Why can sandstone hold oil

Chemical reactions run faster at higher temperatures. At lower temperatures, or smaller depths, the oil may take millions of years to form.

If a source rock has not been buried deep enough for a long period of time, the conventional oil does not form. An example is the shale oil rocks. After the oil is created, it is pushed by pressure upwards. Under the pressure from the oil the shale rock which is a typical source rock can form cracks and allow the oil to flow either upwards or in horizontal directions. The rocks that can both store and transmit the oil are the reservoir rocks.

Usually, these two characteristics are correlated: the higher the porosity the higher the permeability. A lot of reservoir rocks are sandstone and carbonates. Sandstone is a rock composed of sand size grains. Carbonates consist of calcite and dolomite. Calcite is accumulated at the bottom of the ocean from the remains of the sea shell organisms.

The density of dolomite is higher than the density of calcite, as a result, pores form around the dolomite crystals inside calcite. The second effect is solubility of calcite in mildly acid water the acidity of water may be due to presence of CO 2 , while dolomite is only soluble in highly acidic water.

Thus the calcite around the dolomite crystals can dissolve which increases the size of the pores. The last but not the least is that there should be a cap rock that prevents the oil from seeping into the surface of the Earth. The cap rock is a rock that cannot transmit oil.

Examples of cap rock are shale rocks or limestone and sandstone rocks immersed in shale. An example of a trap is an anticline which looks like a dome of a cap rock over the reservoir rock Fig. Natural gas, which is usually formed together with the oil, is the lightest element, it will be at the top of the dome unless it completely dissolves in the oil. Then there is a deposition of oil. The scale is equal to one millimeter. The rock sample was injected with blue-colored epoxy that is seen here filling pores which are interconnected permeable.

After plastic is injected and solidified, the rock sample is cut and polished on a glass slide to a thickness of 35 thousandths-of-an-inch. The "thin section" of the rock is thin enough to permit light to be transmitted through it as in this photomicrograph. This particular sandstone contains grains of quartz white , calcite, and feldspar shades as brown.

The grains originally came from other rocks that had been eroded. The sample is exceedingly porous and permeable. Thegrains are loosely packed and there is very little cement filling the space between the grains. The arrow indicates possible pathways for fluid movement. Figure 2b --A thin-section photomicrograph of a Pennsylvanian limestone taken from a core sample of a producing zone in Victory field, haskell County, Kansas.

This particular sample comes from an interval that is not a good reservoir rock. Circular grains composed of calcite finely crystalline, reddish-stained areas in a grain and dolomite clear, coarse crystals are completely cemented by medium crystalline calcite. No porosity is visible. The cap rock prevents upward migration of these fluids. These types of traps are formed by a folding of rock. Specifically, a sandstone bed covered with low permeability shale is folded into a trap that contains petroleum products.

Hydrocarbons are trapped in the peak of this fold. Most anticline traps are created as a result of sideways pressure, folding the layers of rock, but can also occur from sediments being compacted. For more information on anticline traps, click here. These types of traps are formed when reservoir rock is split along a fault line. Between the walls of the split reservoir, clay traps oil and prevents it from leaving the trap.

Although faulting is common in many petroleum fields, traps that result from faulting alone are not very common. Generally speaking, faulting creates some other type of structure that creates the main trap. Salt dome traps are formed as a result of below ground salt - which is less dense than the rock above it - moving upwards slowly.

This upward migration of the salt can deform and break up rock along the way. A salt dome trap is shown on the lower left of Figure 1.

These traps are formed as a result of the deposition in sedimentary rocks. When the sediment that creates the reservoir rock is deposited in a discontinuous layer, the seals are created beside and on top of the reservoir.