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Energy Glossary




Salt dome A subsurface mound or dome of salt.
SWD Salt-Water Disposal Well
Sample Cuttings of a rock formation broken up by the drill bit and brought to the surface by the drilling mud. These are examined by geologists to identify the formation and type of rock being drilled.
Sample log A record of rock cuttings made as a well is being drilled. A record is then kept that shows the characteristics of the various strata drilled through.
Sandstone Rock composed mainly of sand-sized particles or fragments of the mineral quartz.
Scout An individual who observes and reports on competitor’s leasing and drilling activities.
Secondary recovery The introduction of water or gas into a well to supplement the natural reservoir drive and force additional oil to the producing wells.
Section A square tract of land having an area of one square mile (=640 acres). There are 36 sections in a township.
Sedimentary basin A large land area composed of unmetamorphized sediments. Oil and gas commonly occur in such formations.
Sedimentary rock Rock formed by the deposition of sediment, usually in a marine environment.
Seismic An earthquake or earth vibration including those that are artificially induced.
Seismic exploration A method of prospecting for oil or gas by sending shock waves into the earth. Different rocks transmit, reflect, or refract sound waves at different speeds, so when vibrations at the surface send sound waves into the earth in all directions, they reflect to the surface at a distance and angle from the sound source that indicates the depth of the interface. These reflections are recorded and analyzed to map underground formations.
Seismograph A device that records natural or manmade vibrations from the earth. Geologists read what it has recorded to evaluate the oil potential of underground formations.
Separator A pressure vessel used to separate well fluids into gases and liquids.
Service well A well drilled in a known oil or natural gas field to inject liquids that enhance recovery or dispose of salt water.
Set casing To cement casing in the well hole, usually in preparation for producing a commercial well.
Severance The owner of all rights to a tract of land can sever the rights to his land (vertically or horizontally). In horizontal severance, for example, if he chooses to sell all or part of the mineral rights, two distinct estates are created: the surface rights to the tract of land and the mineral rights to the same tract. The two estates may change hands independently of each other.
Severance tax Tax paid to the state government by producers of oil or gas in the state.
Shale A type of rock, the origins of which were common clay or mud.  The clay and mud were subjected to heat and pressure over long periods of time and subsequently turned into shale.
Shale gas Natural gas produced from wells that are open to shale formations.Shale is a fine-grained, sedimentary rock composed of mud from flakes of clay minerals and tiny fragments (silt-sized particles) of other materials.The shale acts as both the source and the reservoir for the natural gas.
Shale oil The substance produced from the treatment of kerogen, that hydrocarbon found in some shales, which is difficult and costly to extract. About 34 gallons of shale oil can be extracted from one ton of ore.
Shale shaker A vibrating screen or sieve that strains cuttings out of the mud before the mud is pumped back down into the borehole.
Sharing arrangement An arrangement whereby a party contributes to the acquisition, or exploration and development, of an oil and gas property, and receives as compensation, a fractional interest in that property.
Shoestring sands Narrow strands of saturated formation that have retained the shape of the stream bed that formed them. In the United States, such a formation is located in Kansas.
Shoot a well A technique that stimulates production of a tight formation by setting off charges downhole that crack open the formation. The early wells were shot with nitroglycerin; then dynamite was used. The nitro man has been replaced today by acidizers and frac trucks.
Show An indication of oil or gas observed and recorded during the drilling of a well.
Shut-down well A well is shut down when initial drilling ceases for one reason or another. A well is shut in when the wellhead valves are closed, shutting off production, often while waiting for transportation or for the market to improve.
Shut-in To stop a producing oil and gas well from producing.
Shut-in pressure The pressure at the wellhead when valves are closed.
Shut-in Royalty A special type of royalty negotiated in the leasing of a property.
Side track When fishing operations have been unable to recover an object in the hole that prevents drilling ahead, the borehole can often be drilled around the obstacle in the original hole.
Skidding the rig Moving a derrick from one location to another on skids and rollers.
Slickline A form of wireline that lacks an electrical conduit and is used only to perform mechanical tasks such as setting or retrieving various tools.
Solution gas Natural gas that is dissolved in the crude oil in a reservoir.
Sour Crude or Gas Oil or natural gas containing sulfur compounds, notably hydrogen sulfide a poisonous gas.
Source rock Sedimentary rock, usually shale containing organic carbon in concentrations as high as 5-10% by weight.
Spacing unit The size (amount of surface area) of a parcel of land on which only one producing well is permitted to be drilled to a specific reservoir.
Speculative Relating to an asset or a group of assets with uncertain returns. The greater the degree of uncertainty the more speculative the asset.
Spot market A short-term contract (typically 30 days) for the sale or purchase of a specified quantity of oil or gas at a specified price.
Spud To spud a well means to start the initial drilling operations.
Squeeze The procedure of pumping a slurry of cement into a particular space in the borehole (often the annulus between the borehole and the casing), so that the cement will solidify to form a seal.
Step-out well A well drilled near a proven well, but located in an unproven area, that determines the boundaries of the producing formation.
Stopper oil well An oil well capable of producing no more than 10 barrels of oil per day.
Stocktank barrel A barrel of oil at the earth’s surface.
Stratigraphic test A hole drilled to gather information about rock strata in an area.
Stratigraphic trap A porous section of rock surrounded by nonporous layers, holding oil or gas. They are usually very difficult to locate, although oilmen believe that most of the oil yet to be discovered will be found in these traps.
Stripper well A gas well that produces 6 Mcf a day or less, or an oil well that produces 10 barrels a day or less.  Thousands of such wells are found in 29 producing states. (See National Stripper Well Association at nswa.us.)
Structural trap A reservoir created by some cataclysmic geologic event that creates a barrier and prevents further migration. The most common structural traps are anticlines, in which at lease 80 percent of the world’s oil and gas have been discovered.
Structure Subsurface folds or fractures of strata that form a reservoir capable of holding oil or gas.
Submersible pump A bottom-hole pump for use in an oil well when a large volume of fluid is to be lifted.
Substructure A platform upon which a derrick is erected.
Surface rights Surface ownership of a tract of land from which the mineral rights have been severed.
Swab A hollow rubber cylinder with a flap (check valve) on the bottom surface. It is lowered below the fluid level in the well. This opens the check valve allowing fluid into the cylinder. The check valve flap closes as the swab is pulled back up, lifting oil to the surface.
Swath Shooting Is when multiple seismic receiver lines are laid out in an array or pattern, which yields a swath of data for every shot line.
Sweet crude Crude oil with low sulfur content which is less corrosive, burns cleaner, and requires less processing to yield valuable products.
Syncline A downfold in stratified rock that looks like an upright bowl. Unfavorable to the accumulation of oil and gas.
Synfuels Fuels produced through chemical conversions of natural hydrocarbon substances such as coal and oil shale.
Synthetic gas Gas produced from solid hydrocarbons such as coal, oil shale, or tar sands.


