glossary

Salt domes form as a consequence of the relative buoyancy of salt when buried beneath other types of sediment. The salt flows upward to form salt domes, sheets, pillars and other structures. Hydrocarbons are commonly found around salt domes because of the abundance and variety of traps created by salt movement and the association with evaporite minerals that can provide excellent sealing capabilities.

A clastic sedimentary rock whose grains are predominantly sand-sized. The term is commonly used to imply consolidated sand or a rock made of predominantly quartz sand, although sandstones often contain feldspar, rock fragments, mica and numerous additional mineral grains held together with silica or another type of cement. The relatively high porosity and permeability of sandstones make them good reservoir rocks.

The unconsolidated grains of minerals, organic matter or preexisting rocks that can be transported by water, ice or wind, and deposited. The processes by which sediment forms and is transported occur at or near the surface of the Earth and at relatively low pressures and temperatures. Sedimentary rocks form the accumulation and lithification of sediment (see also Sedimentary basin).

A depression in the crust of the Earth formed by plate tectonic activity in which sediments accumulate. Continued deposition can cause further depression or subsidence. Sedimentary basins, or simply basins, vary from bowl-shaped to elongated troughs. If rich hydrocarbon source rocks occur in combination with appropriate depth and duration of burial, hydrocarbon generation can occur within the basin.

Seismology is the scientific study of earthquakes, i.e. the physical exploration of elastic waves inside the earth (seismic waves) and/or examination of the earth by artificially generated seismic waves. Prior to drilling, seismological procedures are used to identify the geological formations and reservoirs (see also LWD).

A fine-grained, fissile, detrital sedimentary rock formed by consolidation of clay- and silt-sized particles into thin, relatively impermeable layers (see permeability). It is the most abundant sedimentary rock. Shale can include relatively large amounts of organic material compared with other rock types and thus has potential to become a rich hydrocarbon source rock, even though a typical shale contains just 1% organic matter. Its typical fine grain size and lack of permeability, a consequence of the alignment of its platy or flaky grains, allow shale to form a good cap rock for hydrocarbon traps. The quality of shale reservoirs depends on various factors, such as formation thickness, organic content, thermal maturity, depth and pressure, fluid saturations (see "oil saturation"), permeability, porosity etc.

Natural gas produced from shale formations.

Oil produced from shale with technologies similar to produce shale gas, i.e. horizontal drilling and multistage hydraulic fracturing.

Shocks are large and sudden forces applied to the bottomhole assembly (BHA). Shocks are often associated with either resonant vibrations (accumulating large amounts of energy) or chaotic motion of the BHA.

The drilling of a new and different hole from an existing wellbore.

The process of drilling without rotating the drillstring

A crude oil containing hydrogen sulfide, carbon dioxide or mercaptans.

A gas (see natural gas) containing hydrogen sulfide, carbon dioxide or mercaptans, all of which are extremely harmful to health.

A rock rich in organic matter which, if heated sufficiently, will generate oil or gas. Typical source rocks are shales or limestones, containing about 1% organic matter and at least 0.5% total organic carbon (TOC), although a rich source rock might have as much as 10% organic matter. Rocks of marine origin tend to be oil-prone, whereas terrestrial source rocks (such as coal) tend to be gas-prone. Under the right conditions, source rocks may also be reservoir rocks, as in the case of shale gas reservoirs.

This tool is one part of the bottomhole assembly with the purpose to mechanically stabilize the bottomhole assembly (BHA) in the borehole, to eliminate vibration, centralize and prevent differential sticking of the BHA, and to control the directional tendencies of the drilling process.

A pipe used for drilling fluid circulation that extends part the way up the derrick to a height suitable for attaching to the rotary hose.

A treatment performed to restore or enhance the productivity of a well. Stimulation treatments fall into two main groups, hydraulic fracturing treatments and matrix treatments. Fracturing treatments are performed above the fracture pressure of the reservoir formation and create a highly conductive flow path between the reservoir and the wellbore. Matrix treatments are performed below the reservoir fracture pressure and generally are designed to restore the natural permeability of the reservoir following damage to the near-wellbore area. Stimulation in shale gas reservoirs typically takes the form of hydraulic fracturing treatments.

A storage tank for oil production after the oil has been treated. A stock tank barrel is a measure of the volume of treated oil stored in stock tanks. A stock tank barrel is commonly abbreviated as STB.

A form of corrosion in which susceptible types of metals will break by a combination of stress within the metal and the specific type of corrosion. Sulfide corrosion of ferrous alloys and chloride corrosion of stainless steels are two common type of stress corrosion.

The portion of the drillstring that cannot be rotated or moved vertically.

A short section of pipe run on the drillstring between or below the drill collars.

A particular type of floating vessel, usually used as a mobile offshore drilling unit (MODU) that is supported primarily on large pontoon-like structures submerged below the sea surface. The operating decks are elevated 100 or more feet (30 m) above the pontoons on large steel columns. Once on the desired location, this type of structure is slowly flooded until it rests on the seafloor. After the well is completed, the water is pumped out of the buoyancy tanks, the vessel refloated and towed to the next location. Submersibles, as they are known informally, operate in relatively shallow water, since they must actually rest on the seafloor.

An exploration and production play type in which prospects exist below salt layers. Until relatively recently, many explorationists did not seek prospects below salt because seismic data (see seismology) had been of poor quality below salt (i.e., it was not possible to map traps accurately) or because they believed that reservoir-quality rock or hydrocarbons did not exist below salt layers. Advances in seismic (See 3-D seismic) processing and compelling drilling results from exploration wells encouraged companies to generate and drill prospects below salt layers, salt sheets and other previously disregarded potential traps. The offshore Gulf of Mexico contains numerous subsalt-producing fields, and similar areas are being explored internationally.

Depth measurements that have been adjusted to a zero reference at sea level.

Crude oil containing low levels of sulfur compounds, especially hydrogen sulfide.

Natural gas that does not contain hydrogen sulfide or significant quantities of carbon dioxide.