RESERVIOR ROCKS

Igneous Rocks

Igneous Rocks are formed when molten rock (magma) cools and solidifies, either below the surface as intrusive or on the surface as extrusive. Igneous rocks make up approximately ninety five percent of the upper part of the Earth’s crust, but their great abundance is hidden on the Earth’s surface by a relatively thin but widespread
layer of sedimentary and metamorphic rocks. These rocks form the barrier to trap hydrocarbons below the ground. Igneous rocks can be of many types common to these are Granite and Basalt.

Sedimentary Rocks

These rocks are formed in three main ways, by the deposition of the weathered remains of other rocks (known as clastic sedimentary rocks), by the depositions of the results of biogenic activity and by precipitation from solution. Sedimentary rocks include common types such as limestone and sandstone. Sedimentary rocks often form porous and permeable reservoirs in sedimentary basins in which petroleum and other hydrocarbons can be found.

Metamorphic Rocks

Metamorphic rocks are formed from other sedimentary deposits by alteration under great heat and/or pressure. Examples of metamorphic rocks are;

• Marble – metamorphosized limestone.
• Hornfeld – converted from shale or tuff.
• Gneiss – similar to granite but metamorphically consolidated.

Oil and gas are not usually found in igneous or metamorphic rocks as both are so non-porous that hydrocarbons can not accumulate or be extracted from them. The few exceptions are when hydrocarbons have seeped from near-by sedimentary formations through cracks and fractures.

A reservoir rock must be able to contain oil, gas and water, which are the reservoir fluids. Pores in the reservoir rock are first filled with saltwater from the sea. When oil and gas flow into the rock, some of the water is displaced. However, not all of the water is forced out. Therefore, oil drillers usually find water with high concentrations of oil and gas.

Oil and gas travel through pores of the reservoir rock, with the help of water, until they reach an impermeable layer of rock through which they cannot pass. Shales are the most common impermeable rock

Oil traps usually form because of rock movements deep within the Earth’s surface.
Over many years, rock formations break and slide, causing spaces where petroleum is trapped. The most common type of trap is an anticline, where rocks are pushed up to form a dome. Oil and gas might lie in reservoir rock just under the top of the dome, which is capped by an impermeable layer of rock.

Another common type of trap is the fault trap, which is formed by a fault, or fracture, of the layers of rock. The rock on one side of the fault sometimes slips down so that a porous reservoir rock is next to a nonporous rock formation. This creates a seal, and the petroleum is trapped.

When salt or other form of intrusion forces up through the strata to form various types of plug and dome etc.
The formation of unconformity formations relies on the erosion of inclined formation which has more recently been covered up with a impenetrable overlying layer.
Lenticular reservoirs, where the permeability of an oil bearing formation changes from average value to zero. This can take the form of many different occurrences, Shale deposits in the formation and pinch-outs.

Now that the oil has been cooked, moved and trapped, it will stay there until rock formation movement causes a change in its surroundings, or until humans decide to drill a well in that spot.