An oil or gas well that has sections that have been drilled to a horizontal or roughly horizontal inclination from vertical. This allows penetration of a greater length of the reservoir or formation from one single drilling pad, improving efficiencies and productivity. Horizontal wells are prominently used to extract hydrocarbons from shale formations because of hydraulic fracturing technology and advances in directional drilling.

For more information on horizontal drilling and its effect on US oil and gas production, see Practice Notes, Understanding Hydraulic Fracturing: Issues, Challenges, and Regulatory Regime and US Oil & Gas Industry: Overview.