Take-or-pay contract A (long-term) contract between a gas producer and a gas purchaser, such as a pipeline transmission company.
Tank bottoms A mixture of oil, water, and other foreign matter that collects in the bottoms of stock tanks and large crude storage tanks and must be cleaned or pumped out on a regular basis.
Tanker An ocean going ship which hauls crude oil.
Tar sands Rocks (other than coal or oil shale) that contain highly viscous hydrocarbons that are unrecoverable by primary production methods.
Tax preference items Certain items of income, or special deductions from gross income which are given favored treatment under Federal tax law.
TCF Trillion cubic feet.
Tectonic map A geologic map showing the structure of the earth’s crust.
Tertiary recovery The recovery of oil that involves complex and very expensive methods such as the injection of steam, chemicals, gases, or heat, as compared to primary recovery, which involves depleting a naturally flowing reservoir, or secondary recovery, which usually involves repressuring or waterflooding.
Therm A measure of heat content. One therm equals 100,000 Btus.
Third for a quarter Sometimes also known as a “quarter for a third”. A widely used arrangement for promoting an oil deal to another party.
Tight formation A sedimentary layer of rock cemented together in a manner that greatly hinders the flow of any gas through the rock.
Tight gas Recoverable volumes of gas and condensate from development of very low permeability sandstones.
Tight hole A well about which the operator keeps all information secret.
Tight oil Tight oil plays are those shale plays that are dominated by oil and associated gas, such as the Bakken shale in North Dakota.
Tight sand A formation with low permeability. Gas produced from a formation so designated by the Federal Energy Regulatory Commission qualifies for a higher market price.
Title The combination of factors that, together, constitute legal ownership of a property.
Tool pusher The supervisor of drilling rig operations.
Top lease A (conditional) type of lease that may be granted by the mineral-rights owner of a property while a pre-existing recorded lease of that property is nearing expiration, but nonetheless is still in effect. The top lease would become effective only if and when the existing lease expires (or is terminated).
Total depth (TD) The maximum depth of a borehole.
Township A square tract of land six miles on a side, it consists of 36 sections of one square mile each.
Transfer rule When an interest in an oil and gas property already proven to be capable of commercial production is transferred, the transferee taxpayer is generally not entitled to percentage depletion, although he may still be entitled to cost depletion, in computing his depletion allowance deduction from gross income.
Transition Zone A zone of transitional saturations between water and gas, water and oil, or gas and oil. These transition zones can be far larger (e.g., 10 times or greater) than the average conventional reservoir in both areal extent and recoverable reserves.
Trap A natural configuration of layers of rock where non-porous or impermeable rocks acts as a barrier, blocking the natural upward flow of hydrocarbons.
Trip Making a “trip” is the procedure of pulling the entire string of drill pipe out of the borehole and then running the entire length of drill pipe back in the hole.
Tubing Small diameter pipe, threaded at both ends, that is lowered into a completed well. Oil and gas are produced through a string of tubing.
Turnkey A drilling contract that calls for a drilling contractor to drill a well, for a fixed price, to a specified depth. The purpose of drilling a well by turnkey contract may be related to the timing of Federal income tax deductions. For income tax purposes, expenses are deductible from gross income as they are incurred. When a turnkey contract is entered into toward the end of the current tax year, the drilling costs may be pre-paid at that time. The idea is to give a working interest owner (or investor) in the well, the opportunity to deduct the intangible drilling costs from his gross income in the current tax year.


