Both consolidated and unconsolidated geological materials are important as aquifers.
Of the consolidated materials (i.e. bedrock), sedimentary rocks are the most important because they tend to have the highest porosities and permeabilities.
Sedimentary rock formations are exposed over approximately 70% of the earth’s land surface. These sedimentary formations are typically hundreds to thousands of meters thick, and they are underlain by the igneous and metamorphic rocks that make up the rest of the crust.
Suites of karst landforms in limestone and related carbonate rocks evolve through progressive denudation of the land surface, while underground denudation is simultaneously enlarging cave conduits so that ever-larger proportions of the drainage can pass underground. Both surface and underground denudation are largely by the dissolution of the carbonate, at rates dependent on the flow and chemical aggressively of the water. Both these factors are dependent on climate.
If the solution continues, its rate slackens and it eventually ceases when saturation is reached. Therefore, a solution is greatest when the bicarbonate saturation is low. This occurs when water is circulating so that fresh supplies with low bicarbonate saturation are made available continually.
Water flows are largely a consequence of rainfall input (though they can be increased locally by supplies of allogeneic water draining off adjacent outcrops of non-karstic rocks).
From the perspective of hydrogeology, the important characteristics of rocks are how much open space they have, how well connected those open spaces are, how strong the rocks are, and how soluble they are.
Sedimentary rocks are formed close to the surface of the earth at relatively low temperatures and pressures. Clastic sedimentary rocks are comprised of weathered and transported fragments of other rocks and minerals. Depending on the degree of sorting and rounding of those fragments, and the extent to which they are cemented together, clastic sedimentary rocks can be quite porous. Some clastic sedimentary rocks are also relatively soft and weak and are easily susceptible to fracturing.
Most sedimentary rocks also have some bedding features that can enhance porosity.
The empty spaces in between the crystals or fragments that make up a rock represent porosity that can hold water. Porosity is a measure of how much water a body of rock can hold, expressed as a percentage of the rock’s volume. The spaces between the grains or crystals of rock are referred to as intergranular porosity. Fractures in rock also represent porosity, and this is known as fracture porosity. For most naturally occurring media the porosity is between (0.10 to 0.40) although on occasion values outside this range have been observed.
Reservoir Rock & Source Rock Types: Classification
Reservoir rock: A permeable subsurface rock that contains petroleum. Must be both porous and permeable
Source rock: A sedimentary rock in which petroleum forms.
- Reservoir rocks are dominantly sedimentary (sandstones and carbonates); however, highly fractured igneous and metamorphic rocks have been known to produce hydrocarbons, albeit on a much smaller scale
- Source rocks are widely agreed to be sedimentary
- The three sedimentary rock types most frequently encountered in oil fields are shales, sandstones, and carbonates
- Each of these rock types has a characteristic composition and texture that is a direct result of depositional environment and post-depositional (diagenetic) processes (i.e., cementation, etc.)
- Understanding reservoir rock properties and their associated characteristics are crucial in developing a prospect
M. Jaafar Aquifer materials