Oil and Natural Gas Formation
Coal, oil and natural gas typically form in sedimentary rocks. When carbon-rich organic materials, such as leaves, are deposited in stagnant water such as a bog or swamp with a low oxygen content it may not fully decay. If this happens and sediment is deposited on top, a coal bed can eventually form. Many of the largest coal beds in the world are a result of huge Devonian and Carboniferous-age swamps. Similarly, oil and gas are formed when organic material is deposited in marine sediments where the organic material consists mostly of microscopic organisms that convert sunlight into energy such as phytoplankton, dinoflagellates, and/or radiolaria. Plant material such as algae and pollen is another primary source of organic material that is converted into oil and gas. When these organisms and plant materials 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 (i.e., turn it into stone) and convert the organic material into kerogen and ultimately oil and natural gas.
In this video clip, Dr. Terry Engelder describes the depositional environment in which organic-rich shales are deposited.
Video: Depositional Setting for Shale, Terry Engelder (1:48)
The temperature, pressure, and type of organic material in the kerogen determine whether oil or gas or a mixture is formed as shown in the figure below. Type 1 kerogen is typically formed in lacustrine(lake) environments and prone to form oil, Type 2 kerogen is formed in marine environments and prone to form oil and natural gas, and Type 3 kerogen is formed from terrestrial deposits and prone to form natural gas. Relatively lower temperatures and pressures form oil, while higher temperatures and pressures form natural gas, however, a spectrum of hydrocarbons can be formed, including ethane, propane, butane, and other natural gas liquids.
In this video clip, Dr. Terry Engelder of Penn State describes what conditions make a shale formation prospective for oil and gas development.
Video: Geologic Conditions for Shale, Terry Engelder (1:14)
Once oil and gas are formed, the hydrocarbons become pressurized and a portion of them are able to migrate through the rock's porosity, which is the open space between grains, fractures, vesicles, and voids formed by dissolution. Geologists look for rock layers that are sufficiently impermeable and seal off the further migration of oil and gas, which become trapped in the relatively permeable underlying rock and is known as a stratigraphic trap or seal rock. 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, therefore, have a low connectivity of pore spaces. A fault in a rock can also have low permeability and trap hydrocarbons in the underlying permeable rock, which is known as a structural trap. Geologists and energy companies have been producing hydrocarbons for over 100 years out of these relatively permeable formations that contain trapped hydrocarbons, which is known as a reservoir rock. Conventional formations are reservoir rocks, typically sandstones or carbonates, with sufficient porosity to store hydrocarbons that have migrated upward out of source rock, typically an organic-rich, black shale, which is where the hydrocarbons were formed (kind of like a kitchen). Where black shales retain enough hydrocarbons they are considered as both the source rock and the reservoir rock, which is considered an unconventional formation with low permeability and therefore requires hydraulic fracturing, or fracking, to extract the oil and gas from the natural gas- or oil-rich shale. Hydraulic fracturing is the process of injecting liquid at very high pressures into a rock, which causes any fractures or weak planes of the rock to open up, thus forming pathways for the oil and gas to migrate out of the low-permeability shale and flow into a well. The figure below shows the black shale layer that is the source of oil and gas in the overlying sandstone and can be drilled into, often horizontally or directionally. We will explore more on horizontal drilling and hydraulic fracturing in the next lesson.