Oil and gas reservoirs are usually categorized as conventional or unconventional based on the characteristics of the rock and the techniques needed to extract hydrocarbons.

Conventional Reservoirs

In conventional reservoirs, hydrocarbons migrate from a source rock into a porous and permeable reservoir rock such as sandstone or carbonate. These rocks typically have sufficient permeability to allow oil or gas to flow naturally toward the wellbore. As a result, hydrocarbons can often be produced with relatively simple well designs and without extensive stimulation treatments. Although stimulation techniques may still be used to enhance productivity, production in conventional reservoirs is primarily controlled by the reservoir rock’s natural properties.

Unconventional Reservoirs

In contrast, unconventional reservoirs contain hydrocarbons trapped in rocks with very low permeability. In these formations, fluids cannot easily flow through the rock matrix, even when hydrocarbons are present in large quantities. As a result, natural flow toward the wellbore is extremely limited. Examples of these unconventional resources include shale oil and gas, tight oil and gas formations, and coalbed methane reservoirs.

Because of these low-permeability conditions, unconventional reservoirs typically need specialized production techniques to achieve economic production. Technologies like hydraulic fracturing and horizontal drilling are commonly used to create additional pathways for fluid flow and to expand the contact area between the wellbore and the reservoir.

As drilling and stimulation technologies continue to advance, reservoirs that were once considered unconventional will gradually become conventional, making them easier and more profitable to produce.