Unassociated gas Natural gas that occurs alone, not in solution or as free gas with oil or condensate.
Unconventional gas resources Reservoirs in which oil or gas do not flow without the aid of fracturing technology. The main categories are coalbed methane, tight gas, and shale gas, although other categories exist, including methane hydrates and coal gasification.
Unconventional Resource Play Is a play in which hydrocarbon accumulations occur and are more difficult and less economically sound to extract than conventional plays, usually because the technology to reach it has not been developed fully, or is too expensive. The economics of extraction play a role in determining whether or not a particular deposit may be unconventional, or simply to costly to extract.
Updip well A well located higher on a structure where the oil-bearing formation is found at a shallower depth.
Upstream Oil and gas extraction, including exploration, leasing, permitting, site preparation, drilling, completion, and long-term well operation.


Vapor pressure The pressure exerted by a vapor held in equilibrium with its solid or liquid state.
Viscosity A fluid’s resistance to flowing.


Water cut The volume of water produced by a well as a percentage of all fluids produced.
Water drive The most efficient driving mechanism to force oil and gas out of the reservoir.
Water-drive reservoir A reservoir in which the pressure that forces the oil to the surface is exerted by edge or bottom water in the field.
Waterflooding A secondary recovery method in which water is injected into a reservoir to force additional oil into the wells.
Well platform An offshore structure that supports a well’s surface controls and flow piping.
Wellbore That part of a well that is below the surface.  Hole diameters vary with the type and purpose of wells.
Well Casing The equipment at the surface of a well used to control the pressure; the point at which the hydrocarbons and water exit the ground.
Wellhead A device on the surface used to hold the tubing in the well. The wellhead is the originating point of the producing well at the top of the ground.
West Texas Intermediate oil

(WTI or Cushing)

A crude oil produced in Texas and southern Oklahoma that is light (low density) and sweet (low sulfur).  It serves as a benchmark or “marker” for pricing a number of other types of crude streams.  It is physically stored and traded in the domestic spot market at Cushing, Oklahoma, the primary oil trading hub in the U.S. Nymex prices are referenced at Cushing as well, for paper trades.  Nymex, the New York Mercantile Exchange, is a futures market in which a seller promises to deliver a given quantity of a commodity at a specified place, price, a nd time in the future. Oil, natural gas and other related commodities are traded on Nymex.
Wet A reservoir rock is said to be “wet” when it contains water but no hydrocarbons.
Wet gas Includes all the natural gas liquids and typically has a higher Btu (British thermal unit) content of at least 1,500 Btu.
Whipstock A steel blocking device place in a borehole. As drilling is resumed, the whipstock forces the drill bit to veer off at a slight angle.
Wildcat An exploration well drilled to a reservoir, from which no oil or gas has previously been produced in the nearby surrounding area.
Working interest Is the percentage of ownership that the company has in a joint venture, partnership, consortium, project, acreage or well.  A working interest owner pays his share of the well drilling and operating costs, and shares in the cash flow.
Workover To clean out or work on a well to restore or increase production.
Workover rig The rig used when oilmen try to restore or increase a well’s production.
Write-off In common usage: a reduction in taxable income that results when allowable deductions are subtracted from gross income.

X, Y, Z

Zone A specific interval of rock strata containing one or more reservoirs, used interchangeably with “formation.”
Zone isolation Sealing off a producing formation while a hole is being deepened. A special sealant is injected into the formation, where it hardens long enough for the hole to be drilled. Afterward, the substance again turns to liquid, unblocking the formation.