Directional drilling (or slant drilling) is the practice of drilling non-vertical well

Oil well

An oil well is a general term for any boring through the earth's surface that is designed to find and acquire petroleum oil hydrocarbons. Usually some natural gas is produced along with the oil. A well that is designed to produce mainly or only gas may be termed a gas well.-History:The earliest...

s. It can be broken down into three main groups: Oilfield Directional Drilling, Utility Installation Directional Drilling (or H.D.D. or Horizontal Directional Drilling) Directional boring

Directional boring

Directional boring, commonly called horizontal directional drilling or HDD, is a steerable trenchless method of installing underground pipes, conduits and cables in a shallow arc along a prescribed bore path by using a surface launched drilling rig, with minimal impact on the surrounding area....

, and in-seam directional drilling to extract coal bed methane.
Many prerequisites enabled this suite of technologies to become productive.