What is shale gas?
Shale gas is conventional natural gas that is produced from reservoirs predominantly composed of shale with lesser amounts of other fine grained rocks rather than from more conventional sandstone or limestone reservoirs. The gas shales are often both the source rocks and the reservoir for the natural gas, which is stored in three ways:
- adsorbed onto insoluble organic matter called kerogen
- trapped in the pore spaces of the fine-grained sediments interbedded with the shale
- confined in fractures within the shale itself
How is shale gas formed?
There are two theories as to how natural gas is formed. The most widely accepted theory, the organic theory, maintains that natural gas formation begins with photosynthesis, where plants use energy from the sun to convert carbon dioxide and water into oxygen and carbohydrates. The remains of these plants and the animal forms that consume them are buried by sediment and as the sediment load increases, heat and pressure from burial converts the carbohydrates into hydrocarbons. Natural gas formation takes place in fine-grained, black, organic, shale source rocks. Continued pressure from burial forces most of the natural gas to migrate from the organic shales into more porous and permeable rock such as sandstone and limestone. The natural gas remaining in the shales is termed shale gas.
The other theory of natural gas formation is the inorganic theory which speculates that hydrocarbons did not originate from buried plant and animal material, but instead were trapped inside the Earth as it formed. This theory is most likely not applicable to shale gas.
How is shale gas found?
Exploration for gas shales is similar to exploration for conventional reservoirs which, for an unexplored basin, usually includes:
- review of existing information
- aerial surveys to gather data regarding magnetic fields, gravity and radiation
- seismic surveys to locate and define subsurface structures capable of trapping natural gas
- exploration drilling to test subsurface structures for the presence of hydrocarbons
- logging the wells to determine porosity, permeability and fluid composition
In the case of shale gas, the primary targets are shale formations with interbedded porous and permeable fine-grained sediments and natural fracture systems.
Down-hole tools used to find fractures include density compensation, caliper and temperature logs, and formation microscanner imaging.
Low-altitude, airborne multispectral imaging is a new tool used to locate subsurface microfractures and prospectivity of shale formations.
previous | next