How does oil form?The image below shows the Kimmeridge Clay in Dorset, one of the major source rocks
How does oil form?The image below shows the Kimmeridge Clay in Dorset, one of the major source rocks for the North Sea. But how do you produce oil?It all starts with microscopic plankton and algae that live within the water column in oceans. When they die they rain down to the seafloor and accumulate. More often than not they are either scavenged by other organisms or broken down by bacteria,destroying the organic material. If the material is preserved within the sediment this usually occurs under anoxic conditions (less than 0.5 milligrams of oxygen per litre of water). They then become buried by overlying sediment and become lithified, forming the source rock (the rock from which hydrocarbons are produced). These are often shales such as the Kimmeridge Clay and have a high (over ten volume percent) total organic content (TOC).These rocks then need to be heated to 60-100ºC to produce oil and 120ºC - 200ºC to produced gas. With an average geothermal gradient of 25ºC/km, this means the rocks need to be buried to at least 3-4km beneath the surface to start producing hydrocarbons. Any hydrocarbons produced are lighter than the surrounding fluids or rocks and therefore migrate upwards towards the surface. This is only stopped when the hydrocarbons hit a seal (an impermeable rock, usually either mudstone or salt) causing the hydrocarbons to pool.Contrary to popular belief the reservoir rock (the rock that holds the oil and gas) isn’t just a large cavern filled with hydrocarbons. The oil is actually held within the pores spaces (the gaps between grains) and this why so much of it can become trapped and hard to extract when drilling into the reservoir.Also contrary to some rumors you may have seen, oil does not come directly from dinosaurs. Many of the current oil and gas source rocks did form during the cretaceous when dinosaurs existed, but those rocks are marine sediments mostly filled with plankton and algae. The amount of oil actually extracted is only a small fraction of the oil ever produced, and I will describe this in another post.WatsonImage Credit: Ian WestFurther Reading http://www.southampton.ac.uk/~imw/Kimmeridge-Bay.htm -- source link
#geology#science#natural gas#chemistry#fossil fuel