The Origin of Oil and Natural Gas
Reservoir Rock
A reservoir rock must have a high porosity to be able to store significant amounts of petroleum, it must also have a high permeability to allow oil to migrate into it and be extracted. The properties of a reservoir rock are the same as the properties of an aquifer. Suitable reservoir rocks include poorly cemented sandstones, limestones and fractured chalk.
Sapropel
After plankton accumulate on the sea bed anaerobic bacteria cause them to partially decay forming an organic rich mud called a Sapropel. Diagenesis of a sapropel forms an organic rich sedimentary rock called a source rock, for example black oil shales and mudstones. Their dark colour represents their high carbon content and their fine grain size represents the low energy conditions of deposition.
If Source Rocks Are Impermeable How Does The Oil Escape?
As maturation takes place Kerogen turns into Petroleum. Kerogen is a waxy solid composed of many different hydrocarbon compounds, where as petroleum is a viscous liquid. The transformation of a solid into a liquid corresponds with an increase in volume. This increase in volume induces an increase in pore fluid pressure which causes the rock to crack, creating pathways for the oil and gas to escape.
Maturation of Petroleum
As the source rock is buried, pressure and temperature increase. Over time the organic matter breaks down to form a mixture of organic compounds of C, H, O, N and S called Kerogen and finally Petroleum. This process is called maturation
How Can Petroleum Be Destroyed or Lost?
Oil and gas may be destroyed if the temperature exceeds 200°C due to: •Heat from an igneous intrusion or volcanic activity. •Regional metamorphism. •Burial where the geothermal gradient results in high temperatures. Oil and gas may be lost from a trap by: •Erosion or removal of the overlying cap rock. •Escaping upwards along an unsealed fault plane.
Requirements For The Formation of Oil and Gas
Oil and gas originate in sedimentary basins. The main requirements to form economic accumulations are: •A source rock •Maturation •Migration •A reservoir rock •A cap rock •A trap
Migration
Once formed the petroleum undergoes migration from the source rock to a reservoir rock. The main factors controlling migration are: •Permeability of rocks - There must be permeable rocks for the petroleum to travel through. •Pressure - Oil & gas naturally move down the pressure gradient from high to low pressure. •Density differences - Petroleum is less dense than water so will percolate upwards until it reaches an impermeable layer. •Viscosity of the oil - Higher temperatures lower the viscosity of the oil. The lower the viscosity the easier it is to flow.
What is Petroleum?
Petroleum is a naturally occurring, viscous hydrocarbon mixture of Crude Oil, Natural Gas and Asphalt. In addition to fuel petroleum forms the basis of the petrochemical industry. Products include plastics, paints synthetic fibres, rubber, pharmaceuticals and fertilisers.
Source Rock Formation
Petroleum is formed from the remains of microscopic marine organisms called plankton. Plankton live in the photic zone of the oceans and after dying accumulate on the seabed. The environment of deposition must be low energy so they can settle out of the water and anoxic so the plankton won't decay or be scavenged.
Cap Rock
The reservoir rock must be overlain by an impermeable cap rock. The cap rock prevents further migration petroleum. Without a cap rock the petroleum would continue to rise, eventually forming oil seeps or tar pits on the surface. Suitable cap rocks include clay, mudstone, shale and crystalline sedimentary rocks such as evaporites.
Traps
Traps are where the geological setting allows oil and gas to be concentrated in one place, making them economic to extract. All traps require a porous and permeable reservoir rock to be overlain by an impermeable cap rock.
