tar

A naturally occurring black, viscous, residual crude oil containing a large quantity of polar compounds and asphaltenes.

tar sand

Native asphalt, solid and semi-solid bitumen, including tar-impregnated rock or sands from which oil is recoverable by special treatment.

tectonic

An adjective used to relate a particular phenomenon to a structural orogenic concept.

thief zone

Thief formation. A part of a formation which takes drilling mud after it has been penetrated by the borehole. This loss of mud into the formation constitutes lost returns and could result in lost circulation.

tight

Having very low permeability.

tight hole

A well about which information is restricted and passed only to those authorized, for security and competitive reasons.

tool

Downhole tool or downhole instrument package.

toolpusher

The location supervisor for the drilling contractor. The toolpusher is usually a senior, experienced individual who has worked his way up through the ranks of the drilling crew positions. His job is largely administrative, including ensuring that the rig has sufficient materials, spare parts and skilled personnel to continue efficient operations. The toolpusher also serves as a trusted advisor to many personnel on the rigsite, including the operator's representative, the company man.

total depth

TD. The total depth reached by drilling tools.

total dissolved solids

TDS. The total dissolved mineral matter in water (e.g., formation water). Usually measured in mg/litre and often reported in parts per million, sometimes in grains per gallon.

total porosity

The total pore volume occupied by fluid in a rock. Includes isolated nonconnecting pores and volume occupied by adsorbed, immobile fluid. See porosity. Compare effective porosity.

transition zone

A zone of transitional saturations between water and gas, water and oil, or gas and oil.

trap

Any feature or characteristic of a formation which will allow the accumulation, but not the escape, of oil or gas.

travelling block

The set of sheaves that move up and down in the derrick. The wire rope threaded through them is threaded (or "reeved") back to the stationary crown blocks located on the top of the derrick. This pulley system gives great mechanical advantage to the action of the wire rope drilling line, enabling heavy loads (drillstring, casing and liners) to be lifted out of or lowered into the wellbore.

true resistivity

The resistivity of fluid-filled rock where the fluid distributions and saturations are representative of those in the uninvaded, undisturbed part of the rock

turbidite

A sediment deposited by a turbidity current.

turbidity current

A highly turbid, sediment-rich dense current which moves rapidly along the bottom of standing water until it loses its energy.

turbodrill

A drilling tool that rotates a bit attached to it by the action of the drilling mud on the turbine blades built into the tool. When a turbodrill is used, rotary motion is imparted only at the bit; thus it is unnecessary to rotate the drill stem. Although straight holes can be drilled with the tool, it is used most often in directional drilling.

turbulent flow

The high-velocity flow of fluids in which the fluid elements are heterogeneously mixed and confused, and local velocities and pressures fluctuate irregularly.