National Wildlife Refuge System

Exploration and Production

Oil and gas exploration and production typically includes the following phases:

  • Exploration
  • Drilling
  • Production
  • Decommissioning/Reclamation

Seismic operation at Anahuac NWR, TX. Credit: M. Slaughter/USFWSExploration:  Seismic surveys and exploratory drilling are the major activities that occur during exploration for oil and gas. Petroleum geologists use seismic surveys to locate oil and gas underground. Seismic surveys use reflected sound waves created by explosives or shock waves produced by vibroseis equipment to map subsurface geologic formations and potential petroleum reservoirs. Activities and impacts associated with drilling exploratory wells are similar to those for drilling production wells (see below).

Seismic surveys generally cover many square miles. Access roads may be constructed or existing roads upgraded to support exploration. The extensive nature of seismic surveys and access requirements make seismic exploration among the most potentially disruptive and damaging type of oil and gas activity.


Drilling rig at Lacassine NWR. Credit: B. Leonard/ USFWSDrilling: Most oil and gas wells are drilled with rotary drilling rigs that include a host of equipment, including the derrick or mast (the steel tower that supports the drill pipe and hoist system); mud tanks, and diesel engines and generators. The equipment is placed on a well pad that ranges from 4 to 6 acres and perhaps larger. Well completion can vary from 28 to 80 days depending on the site and depth of the well. The average is 40 days.

After drilling the wellbore, the drilling rig and equipment are removed from the well pad and a decision is made on the ability of the well to economically produce oil and/or gas. If not viable, the well may be plugged and abandoned (see Decommissioning/Reclamation below).

If the well is economically viable, pipe or casing is installed and cement is pumped into the well to isolate it from aquifers and other geologic formations.

Horizontal wells drilled in shale oil or shale gas formations typically require fracturing the oil- or gas-bearing rock by injecting large volumes of fluids (water, sand and chemicals) under very high pressure. Hydraulic fracturing (frack) operations use 2-5 million gallons of water per well.  After the well is fracked, 10-70 percent of the fluid (flowback) is temporarily stored in either steel tanks (frack tanks) or an earthen pit (reserve pit). Flowback water is transported off-site for disposal in injection wells or evaporation ponds.


Oil pump jack and well, Hagerman NWR, TX. Credit: P. Ramirez/USFWSProduction: Following drilling and well completion, the well is brought into production. The wellhead is fitted with a series of valves, called a "Christmas tree." Most oil and gas wells will flow under natural formation pressure. However, over time, pressure decreases and requires installation of a pump. A pump jack is the most common pumping system used on oil wells.

Separators and storage tanks are used to process and store hydrocarbon liquids, crude oil, natural gas, and produced water or brine. Small diameter pipelines, or flow-lines, transport the liquids and gases to separators and storage tanks designed to store 3-7 days of production. Liquids from the storage tanks are typically transported off site by tanker trucks.

Well maintenance is periodically conducted using a workover rig to repair mechanical problems or clean out the well. Workover rigs may be brought in for repairs anytime production experiences a problem.  The rigs may be in place for only a few days to a few weeks, depending on the situation.


Workover rig and crew, Anahuac NWR, TX. Credit: M. Slaughter/USFWSDecommissioning/Reclamation: The average life of a natural gas or oil well is 20-30 years, but the life an oil or gas field can be far longer. On Deep Fork National Wildlife Refuge, the first well was drilled in 1901; however, production continues throughout this area today.


Last updated: February 21, 2014