Energy Glossary: The Vocabulary of Oil & Gas

Energy has a dense, acronym-heavy language. This glossary defines the terms you will meet most often across this guide, each in a sentence or two of plain English. For fuller context, follow the links to the relevant explainers.

Volumes and units

• Barrel (bbl) — The standard unit for crude oil, equal to 42 U.S. gallons.
• Mcf / MMcf / Bcf — Thousand, million, and billion cubic feet of natural gas, respectively (the Roman numeral M means thousand here).
• Barrel of oil equivalent (boe) — A way to add oil and gas together by converting gas into the amount of oil with the same energy content.
• Reserves — The portion of a discovered resource that can be produced economically with current technology and prices; distinct from the total resource in place.

Geology and the subsurface

• Source rock — The organic-rich rock in which oil and gas originally form, explained in our basics guide.
• Reservoir — Porous, permeable rock that holds hydrocarbons in its pore spaces.
• Cap rock / seal — An impermeable layer that traps hydrocarbons and stops them migrating further.
• Permeability — How easily fluids flow through rock; low permeability is what makes shale “unconventional.”
• Play — A specific productive formation or group of prospects sharing similar geology, such as the Wolfcamp or Niobrara.

Drilling and completion

• Horizontal drilling — Steering a well sideways to run through a thin target formation, maximizing contact with the rock.
• Hydraulic fracturing — Pumping fluid at high pressure to create cracks in tight rock so hydrocarbons can flow; see the shale guide.
• Proppant — Sand or engineered particles that hold fractures open after the pressure is released.
• Casing — Steel pipe cemented into the wellbore to protect groundwater and stabilize the hole.
• Completion — The work that turns a drilled hole into a producing well, including perforating and fracturing.

Production and operations

• Decline curve — The typical decrease in a well's output over time; steep in unconventional wells.
• Associated gas — Natural gas produced alongside crude oil.
• Pumpjack — The nodding surface pump that lifts oil once natural pressure declines.
• Plug and abandon (P&A) — Permanently sealing a spent well with cement and reclaiming the site.

Industry structure and markets

• Upstream / midstream / downstream — Finding and producing (upstream); transporting and storing (midstream); refining and selling (downstream). See the E&P lifecycle.
• Royalty — A share of production value paid to the owner of the mineral rights.
• WTI / Brent / Henry Hub — The main price benchmarks for U.S. crude, international crude, and U.S. natural gas; see energy markets.
• LNG — Liquefied natural gas, chilled to a liquid for shipping across oceans; see natural gas.
• NGLs — Natural gas liquids such as ethane, propane, and butane, separated during processing.

Keep learning

For authoritative, continually updated definitions and data, the EIA's own glossary is an excellent companion to this one. When a term here still feels abstract, the linked explainers put it back in context.