Most of the United States' highest-producing oil wells were drilled horizontally through hydrocarbon-bearing formations rather than vertically, according to data released by the Energy Information Administration (EIA).Count of U.S. oil wells producing at least 400 barrels of oil equivalent per day (2000–2015). Image credit: EIA.

In 2015, nearly 77% of the most prolific U.S. oil wells, or those producing more than 400 barrels of oil equivalent (BOE) per day, were horizontally drilled wells, EIA says. For approximately 85,000 moderate-rate wells in 2015—defined as those producing between 15 BOE and 400 BOE per day—42% were drilled horizontally. Of the approximately 370,000 lowest-rate, marginal oil wells in 2015, also known as stripper wells, only about 2% were horizontal wells, according to EIA.

Geologic formations are almost always much greater in horizontal extent than they are in vertical thickness, EIA says. For this reason, more oil-bearing rock is exposed for production in horizontal drilling than in vertical drilling.

Although modern horizontal drilling first achieved commercial success in the 1980s, drilling techniques have since improved, and in recent years the technology has become more commonly used. Horizontal wells are often completed using unconventional techniques such as hydraulic fracturing to maximize production along the exposed rock formation.