(3) Metamorphic. These rocks are either igneous or sedimentary rocks, which have been
metamorphosed or transformed by extreme heat and/or pressure.
d. Porous and Permeable Rock.
(1) The three general rock types of significant importance to the formation of a petroleum reservoir
are sandstone, limestone, and dolomite formations.
(2) It is believed that hydrocarbons generally move upward from their place of formation (the source
bed) to their accumulation sites (the reservoir), displacing the seawater that originally filled the pore spaces of the
sedimentary rock. The primary forces causing the migration of petroleum are buoyancy and capillary action.
(3) Potential reservoir rocks require three basic properties: porosity, permeability, and saturation.
(a) Porosity. This is the capacity of the rock to contain fluids. Porosity is normally expressed in
percent, which represents the void volume of the rock divided by the bulk volume. The porosity of any given -
reservoir depends on grain packing, grain sorting, cementation of the grains, grain angularity/roundness, and
1 Packing refers to the configuration in which the grains are geometrically arranged.
Assuming grains me perfectly spherical in shape, a cubic packing (grains stacked directly on top of each other)
can have a maximum porosity of 47.6 percent. Given grains of the same size and spherical shape but in a rhombic
packing (grains stacked in the "hollow" formed by four other underlying grains), the porosity would be reduced to
2 Sorting refers to the sizes of the grains. "Well sorted" reservoirs have grains of uniform
shape and size. "Poorly sorted" reservoirs have grains of different sizes and shapes. Obviously, a poorly sorted
reservoir will have less porosity than the well sorted.
3 Cementation refers to the binding agent that holds the grains together. A well-cemented
reservoir will have low porosity, and a poorly cemented reservoir will have high porosity.
4 Angularity or roundness of the grains will affect porosity due to the interlocking and
subsequent filling of void spaces.
5 Compaction is the degree to which the overlying pressure alters the size and shape of the
reservoir rock. Normally, due to the time stresses on the rock, porosity will be hindered.
(b) Permeability. This defines the ability of a reservoir to permit flow or passage of reservoir
(c) Fluid saturation of a rock. This is the ratio of the volume of fluid within the pores of the rock
to the total pore volume. Fluid saturations are expressed as a percentage of pore volume. Very simply, a water
saturation of 30 percent means that 3/10 of the pore space is filled with water. Water, oil, and gas may be found
simultaneously in hydrocarbon reservoirs. However, due to gravity, fluids tend to segregate within the reservoir.
The reservoir can be put into one of three classifications. These classifications are known as porosity,
permeability, and saturation.