Anthropogenic Caused by human activity. For example, anthropogenic earthquakes are those caused by human actions such as mining or hydraulic fracturing.

A layer of rock with sufficient porosity and permeability to allow water to flow through it. Aquifers are typically tens to hundreds of meters below ground and are a common source of drinking water.

Biocide Chemicals added to the frack fluid to prevent build-up of algae or bacteria in the wellbore that could damage the well casing.
Drilling Rig The equipment used during the construction and drilling phase of a well to drill the borehole. This equipment is removed once well construction is complete, which may take several months.


Flowback Water Because of the pressure of the overlying rocks, a proportion of the frack fluid pumped into the shale rock is forced back up the wellbore once pumping stops. When it reaches the surface it is collected and treated.
Frack Fluid

The mix of water, proppant and chemicals used to create fractures in shale rock. Chemicals used include biocides, lubricants and acids. These chemicals are used mainly to prevent the clogging of the wellbore and to prevent the formation of chemicals that could corrode the well casing.


The process by which gas is extracted from impermeable shale rocks. Water, proppant and chemicals are injected into a horizontal well at pressures high enough to fracture the shale rock and allow gas or oil to flow to the surface through the wellbore.

Fugitive Emissions 

Unplanned releases of gas, predominantly methane, that occur at any stage of the gas production process. This includes leakages from pipelines, abandoned wells, and producing gas wells.


Water contained below the ground surface. In the context of fracking, groundwater usually refers to water stored in aquifers (permeable rocks) tens to hundreds of meters below ground. Freshwater aquifers are a common source of drinking water.

Hydraulic Fracturing See Fracking.
Induced Seismicity 

Earth tremors caused as a result of human activity such as mining or hydraulic fracturing. In the case of fracking, fractures and frack-fluids can interact with existing geological faults causing them to move and generate seismicity.



Seismicity that is generally too small to be felt by humans, but can be measured using specialist equipment. When hydraulic fractures are formed, they generate micro-seismicity which can be detected and used to map the fracture network.

NORM Naturally Occurring Radioactive Materials exist naturally in many different rock types. In shales there are small amounts of radioactive materials such as Radium-226. When shales are fracked, small quantities of these can be brought to the surface. 



Permeable materials have many inter-connected pores or fractures which allow fluids to flow through them. The more connected these spaces are, the more permeable the material. Shales can be relatively porous but usually have low permeability as the pore spaces within the rock are not connected.


The proportion of a material (such as shale) that is made up of pore spaces. Shale can have relatively high porosity but will generally have low permeability.


Small particles, usually sand, that form a small proportion of the frack fluid used to fracture shales. The proppant remains inside the fractures and prevents them from closing, which in turn allows gas to flow through the fractures into the wellbore.

Reserve The amount of oil/gas in a rock formation that can be technically and economically extracted. Reserves are therefore always smaller than resources.


A body of rock, usually several kilometres underground, which contains hydrocarbons. Hydrocarbons can be formed in the reservoir rock (in the case of shale gas) or they can migrate there from source rocks.


The total amount of oil or gas estimated to be held within a rock formation beneath the ground. Only a small percentage of a resource may be extractable. The amount that can be extracted is limited by economics, geology and current technology.

Sedimentary A rock type composed of small particles of older rocks or minerals. Shale is a sedimentary rock primarily composed of clay minerals.

The frequency and distribution of earthquakes in a given region.


A fine grained sedimentary rock composed of clay, quartz silt, calcium carbonate and organic matter. Because of the organic matter content, shales that are buried underground under certain conditions of pressure and temperature can produce hydrocarbons.

Shale Gas Natural gas (primarily methane) that develops within shale rocks when they are buried to certain pressures and temperatures. The gas is trapped in pore spaces within the shale rock.


Unconventional Gas/Oil

Gas or oil that is extracted from the ground using techniques other than conventional drilling. Unconventionals include shale gas, coal bed methane, and tar sands. The gas/oil extracted is no different to that obtained from conventional drilling – it is the method of extraction and the location of the oil/gas that is unconventional.

Well Casing 

The layer of metal that surrounds the wellbore to provide structural integrity and to prevent loss of fluids. Multiple layers of casing and cement are used to prevent loss of fluids into sensitive rock layers such as aquifers.

Well Integrity 

A well is said to have lost integrity if one or more well barriers fail, allowing release of gas or liquid to the surrounding environment. These releases vary from slow leaks of gas to rare blowouts.

Well Pad 

The area of land at the surface that contains the wellhead and associated infrastructure. Well pads are usually approximately 1 hectare in size and can contain multiple wells. 


The drilled hole that forms the basis of the oil/gas well. Oil/gas flows from the reservoir to the surface through the wellbore.


The system of valves at the surface of a wellbore that allows control over the flow and pressure of liquids/gas in the wellbore. A typical wellhead is several meters tall.

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