Oil and Natural Gas

Oil and natural gas typically form in sedimentary rocks. When carbon-rich organic material, like leaves, are deposited terrestrially in water with a low oxygen content (stagnant water), it may not fully decay. Think about swamps and bogs! If this happens and sediment is deposited on top, a coal bed can form. Many of the largest coal beds in the world are a result of huge Devonian and Carboniferous swamps . Similarly, oil and gas are formed when organic material is deposited in deep marine sediments. In marine deposits the organic material is mostly phytoplankton, microscopic animals that convert sunlightinto energy. When they die and settle to the ocean floor some of this energy remains in the form of carbon molecules in their bodies. Other elements that settle to the ocean floor are nitrogen,oxygen, and hydrogen. All of these molecules mix with very fine grained sediment and form an organic-rich ‘ooze’. As sediments continue to be deposited, burying the ooze (remember burial is vital to preservation!), the weight of the overlying sediment will cause an increase in temperature and pressure, which will in turn lithify the ooze. It also provides enough energy for the carbon and hydrogen molecules to bond, forming hydrocarbons: oil or natural gas. The temperature,pressure, and type of organic material determine whether oil or gas is formed. Relatively lower temperatures and pressures form oil, while higher temperatures and pressures form gas.

graph showing depth below surface vs. relative amount of petroleum formed.
Figure 1: The relationship between depth of burial, temperature and the relative amount of crude oil and natural gas formed from Type II kerogen in an area with a geothermal gradient of about 35 °C km−1
Source: OpenLearnWorks
 
Once oil and gas are formed, they are able to migrate through open spaces in rock called pore space. The spaces within a rock that are unoccupied by solid material. These include the space between grains, fractures, vesicles, and voids formed by dissolution. Geologists look for a trap,or impermeable rock that can catch migrating oil and gas. When pore spaces are well connected,a rock is considered to be relatively permeable, since it is easy for liquids to flow through it. An impermeable rock will have therefore have a low connectivity of pore spaces. The rock that the oil or gas resides in below the impermeable rock is called the reservoir rock, while the rock the oil and gas formed in is called the source rock. Sometimes, the source rock and the reservoir rock are the same - it just depends on where the impermeable rock layer is. Shale makes a good reservoir rock because the pore spaces are so small that oil and natural gas become trapped within and can’t migrate out. In fact, the very low permeability of shale is why hydraulic fracturing, or fracking, is the preferred method to extract the oil and gas from natural gas-rich layers like the Marcellus Shale. Fracking is the process of injecting liquid at very high pressures into a rock. The pressure causes any weak sections of the rock to break, forming pathways for the oil and gas to escape

Pore space and permeability image.

Trap/reservoir etc image.

Short video showing phytoplankton to reservoir rock?