Hydrocarbons accumulate when three conditions are met
- A sedimentary basin must be formed. This phenomenon is the result of the movement of the earth’s crust, which results in huge depressions, which are filled with sediments from surrounding areas which are elevated, over time.
- These basins must be filled with high levels of organic material, which becomes part of the sedimentary material to create source rock.
- Due to elevated temperature and pressure, and over millions of years, the material in the source rock was converted into oil and gas. Thick oil is considered to be immature and generated at relatively low temperatures, whereas, lighter or less viscous oil, which forms at higher temperatures is considered mature.
A process which happens after, called migration, also play an important role. Hydrocarbons move out of the source rock through cracks, faults and fissures, into porous and permeable reservoir rock. The reservoir rock is modified in a specific way that immobilizes hydrocarbons within structures called traps, this allows oil and gas to accumulate in sufficient volumes for commercial use.
Petroleum geochemists and geologists agree that crude oil comes from ancient organic matter. Ranging from single-celled planktons to complex aquatic plants like algae, and even invertebrates and fish, which are buried and preserved in sediments aeons ago at the bottom of ancient oceans. Rivers also carry organic materials into the oceans, these materials are also deposited and buried in a similar way.
As millions of years pass, the organic matter undergoes a transformation. Initial microbial action (in the presence of oxygen dissolved in seawater) returns part of the sediments carbon to the atmosphere in the form of carbon dioxide. When sediment layers get deeper the bacteria have to work in deep seabed mud with little or no oxygen present. In this case, the organic matter is converted into kerogen, a waxy material. Kerogen mixture consists of large organic molecules and their appearance depending on the kind and concentration of materials such as algae, plankton, pollen, resin, cellulose and bacteria. Oil and gas are generated from kerogen.
Our world has been through many geologic eras. Throughout these eras organic matter was not laid down evenly, accumulations occurred in limited intervals whose duration was determined by the movement of earth’s crust and changes in climate. The levels of world’s kerogens were contributed based on different periods:
- Middle Cretaceous (about 100 million years ago): nearly 30%
- Late Jurassic (150 million years ago): 25%
- Late Devonian (350 million years ago): less than 10%
- Silurian (420 million years ago): less than 10%
- Early Cambrian (550 million years ago): less than 10%
A rough estimate of the total amount of organic matter stored in the earth’s crust is 10 quadrillion tons. However, only 1% is in organic-rich rock. Usually, shales contain at least 3% of organic matter. Click here to continue reading about oil and gas origins.