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0–5% insignificant
5–10% poor
10–15% fair
15–20% good
20–25% excellent

Two types of pores are primary and secondary pores. Primary pores are formed on the
surface when the sediments are deposited, for example, between the sediment grains. Secondary
pores are formed in the subsurface by processes such as solution and fracturing.
Porosity is a relatively easy and accurate measurement to make. Because of this, a
porosity cutoff (a minimum porosity value) is often used to help decide whether to complete an
oil well. For sandstones, a typical porosity cutoff is 8 to 10%. Limestones often have less
porosity than sandstones but typically have fractures that drain larger areas. For limestones, a
typical porosity cutoff of 3 to 5% is used. These values vary depending on the depth and
economics of the well.
Permeability is a measure of the ease with which a fluid can flow through a rock. It is
measured in units of darcys (D) or millidarcys (md). A millidarcy is 1⁄1,000 of a darcy. The
greater the permeability of a rock, the easier it is for the fluids to flow through the rock.
The only way to make a quantitative permeability measurement is to drill a core of the
reservoir rock and cut a plug. The plug is dried to remove any liquids. An instrument called a
permeameter is used to measure the permeability of the dried plug by measuring the flow of air
or nitrogen through it.
Porosity and permeability in a single sedimentary rock layer are related. In general, the
higher the porosity, the greater the permeability. Permeability, however, is also controlled by the
grain size. The oil or gas flow in rock is most difficult through the narrow connections (pore
throats) between the pores. The smaller the pore throats, the harder it is for the oil or gas to flow.
Smaller grain sizes have smaller pore throats. Because of this, porous, coarse-grained rocks such
as sandstones that have large pore throats are usually very permeable. A porous, fine-grained
rock such as shale or chalk has small pore throats and little or no permeability.
The two most common petroleum reservoir rocks are sandstones and limestones. Most
sandstones and limestones, however, are not reservoir rocks. A very low or no permeability
sandstone or limestone is called tight or tight sands.
Text 2
Saturation
In an oil or gas reservoir, the oil or gas always shares pore spaces with water. The relative
amount of the water and oil or gas sharing the pores of the reservoir will vary from reservoir to
reservoir and is called saturation. It is expressed as a percent and always adds up to 100%.
Saturation is why most oil wells pump not only oil but also water, called oil field brine. Oil field
brine is very salty water that shared the pores with the oil. The fluid that occupies the outside of
the pore and is in contact with the rock surface is called the wetting fluid. Sandstones usually
have oil in the center of the pore, and water is on the outside of the pore in contact with the sand
grains. Because of this, most sandstones are water wet (water coats the sand grains). In contrast,
limestones are usually oil wet (oil coats the rock surfaces). The percentage oil recovery tends to
be greater in sandstone reservoirs than in limestone reservoirs. This is because the fluid in the
center of the pore will flow more easily than the fluid on the outside of the pore, which is being
held to the rock surface by surface tension. Below the oil-water or gas-water contact, the
reservoir is saturated with 100% water.

Task 3. Copy out all the terms from the text, translate them and provide a glossary
for them.

Task 4. Answer the following questions, using the vocabulary from Task 1.
1. What kind of a rock can both store and transmit fluids?
2. What are the two characteristic features of a reservoir rock?