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Oil and Gas Glossary

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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 sand - A sandstone in which the spaces between grains are filled with a highly viscous tar.

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.

Tender - 1. A permit issued by a regulatory body for the transportation of oil or gas. 2. A barge or small ship that serves as a supply ship and/or storage facility for an offshore rig.

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 hole - A well about which the operator keeps all information secret.

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.

Time value of money - The concept that a dollar in hand today is worth more than a dollar that will be received in some future year.

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 an 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.

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.


ULCC (Ultralarge crude carrier) - A large tanker built especially to carry 500,000 dwt and up of crude oil.

Unassociated gas - Natural gas that occurs alone, not in solution or as free gas with oil or condensate.

Underwriter - One who guarantees the sale of securities to investors. He is at risk to the extent he assumes the responsibility of paying the net purchase price to the seller at a pre-determined price. He charges a fee for this service.

Undiscovered recoverable resources - Resources outside of known fields, estimated from broad geologic knowledge and theory.

Updip well - A well located high on a structure where the oil-bearing formation is found at a shallower depth.

Upstream - Activities concerned with finding petroleum and producing it, compared to downstream which are all the operations that take place after production.


Vapor pressure - The pressure exerted by a vapor held in equilibrium with its solid or liquid state.

Viscosity - A fluid's resistance to flowing.

VLCC (very large crude carrier) - A tanker built to carry 200,000 to 350,000 dwt of crude oil.


Wall sticking - A condition in which a section of the drillstring becomes stuck on deposits of filter cake on the wall of the borehole in a well.

Wasting assets - Assets that will eventually lose their value.

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.

Well program - The procedure for drilling, casing, and completing a well.

Wellbore - Physically, wellbore refers to a borehole, in other words a completed well.

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 - Refers to a grade of crude oil produced in the Permian and Midland basin areas of west Texas. The price paid for crude oil varies according to quality.

Wet - A reservoir rock is said to be "wet" when it contains water but no hydrocarbons.

Wet gas - Natural gas containing liquid hydrocarbons - commonly condensate.

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.

Wildcatter - An operator who drills the first well in unproven territory.

Working interest - An interest created by the execution of an oil and gas lease.

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.


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