In order to have economic reserves of hydrocarbon there has to be five aspects of a petroleum system. A conventional hydrocarbon reserve will have a source rock, hydrocarbon migration pathways, a reservoir or host rock, a seal, and a trap. An unconventional hydrocarbon reservoir is simplified in some aspects. With an unconventional oil play, the entire hydrocarbon system can be within one given formation. The source, reservoir, and in some cases the seal and trap can all be the same. These can be a super complicated systems and are studied by petroleum geologists and petroleum engineers for years. This article will outline why the Permian Basin is such a prolific source of hydrocarbon.
Without a really good source, it is impossible for economic reserves of hydrocarbon to accumulate. There are many different source rocks that are found in the Permian Basin, but most of them have similarities. The source is often an organic rich calcareous mudstone that has undergone a great deal of heat and pressure. The organic content most often results in plankton and algal blooms. The organic content must then be in an anoxic environment. The anoxic environment ensures that the organic content does not decompose. When the rock goes through lithification and is buried the organic material can then become cooked and the kerogen can then crack. There are many different types of kerogen, but light crude is most sought after.
When organic material is cooked, it expands and can cause microfractures in the rock. This allows the oil to migrate to the source. In the Permian Basin the reservoir is usually a carbonate rock. Limestone or dolomite can make a fantastic reservoir rock. Limestone is made of calcite and aragonite which are very susceptible to diagenesis, which is alterations, after deposition. diagenesis can greatly increase the porosity and permeability of the reservoir. Porosity and permeability is simply how easily oil can move through the rock. The higher the porosity and permeability the easier it is for the oil to move.
The oil would simply continue to migrate if it were not for a seal. The seal in the Permian Basin is as perfect as it possibly could be. The seal is most often a type of evaporite such as halite, gypsum, or anhydrite. The reason these are such great seals is they have no porosity or permeability. They also have the ability to flow, and fill any microfracture that could have occurred.
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