We often hear about oil and gas, but it seldom crosses our mind that these two terms represent very complex characteristics and that we can classify the sources, reservoirs, and even the fluids and mechanisms of recovery, as unconventional or conventional.
Conventional reservoirs refer to the more frequently found type of hydrocarbons natural storages or reservoirs, as crude oil from low to medium specific gravity (high to medium API gravity), with generally normal pressure conditions, not so high or not so low temperatures, among other characteristics that can also appear on unconventional environments. This to end in more concretely defined unconventional hydrocarbons as those hydrocarbons being ‘difficult to produce’ in comparison with conventional ones, and therefore, they are alternative sources of Fossil Energy.
For now, we can start listing the main unconventional sources of oil and/or gas by differentiating first between liquids and gases, for easier understanding of this matter.
- Natural Bitumen (Oil Sands) – Generally defined as being of less than 10° API gravity and having a viscosity greater than 10,000 centipoise. See More.
Tar Sands Example
- Extra-Heavy Oil – Extra-heavy oil is generally defined as being of less than 10° API gravity but having a viscosity less than 10,000 centipoise. See More.
- Shale Oil – Shale oil is produced from fractures within a mature source rock that lies within the oil window, i.e. the shale is source, reservoir and seal. See More.
- Coal bed Gas – Coalbed gas is methane-rich gas occurring in self-sourcing reservoirs in undeveloped coal beds and worked coal seams. See More.
- Anomalously-Pressured Basin-Centred Gas («Tight Sand Gas») – This type of unconventional gas play comprises gas dissolved in abnormally-pressured low-permeability aquifers in the central (generally deeper) part of basins. They tend to be overpressured in subsiding basins and underpressured in uplifted and eroded basins. See More.
Shale Gas – As in the case of shale oil, shale gas production comes from a tight self-sourcing shale reservoir. In this instance, however, the shale source/reservoir is over-mature for oil. In fact, the presence of liquid hydrocarbons impedes the relative permeability of gas and is detrimental to gas production. See More.
Shale Gas Rock Sample
- Gas Hydrates – Gas hydrates or clathrates are methane trapped in a lattice of ice. They exist onshore in permafrost regions at depths from 130 – 2,000 m and on continental margins where water depths exceed 300 m and bottom water temperatures are around 0°C. Where present, hydrates themselves may act as a top seal for underlying free gas accumulations. See More.
From a geological point of view, “unconventional hydrocarbons are considered to be naturally-occurring hydrocarbon accumulations that are not significantly affected by hydrodynamic influences and that lack welldefined downdip water contacts. Generally referred to as «resource plays», these accumulations are often pervasive throughout a large area. The limits of such occurrences tend to be those of the containing lithology, and exploitation focuses on the identification and development of «sweet spots».”
This geological definition therefore excludes non-fossil, renewable and conversion sourced hydrocarbons such as landfill gas, bioethanol/biodiesel and coal-, shale- and gas-to-liquids conversion.