Exploration Glossary

Anticline
A fold, generally convex upwards, whose core contains stratigraphically older rocks.

Core data
A solid column of rock up to four inches in diameter taken from the wellbore so geologists may study the rock formation for clues as to whether oil or gas is present.

Drillstem test
A test of the productive capacity of an oil or gas reservoir when the well is uncased. The test is conducted through the drill pipe to see if oil or gas is present in a certain formation; preliminary sampling aids the decision to complete or abandon the well.

Fault
A fracture or fracture zone along which the sides have been displaced relative to one another. A break or fracture in the Earth’s crust that causes rock layers to shift.

Field
An area in which a number of wells produce from a reservoir. There may be several reservoirs at various depths in a single field.

Horst
An elongate, uplifted block that is bounded by faults on its long sides.

Hydrocarbon
An organic compound consisting of carbon and hydrogen. Hydrocarbons can be gaseous, liquid or solid.

Log
A continuous record as a function of depth of information on the rocks and fluids encountered in a wellbore. The readings are commonly obtained by equipment lowered by wireline into the wellbore. Acoustic, radioactive and electrical readings are used to identify the types of rocks and their characteristics. Measurement-while-drilling (MWD) tools can accumulate data as the drill bit drills through the rock formation.

Permeability
The capacity of a rock to transmit fluids. A tight rock, sand or formation will have low permeability and thus, low capacity to produce oil or gas, unless the well can be somehow fracture-stimulated to increase production. Expressed in millidarcies for tight reservoirs and darcies for extremely permeable reservoirs.

Porosity
The volume of small to minute openings in a rock that allow it to hold fluids. Measured in percentages, typically from near zero to about 35%.

Prospect
An area that is the potential site of an oil or gas accumulation. A lease or group of leases upon which an operator intends to drill.

Reserves
The volumes of oil and gas that can be profitably recovered from a well with existing technology and present economic conditions.

Sedimentary
rock A layered rock resulting from the consolidation of sediment. Sediments are materials that are transported and deposited by wind, water or ice, chemically precipitated from solution or deposited by organisms.

Seismic
An earthquake or earth vibration, including those that are artificially induced.

Strike
The direction taken by a structural surface such as a bedding or fault plane.

Wellbore
That part of a well that is below the surface. Hole diameters vary with the type and purpose of wells; a common wellbore diameter is a little less than nine inches.