Conventional vs. Unconventional Plays
Contrast discrete buoyancy-driven accumulations with pervasive, continuous hydrocarbon deposits.
The distinction between conventional and unconventional resources is not about the oil itself, but the rock it sits in and how we get it out.
Conventional Plays
In a conventional play, oil and gas migrate from the source rock into a permeable reservoir rock, where they are trapped by a seal and accumulate due to buoyancy. These accumulations are discrete, localized in structural or stratigraphic traps. Because the reservoir rock has high permeability (often in the darcy to millidarcy range), the hydrocarbons flow easily to the wellbore under natural reservoir drive mechanisms.
Unconventional plays represent a fundamental shift: the source rock is the reservoir. In these plays, hydrocarbons never migrated out of the low-permeability rock where they were generated. Examples include shale oil and gas, tight oil and gas, and coalbed methane. These resources are pervasive and continuous across large areas, rather than discrete accumulations. The permeability is extremely low (micro- to nanodarcy range), meaning fluids cannot flow naturally at commercial rates.
The Role of Stimulation
Because unconventional reservoirs have permeability orders of magnitude lower than conventional ones, they require advanced technology to produce. Wells are typically drilled horizontally through the productive layer to maximize contact. Then, hydraulic fracturing is used to create artificial fractures in the rock, propping them open with sand or other materials. This stimulation bypasses the rock's low permeability, allowing hydrocarbons to flow into the wellbore. (The engineering mechanics of this process are covered in Module 8).
Resource Distribution
Conventional fields are relatively small in footprint but highly concentrated, making them efficient to develop once found. Unconventional plays cover massive geographic areas but yield lower production rates per well, requiring dense networks of well pads to develop the resource continuously. The distinction is largely technological and economic: what is 'unconventional' today may become conventional as extraction methods evolve.
- Analyze Prospect A: High permeability (200 md), structural trap (anticline). This fits the definition of a conventional play.
- Analyze Prospect B: Very low permeability (0.001 md), source rock is reservoir. This fits an unconventional play.
- Determine stimulation need: Prospect B's permeability is too low for natural flow; it requires hydraulic fracturing.
- Contrast: Prospect A will flow naturally under reservoir drive.
Check your understanding
- Conventional plays involve discrete accumulations in permeable reservoirs with buoyancy-driven traps.
- Unconventional plays involve pervasive hydrocarbons in low-permeability source rocks (shale, tight, coalbed).
- Permeability in conventional reservoirs (darcy/md) is orders of magnitude higher than in unconventional (micro/nanodarcy).
- Unconventional plays require hydraulic stimulation to produce hydrocarbons at commercial rates.