Fluids in Rocks

Fluids in Rocks

Permeability and Porosity

  • Porosity is the measure of the volume of open spaces or ‘pores’ within a rock.
  • Permeability, however, refers to the capacity of a rock or sediment to transmit fluids through its pores and fissures.
  • High porosity does not always equate to high permeability since it depends on how interconnected the pores are.
  • Examples of rocks with high porosity and permeability include sandstone and limestone.

Fluid Migration

  • Fluids within rocks such as water or hydrocarbons can migrate from a source to a reservoir rock, influenced by factors like pressure, temperature, and the rock’s permeability.
  • Fluid migration can also be driven by gravity (buoyancy) and capillary action.
  • Understanding fluid migration is key in the economic extraction of fossil fuels.

Hydrothermal Fluids

  • Hydrothermal fluids are heated water solutions which circulate through the Earth’s crust, usually associated with igneous activity.
  • Hydrothermal fluids can dissolve minerals from the rocks through which they pass and later deposit them in a different location, creating ore deposits.
  • These fluids are predominantly responsible for the formation of economic metallic mineral deposits like copper, zinc and gold.

Fluid-Rock Interaction

  • Fluids can greatly impact the chemical and physical characteristics of the rocks through which they migrate.
  • Metasomatism is the process of mineral alteration, where the minerals in a rock are altered or replaced by new minerals due to the action of hydrothermal fluids.
  • Metasomatism can create economically important mineral deposits.

Economic Importance

  • Understanding the role of fluids in rocks is very important in the search for and extraction of hydrocarbon fuels (like petroleum and natural gas), as well as geothermal energy resources.
  • Porous and permeable rocks are often exploited to store water, oil, gas, and even waste products.

Fluid Inclusions

  • Fluids trapped within minerals as they crystallise are called fluid inclusions.
  • Studying these fluid inclusions can reveal valuable information about the conditions under which the mineral formed and the composition of the fluid from which it crystallised.
  • They often give us clues about the mineralizing fluids responsible for ore deposits.