering drillpipe and casing; a drilling rig.
DEVELOPMENT WELLS - Wells drilled in an area already proved to be productive.
DIAPIR - A mass of rock, usually salt, which has come from a slightly deeper part of the earth's surface by piercing through overlying layers of sediment through a zone of weakness.
DIRECTIONAL DRILLING - The technique of drilling at an angle from the vertical by deflecting the drill bit. Directional wells are drilled to develop an offshore lease from one drilling platform; to reach a pay zone where drilling cannot be done, such as beneath a shipping lane.
DISCOVERY WELL - An exploratory well that encounters a new and previously untapped petroleum deposit; a successful wildcat well. A discovery well may also open a new horizon in an established field.
DOME - A roughly symmetrical upfold of the layers of rock in which the beds dip in all directions more or less equally from a common point; any deformation characterized by local uplift and approximately circular in outline; e.g. the salt domes of Louisiana and Texas.
DOWNHOLE - A term to describe tools, equipment, and instruments used in the well bore; also conditions or techniques applying to the well bore.
DRILL CUTTINGS - Chips and small fragments of drilled rock that are brought to the surface by the flow of the drilling mud as it is circulated.
DRILL PIPE - Heavy, thich walled, hollow steel pipe used in rotary drilling to turn the drill bit and to provide a conduit for the drilling mud.
DRILLING CONTRACTOR - A person or company whose business is drilling wells. Wells are drilled on several contract specifications: per foot, day rate, or turnkey (that is, upon completion). Most major oil companies do not own drilling rigs. Exploration and development drilling is contracted. Personnel manning the rigs work for the contractor.
DRILLING MUD - A special mixture of clay, water, or refined oil, and chemical additives pumped downhole through the drill pipe and drill bit. The mud cools the rapidly rotating bit; lubricates the drill pipe as it turns in the well bore; carries rock cuttings to the surface; serves as a plaster to prevent the wall of the borehole from crumbling or collapsing; and provides the weight or hydrostatic head to prevent extraneous fluids from entering the well bore and to control downhole pressures that may be encountered.
DRY HOLE - A well drilled to a certain depth without finding commercially exploitable hydrocarbons.
DRILLING PERMIT - In states that regulate well spacing, a drilling permit is the authorization to drill at a specified location; a well permit.
DRY GAS - A natural gas from the well free of liquid hydrocarbons; gas that has been treated to remove all liquids; pipeline gas.